ralphonz 3/16/2018 9:36 AM
Fender Stage 112 repair - how to tackle burnt PCB
Hi guys, This is my first post and also my first time repairing an amp. Please go easy :) I'm taking a look at my band-mates fender 112. It has two burnt resistors (R75 & R76) which have almost burnt a hole on the top and on the bottom the pads and traces have lifted. [ATTACH=CONFIG]47620[/ATTACH] [ATTACH=CONFIG]47621[/ATTACH] Can anyone please advise me on the best way to treat this and get the amp working again. I alredy tried replacing the resistors using the bits of trace and pad still left there, and although the amp was better for a short while it still didn't work properly - the problem being that the volume would suddenly drop after 30 seconds or so of playing and become really distorted, then intermittently would go back up again.... If anyone can tell me why the resistors burnt in the first place and if there is anything else I should check (though all caps look fine) then I'm all ears! I'm here to learn :)
 
Mick Bailey 3/16/2018 11:04 AM
Those resistors along with C49 form a 'Zobel network'. If you look at the schematic, that capacitor hangs off the +ve speaker connection and this is connected to the paralleled resistors that then connect to ground. Maybe C49 is shorted/shorting, or you have an issue that's putting excessive high-frequency across the network. Make sure the connections to L1 and R82 are good. In fact, while you're at it inspect the board around those components for bad solder joints. With components like the capacitor if I suspect them I replace them and don't bother testing. To repair your board you need to cut away the lifted section cleanly with a scalpel. I would also clean away any carbon. It's convenient that the resistors are paralleled because you can twist the leads together on the copper side of the board to give a secure mounting. Solder the twist and clip it neatly, then attach insulated wires to where the tracks originally ran to the next solder pad. I secure the leads with a wire-tack adhesive, but a dab of silicone or hot-melt can be used sparingly. Just enough to secure the leads against the PCB
 
bob p 3/16/2018 11:14 AM
wire tack adhesive?
 
ralphonz 3/16/2018 11:18 AM
[QUOTE=Mick Bailey;483122]Those resistors along with C49 form a 'Zobel network'. If you look at the schematic, that capacitor hangs off the +ve speaker connection and this is connected to the paralleled resistors that then connect to ground. Maybe C49 is shorted/shorting, or you have an issue that's putting excessive high-frequency across the network. Make sure the connections to L1 and R82 are good. In fact, while you're at it inspect the board around those components for bad solder joints. With components like the capacitor if I suspect them I replace them and don't bother testing. To repair your board you need to cut away the lifted section cleanly with a scalpel. I would also clean away any carbon. It's convenient that the resistors are paralleled because you can twist the leads together on the copper side of the board to give a secure mounting. Solder the twist and clip it neatly, then attach insulated wires to where the tracks originally ran to the next solder pad. I secure the leads with a wire-tack adhesive, but a dab of silicone or hot-melt can be used sparingly. Just enough to secure the leads against the PCB[/QUOTE] Thanks Mick, very helpful info. I had looked at the schematic and suspected something funky was going on with that little circuit :) What exactly do you mean by "clean away any carbon"? The black charred stuff? Time to read up on "Zobel Network" and order the bits and bobs i need to fix it. Thanks again, I think I'm going to enjoy this forum.
 
ralphonz 3/16/2018 11:40 AM
[QUOTE=bob p;483123]wire tack adhesive?[/QUOTE] This sort of thing? [URL="https://uk.rs-online.com/web/p/cyanoacrylate-adhesives/1987144/"]https://uk.rs-online.com/web/p/cyanoacrylate-adhesives/1987144/[/URL] Seems pricey, is it worth investing in some?
 
52 Bill 3/16/2018 11:50 AM
Welcome to the place. Often when I find those resistors burned or overheated like that, I find that some of the small value feedback loop caps in the preamp section have broken off or have come unsoldered. Caps like C13 or C19. You should grind away the burnt, carbonized part of the pc board as much as possible without damaging the underside copper traces. Carbon is conductive and while the amp you are working on will probably not be effected by it, it is good practice to clean it out when it occurs.
 
bob p 3/16/2018 12:57 PM
[QUOTE=ralphonz;483128]This sort of thing? [URL="https://uk.rs-online.com/web/p/cyanoacrylate-adhesives/1987144/"]https://uk.rs-online.com/web/p/cyanoacrylate-adhesives/1987144/[/URL] Seems pricey, is it worth investing in some?[/QUOTE] LMAO. Ethyl cyanoacrylate is Crazy Glue.
 
Mick Bailey 3/16/2018 5:19 PM
The stuff I use is rubberized and specifically used to secure wiring in aircraft assemblies. Super-expensive but I get it cheap due to it being close to its expiry date. It still lasts for ages and works perfectly. Just like you can still eat a can of peaches three years out of date. I've seen those resistors burn when there's high-frequency oscillation. Oscillation can produce a lot of energy that causes distortion and saps the amp of power, though you can't hear it. Above a certain frequency C49 will act like a straight piece of wire and then there are just the resistors in the way to cook. It can also damage your speaker. Boards in amps can suffer solder cracks and I use an illuminated bench magnifier to inspect the solder joints. Your eyesight may be better than mine.
 
bob p 3/16/2018 6:21 PM
I'd like to know more about the rubberized stuff. I have an amp with a burned ground trace that needs to be fixed and I've been putting that off...
 
ralphonz 3/17/2018 6:37 AM
[QUOTE=Mick Bailey;483122]Those resistors along with C49 form a 'Zobel network'. If you look at the schematic, that capacitor hangs off the +ve speaker connection and this is connected to the paralleled resistors that then connect to ground. Maybe C49 is shorted/shorting, or you have an issue that's putting excessive high-frequency across the network. Make sure the connections to L1 and R82 are good. In fact, while you're at it inspect the board around those components for bad solder joints. With components like the capacitor if I suspect them I replace them and don't bother testing.[/QUOTE] I've tested C49 which looks to be within tolerance - I read 107nf. All the joints for C49, L1 and R82 look good so I'm assuming the problem was in the bad pad/track from R76 to C49 (or R76 to ground?). In your experience would there be anything else to check before I put it back together to test (it's not the easiest board to get out of the chassis!)? Thanks for the responses everyone.
 
Mick Bailey 3/17/2018 8:27 AM
Whilst the value reads OK, this doesn't tell you if a cap shorts out in operation. Cap value is not an indication of whether its good or not, though no reading or zero is clearly a problem. Whatever caused those resistors to overheat in the first place was most likely not a bad pad or bad track. The resistors cooked because of excessive voltage (resulting in their wattage rating being exceeded). So even if you replace the resistors and wire them in you're no further forward - you still have the original fault. What level of equipment do you have to hand - DMM/scope/variac? At this stage I would repair the original damage (and throw in a new C49 just to be sure) and be scoping around the output looking for oscillation. In the absence of a scope a decent DMM will detect oscillation. The schematic has useful test voltages and outlines the test conditions in the notes section. Again, did you thoroughly check the board for cracked joints? A consistent problem with amps is circular cracks around component legs. Sometimes the leg is loose.
 
Enzo 3/17/2018 11:05 AM
In my experience, there is only one thing that bur s up those resistors - HF oscillation in the amp. If teh cap shorted, they would be across teh output, but I don't recall ever finding a shorted cap there.
 
ralphonz 3/18/2018 8:18 AM
Thanks Mick, Makes perfect sense, yes the original fault that caused the resistors to burn must still be present. I don't have a scope or variac but I do have a DMM. I've looked all over the board and all the joints look shiny and well-shaped, nothing seems loose or cracked. To be honest I don't know where to start looking for HF oscillation with a DMM, this is my first amp repair. When I had a guitar plugged into the amp prior, the power amp input worked fine, only input 1 and 2 exhibited the behaviour I described in my first post - I should have mentioned that before.
 
Mick Bailey 3/18/2018 10:52 AM
Ah. So that means the power amp is OK and probably what we're looking at is a fault that lies in the preamp. I'm thinking that 52 Bill maybe had the answer in post #6 and those caps should be examined in more detail. So that would be C13, C19, C30, C34, C37. These are in the feedback loops of the opamps and serve both to stabilize the circuits to prevent oscillation and also to prevent broadcast radio from being picked up and amplified. A good DMM should be able to 'read' oscillation - certainly my Fluke does this when set to AC volts mode (mine autoranges but manually you could begin with 200mV). You see on the schematic where those caps are located? Pins 1 or 7 respectively are the opamp outputs. If you insert a plug into the 'power amp in' socket you'll disconnect the power amp to prevent further damage. Ground the negative lead of your DMM and check with the +ve lead to see if there's any AC voltage on the outputs of those opamps. There's usually a little due to power supply ripple and noise, but should be no more than a few mV at most. Sometimes an oscillating opamp will get much hotter than the rest. Turning up the controls will give you the best chance of spotting anything wrong. The only caveat with this is the multimeter itself (or scope probe) can sometimes stop any oscillation.
 
Enzo 3/18/2018 11:07 AM
The zobel network is there to protect the PA from itself. If the PA turns into a power oscillator is can shake itself to death. Just sending it a steady HF signal from the preamp won't hurt the PA. The PA can break into oscillation at 50kHz even, and my meter won't read that.
 
J M Fahey 3/18/2018 2:03 PM
[QUOTE=bob p;483139]LMAO. Ethyl cyanoacrylate is Crazy Glue.[/QUOTE] Crazy Glue is *one* Cyanoacrylate adhesive. There are [B]many[/B] more out there, catering to specific needs ;)
 
Mick Bailey 3/19/2018 3:13 AM
[QUOTE=Enzo;483319]The zobel network is there to protect the PA from itself. If the PA turns into a power oscillator is can shake itself to death. Just sending it a steady HF signal from the preamp won't hurt the PA. The PA can break into oscillation at 50kHz even, and my meter won't read that.[/QUOTE] I was thinking here if the frequency was high enough on the input those resistors could see a higher voltage. Just looking at the Zobel network on its own as a simple CR filter the corner frequency works out at roughly 68kHz. If the PA works fine with a signal plugged straight in I can only think of the preamp oscillating. A trick with DMMs to read oscillation where no scope is available is use a signal diode in series with a small-value film cap (1uf or greater) across a pair of probes and set the meter to read DC volts, measured across the cap. Many DMMs are flaky when it comes to HF readings on AC volts and this gets round it by reading the rectified voltage. You can't read lower than the forward voltage drop of the diode, though. Usually oscillation is severe so this isn't a problem - I have an amp on my bench this morning where one of the opamps is oscillating near-enough rail-to-rail.
 
Jazz P Bass 3/19/2018 9:36 AM
So what would cause a guitar amp opamp circuit to oscillate? Phase shift?
 
bob p 3/19/2018 9:45 AM
[QUOTE=J M Fahey;483329]Crazy Glue is *one* Cyanoacrylate adhesive. There are [B]many[/B] more out there, catering to specific needs ;)[/QUOTE] Thanks, Juan. I didn't know that you were a polymer chemist. :D That there are many derivatives is not the point. The point is that one specific formulation, *ETHYL* cyanoacrylate, is [I]Krazy Glue[/I]. It's also the exact same formulation that was linked to in post 5. MSDS: [url=http://www.krazyglue.com/docs/default-source/MSDS-Sheets/mkg0925.htm?sfvrsn=2]KG0925 KRAZY GLUE ALL PURPOSE BRUSH-ON[/url] That's relevant because you can buy generic ethylcyanoacrylate at hadware stores in the USA for 99-cents, or you can pay [URL="http://uk.rs-online.com/web/p/cyanoacrylate-adhesives/1987144/"]44 pounds[/URL] (!) for it in the UK if you need to have the Loc-Tite brand. [U]It's the exact same chemical at both prices[/U]. All that differs is the packaging and the price. Caveat emptor.
 
bob p 3/19/2018 9:49 AM
[QUOTE=Jazz P Bass;483421]So what would cause a guitar amp opamp circuit to oscillate? Phase shift?[/QUOTE] Yes. The typical push-pull amp that uses negative feedback from the OT secondary to the phase inverter has three frequency poles where phase shift occurs in the feedback loop: the OT, the grids of the power tubes, and the grids of the PI. All of these impart frequency dependent phase shifts. Negative feedback is optimized when the phase angle is 180*. It becomes neutral when it shifts enough to become 90* or 270*. It becomes positive feedback when the angle is in between 90* and 0* or 270* and 0*. Positive feedback causes oscillation.
 
Enzo 3/19/2018 10:16 AM
[QUOTE]If the PA works fine with a signal plugged straight in I can only think of the preamp oscillating. [/QUOTE] And if we find the preamp is not oscillating either?
 
Mick Bailey 3/19/2018 11:10 AM
Then we're down to troubleshooting the complete amp as a system - maybe one time where divide and conquer doesn't work.
 
J M Fahey 3/19/2018 12:51 PM
Or in other words: **any** amplifier with positive feedback, and loop gain >1 will oscillate.
 
Enzo 3/19/2018 1:30 PM
And some circumstance causing the oscillation may no longer be present. We could have an intermittent ground connection somewhere, or for that matter in many other places in the circuit.
 
Mick Bailey 3/19/2018 5:28 PM
I can't recall working on one of these amps where I didn't find at least one cracked solder joint - often the inductor L1 is loose, but can be anywhere. The OP said the amp intermittently played OK, so that could indicate a crack somewhere. Sometimes a crack can be obvious, but equally can be elusive. How many times do you find an unsupported TO-220 device where the leg has cracked at the junction of the top-side of a single-side board and the joint itself is good? Or those early Roland cubes where the little SIL reverb driver IC cracks the legs but the joints are good? You pull them and find half of the legs have broken right through.
 
g1 3/19/2018 6:57 PM
[QUOTE=J M Fahey;483329]Crazy Glue is *one* Cyanoacrylate adhesive. There are [B]many[/B] more out there, catering to specific needs ;)[/QUOTE] Agreed. There are many variants of products using specifically [I]Ethyl[/I]-cyanoacrylate, but unless they are specified as 100% such, they are not identical. Different formulations of additives for many different applications. For example, krazy glue and Epiglu are not identical. ;)
 
nevetslab 3/19/2018 8:57 PM
After I downloaded the schematic for this Stage 112 amp, I see this isn't a conventional power amp design, where the speaker sits between the output buss of the complimentary semiconductor output stage and ground (with the zobel network between them). Instead, it's passing the output audio current thru the power supply caps as has been done by numerous QSC amps over the years. So, you have the filter cap's non-linear impedance characteristics in the open loop GBW response and it's contribution to phase margin. I wonder if that is a contributing factor to find HF oscillation present under output drive. I don't recall ever having one of these on my bench, though I could be wrong. If so, I never ran across the flame-out as we see here.
 
Enzo 3/19/2018 10:11 PM
Lots of amps are flying rail topology these days, PV has done it for years. Imagine you built the power amp on a wooden plank. The output transistor emitters connected to one side of the speaker, the power supply common connects to the other side of the speaker. There is no "ground". Now in the grand scheme of things, does it really matter which end of the speaker you connect to chassis and call it ground? We are used to seeing the power supply common grounded, but either way works. Your filter caps maintain a steady voltage between power common and rail. In a flying rail amp, the voltage at the filter cap moves all over with the signal with respect to ground, but always remains steady with respect to its common. I can't say I have seen burnt zobel resistors in flying rail amps any more than in conventional amps.
 
52 Bill 3/20/2018 10:14 AM
Another "problem" with that series of amps is lead dress of the wires inside the chassis. Most have a punched loop in the rear panel that holds a wire tie that keeps the speaker leads away from the pc board. I often find that a previous repair will leave these wires floating or laying on top of the pc board.
 
bob p 3/20/2018 10:52 AM
[QUOTE=g1;483515]Agreed. There are many variants of products using specifically [I]Ethyl[/I]-cyanoacrylate, but unless they are specified as 100% such, they are not identical. Different formulations of additives for many different applications. For example, krazy glue and Epiglu are not identical. ;)[/QUOTE] I'm having trouble understanding your point. What I originally mentioned in this thread was that the Loc Tite product in question, and Crazy Glue, are exactly the same chemical. Their chemical identity was verified by reading the formulation specified in the Material Safety Data Sheet for each product. The MSDS show that the formulations are identical.
 
g1 3/20/2018 1:26 PM
What does the identification of hazardous items listed in the MSDS have to do with a products formulation? It tells you 2 products contain some of the same items (the hazardous ones) but that is all.
 
bob p 3/20/2018 2:08 PM
What does the MSDS have to do with formulation? A lot more than you think. I can see where someone who isn't a chemist by training may believe that there's some magic hidden ingredient that renders brand formulations different, but there isn't. Ethyl cyanoacrylate preparations are monomers that react with water. They have to be stored under anhydrous conditions to prevent premature polymerization. These products are activated by exposure to ambient atmospheric humidity, which explains why they have a short shelf-life after opening them. Their susceptibility to exposure to water rules out water as being used as a solvent. That's significant because water is the only solvent that can evade being listed on an MSDS. ECA glue formulations are typically presented as pure cyanoacrylate (Krazy Glue), or gelled cyanoacrylate (Krazy Glue Gel) which is manufactured as a colloidal suspension of ECA with pyrogenic silica. If some other formulation were desired, the only common solvents in which ECA is soluble are MEK, acetone, nitromethane and methylene chloride, each of which is defined by law as a hazardous organic solvent and is required to be listed as a hazardous substance on the MSDS for any product that uses them as a solvent. If they aren't on the MSDS, then the ECA formulation has not been diluted and it does not contain them. The MSDS for the two formulations under discussion only list ECA as sole the hazardous organic content. If the formulation was anything other than pure ECA or gelled ECA then one of the previously cited organic solvents would be required in the mixture and by law it would have to be listed on the MSDS. Neither MSDS lists an organic solvent, hence the product formulations can be deduced as being purified ECA. From a practical standpoint, a diluted ECA mixture is impractical and undesirable for use in the "Krazy Glue" application as the viscosity of the diluted liquid is too low, which would result in runaway as the product is applied to the substrate being glued. Commercial ECA preparations go the other direction -- instead of thinning out a pure ECA formulation with a solvent, they either use pure ECA which is somewhat viscous, or they thicken the ECA formulation by creating a colloidal suspension with pyrogenic silica to make it highly viscous and easier to handle. High viscosity keeps the glue where you put it. It's why Krazy Glue gel will form a beaded droplet rather than running off of the workpiece like a drop of water, MEK, acetone, etc. I'm sorry, but your assumptions that the two brands of ECA were different was based upon unsupported speculation and not organic chemistry.
 
Mick Bailey 3/20/2018 2:21 PM
The same chemical. Not necessarily the same quality, purity, viscosity, bond strength, specification compliance, cure-time, traceability, substrate-adhesion, shelf-life, activator response etc. Industrial products have to meet defined criteria and have published specifications. A critical assembly may have time and handling constraints that need predictable, repeatable results that don't matter from a DIY perspective. I've used Loctite activator/primers with Loctite products and the results are consistent. With high-street superglue I get foaming, excess blooming, overheating, embrittlement and other issues. This alone suggest there is a difference in the composition of the respective adhesives. Industrial products have published specifications that are achievable in a production environment. Contaminants in the base materials, processing, or post-process environment can play a big part in bond strength and can impact on curing and shelf-life. I wonder if the manufacturers of Ģ/$ store products meet the same quality acceptance criteria as Loctite or some other industrial product.
 
bob p 3/20/2018 4:11 PM
At the molecular level, two samples of the same pure chemical will, by definition, have physical properties that are identical. It's not possible for the physical properties of two samples of the same chemical to be different unless one sample is contaminated or the samples are comprised of two different chemicals. The physical properties of the chemical itself, such as melting point, boiling point, vapor pressure, viscosity, etc. are CONSTANT. As in INVARIANT. They're so constant that the reference values are published in the CRC Handbook of Chemistry and Physics. (You'll need a subscription to look them up.) [url=http://hbcponline.com/faces/contents/ContentsSearch.xhtml]Handbook of Chemistry and Physics 98th Edition[/url] If the chemical is pure then all of it's physical properties are going to be consistent. So what's the difference that makes the industrial-grade products special? It's the special attention that is provided to the product by the seller. Commercial products have to meet defined criteria and they're guaranteed to have a stated level of purity, there is traceability in their provenance, there are tighter controls on freshness, storage, and much better packaging. And they go through a process of grade certification and that adds to their cost. You don't get that with the 99-cent tube of generic ECA. Packaging is significant. the 99-cent tube of dollar store ECA is made as cheaply as possible. The tube is not designed to eliminate bubbles, it foams, it drips, and it cannot be resealed well enough to prevent degradation of the product after it has been opened. The result is that a tube of generic ECA typically squirts out a mess. It can't be effectively resealed so it gives you one application and then it's dried up by the next time you go to use it. In contrast, the industrial preparations have much better packaging that's designed to eliminate bubbles, foaming, drips, provide consistent application and can effectively be resealed. Those qualities lead to improved success and prolonged service life after opening, even if the formulation in the dispenser is the same. There are many factors that make the 44-pound tube of glue cost 61x more than the 99-cent tube. Formulation isn't necessarily the biggest difference. It would probably surprise people to know that there are not a lot of manufacturing lines making specific specialty chemicals in the US. Even once a product goes off patent, in most circumstances chemicals like this are made by only a handful of suppliers, with intermittent production runs and consistent sales into the secondary market. One production run ultimately leads to huge amounts of product, and packaging of the chemical from the same production run under a variety of brand names. The formulation of the lot ends up being the same, though different grades may be assigned to sublots depending upon the how those specific subplots are packaged, handled and assayed to provide the types of guarantees that were mentioned previously. All of those grading certifications cost money. Here in the USA the supplier of ECA for LocTite is Henkel North America. In addition to selling it's own name-branded products Henkel also sells their chemicals in the secondary market. in the big scheme of things, most of the difference in the different ECA products isn't based on formulation per se, it's based on packaging and ancillary certifications that are provided for the product. Putting this into it's proper perspective -- none of us are manufacturing mission critical components. If the OP wants to secure a lifted trace back down to the board, both the $61 tube of Loctite and the 99-cent tube of Krazy Glue would do the job. My experience is that Krazy Glue holds a trace down pretty well, at about 1/60th the price of the Loc Tite branded product linked previously. But it is indeed a PITA to work with. > I wonder if the manufacturers of Ģ/$ store products meet the same quality acceptance criteria as > Loctite or some other industrial product. They may meet the same quality criteria, but of course they don't guarantee them at the lower price. When you buy the dollar-store product you're getting the cheapest packaging possible, with none of the ancillary certifications. When you buy the Loctite branded product you are buying a value added product, and paying for things that are not being sold with the dollar-store generic product, which go beyond the formulation of the chemical itself, like a brand name's guarantee of quality and presumably other characteristics such as guaranteed performance specifications, a guarantee of a high degree of purity, guaranteed freshness, support from your factory representative, etc. At most, you might get a guarantee of a 99-cent refund if the dollar store product is "bad", but the formulation should not be expected to be any different. ECA adhesives are typically sold as non-diluted ECA. When you buy the name brand you're paying for a lot more than just the chemical in the tube.
 
J M Fahey 3/20/2018 4:40 PM
Long ago I tried using Loctite or Hernon "speaker glue" to assemble my own, but results werenīt good enough, at least in heavily abused Guitar speakers. One of them was black and boasted of having black rubber microspheres in suspension. Went back to old reliable oven cured Industrial Epoxy.
 
g1 3/20/2018 6:08 PM
The MSDS are not some secret, so I'll share them. If we're going to discuss them perhaps they are worth a glance? ;) My eyes may be old and tired, but they seem to list different ingredients. In any case, it seems a bit of a stretch to call the two the 'exact same formulation'.
 
bob p 3/21/2018 9:56 AM
Your MSDS do indeed show other ingredients. You've chosen two products that do indeed have different formulations, so your point on those products is correct. As it turns out, neither one of those MSDS are for the same products as the MSDS that were linked earlier on the thread, which did not show different formulations. Anyone can find an MSDS for a different product if they look hard enough; doing that begins an apples & oranges discussion.
 
Mick Bailey 3/21/2018 10:10 AM
MY understanding of MSDS is that it is not an ingredients list and non-hazardous chemicals are not required to be listed. There are also thresholds for hazarous chemicals below which their listing is not mandatory, and a lower threshold for carcinogens. Even so, the listings usually (but not always) present the composition within a range, such that with two seemingly identical products their ingredients could lie at the extremes of their respective ranges and therefore perform differently. I bet the OP wonders what happened to his original question. Maybe Elvis has left the building.
 
bob p 3/21/2018 11:02 AM
Oh yeah, Elvis left the building a long time ago! My apologies to the OP for the hijack, but that’s the norm around here once a thread achieves a critical number of posts. But I’m happy to keep the chemistry hijack going if anyone is interested… In response to Mick’s comment about the MSDS, he’s 100% correct – it’s not an ingredients list or a recipe. It’s a list of hazardous chemicals in the product that people need to be aware of. And as we’ll find out in looking at the latest 2 data sheets that have been posted, a hazardous chemical can be present in very small amounts ( on the order of 0.1% ) yet it’ll still be required to be listed on the MSDS. Kudos to g1 for posting those two additional data sheets. They shed some light onto the discussion that the original data sheets that have been the subject of this discussion previously, did not shed. Specifically – the two MSDS that were originally cited in this thread showed a pure ECA products without listed additives. The conclusions drawn from them were correct – those products had the same formulations. That’s not a surprise, as ECA glues are monomeric liquids that form polymers when they get exposed to a nucleophile, which is typically water obtained from environmental humidity. It's common knowledge for anyone who's taken a college level organic chemistry course that their formulations do not require anything other than pure ECA for them to work as intended. Now we have two new data sheets to look at, which list other ingredients. This is where it gets interesting for chemistry geeks. Elmer’s Crazy Glue: 0.10% 2,2'-Methylenebis(4-methyl-6-tertbutylphenol) Loctite 382: 0.10 to 1% Phtalic anhydride 0.10 to 1% hydroquinone 0.10 to 1% 2,2'-Methylenebis(4-methyl-6-tertbutylphenol) These additives are interesting. We can discuss what each one of these individual additives does, if anyone is interested, but I won't go there unless someone asks. Just in case someone doesn't ask, here's the spoiler: none of these additives are directly incorporated into the ECA polymerization reaction that provides the adhesive properties and structural stability of ECA glues. They aren’t part of the adhesive properties of the product, which is 100% based on ECA polymerization, so the additives can be considered irrelevant when it comes to bond strength. Instead, these additives act as preservatives, like BHA and BHT that get added to a loaf or bread, which acts to prolong the product’s shelf life without changing the nature of the product itself. They don’t exist in sufficient quantity to have an effect on the issues that Mick was concerned about: bubbling, foaming, viscosity, blooming, embrittlement, heating, etc., but they clearly show that the Loctite 382 product is a superior formulation that will have a longer shelf life than Crazy Glue. Will it result better workability or stronger bonding? Bonding, no. Workability? Maybe. One of the additives is actually used to effect the speed of the ECA polymerization reaction without being directly incorporated into the ECA polymer itself. I know we have at least one other chemist on the board, but I don't remember who it is. Maybe he'll chime in...
 
ralphonz 3/21/2018 1:19 PM
Haha. Elvis would have left but someone glued him down... I do plan on posting here again re the original topic once I've had a chance to explore the problem further :) I'm afraid other things have gotten in the way. You'll be glad to know I did not buy any glue (yet!)
 
ralphonz 3/21/2018 1:24 PM
I'm new here so pardon me for being cheeky and asking the discussion about glue happen on another thread though if that's doable! :offtopic: I'm trying to diagnose an amp and instead find my self pondering the chemical make-up of adhesive! I had not expected that :p
 
bob p 3/21/2018 1:58 PM
Elvis has definitely left the building on that one, sorry. Your request does not sound unreasonable, but the way things work around here hijacks are so common that we just grin and bear it. it would take so much effort to clean up all of the threads that we just live with it for the most part. :calm:
 
ralphonz 3/21/2018 2:01 PM
Fair doos :) I'll just have to live with the scrolling and thoughts about adhesive! Worse things happen at sea. :thumbsup:
 
Enzo 3/21/2018 5:17 PM
In our defense, we have resisted the temptation to talk about bringing charcoal to Newcastle.
 
bob p 3/21/2018 5:28 PM
But why? If you think of it when you're reading this thread then it's fair game. :p
 
g1 3/21/2018 11:06 PM
ralphonz, sorry about the tangent we are off on here, if you can bear with us a little more, I assure you we'll get back on track when you get back into the amp. :D The datasheet that implied the loctite was pure ethyl-CA was not the MSDS, but a 'technical data sheet' which I imagine was only concerned with the most active ingredient. I could not see how a flexible CA could be the exact same formulation as the brittle stuff, nor could I believe Loctite was simply gouging; thus my objection to the idea they were the exact same product (just at radically different prices). I am interested in what gives it flexibility. I think tedmich was the other member here working in chemistry, so I've flagged him and hopefully he will shed more light on the issue.
 
tedmich 3/22/2018 12:35 AM
[QUOTE=bob p;483729] I know we have at least one other chemist on the board, but I don't remember who it is. Maybe he'll chime in...[/QUOTE] Elmer Ph(u)D here ;) In my experience while MSDS (now known solely as SDS) can provide valuable clues as to the general formulation of a product they cannot be used to definitively claim any two formulations are functionally identical. There simply is not enough data contained therein; the SDS is just to be used by medical personnel to guide exposure treatment and emergency responders and Hazmat crews for the rare tanker car spill. Low level additives and indeed "proprietary components" may be listed or omitted as the company sees fit, guided only by potential legal repercussions and secrecy concerns often trump these. The purity level of all the components is not dictated by its listed CAS number, just the main component chemical deemed to be a potential hazard. One grade of ethyl 2-cyanopropenoate will produce an entirely different N number when polymerized with a decidedly different peel strength, hardness, resistance to oxidation/UV etc. than an inferior (less pure or higher contaminate level) grade of the same ethyl 2-cyanopropenoate. In a related example some of the available epoxys retain yellowing impurities which require re-distillation or other purification strategies to produce optically clear polymerized final product, and you will pay for this added processing even though it would never show up on a blunt instrument doc such as an SDS. A decent analogy for this exists in the 90's bad capacitor plague, which saw umpteen Asian companies copy the general formulation of the water based electrolytic gels, perhaps to a level of resolution seen in their SDS listing, but omitting specific low level corrosion inhibitors to stop the water catalyzed corrosion of the aluminum metal, which creates gas and pops the caps. Even detailed chemical patents (which I am far too versed in) usually fail to give specific enough information to actually reproduce the product they describe. Lots of testing and numbers are needed to judge if two formulations are indeed functionally identical (or different) providing steady employment for grizzled old chemists such as myself. The SDS (and the price) are merely clues.
 
Enzo 3/22/2018 1:32 AM
Tangent on a tangent tedmich: are you familiar with Joe Schwarcz?
 
Steve A. 3/22/2018 2:22 AM
[QUOTE=bob p;483729]Oh yeah, Elvis left the building a long time ago!* My apologies to the OP for the hijack, but that’s the norm around here once a thread achieves a critical number of posts.* But I’m happy to keep the chemistry hijack going if anyone is interested… [/QUOTE] This thread like many others has been hijacked by you, Bob, in your arguments with other members regarding product MSDS forms. As several people pointed out MSDS's list [I][B]only [/B][/I] the chemicals or ingredients deemed to be hazardous so while most of the CA adhesives might have the same hazardous chemical (ethyl 2-cyanoacrylate) the exact formulations can be different. [QUOTE=bob p;483422][QUOTE=J M Fahey;483329] [QUOTE=bob p;483139]LMAO. Ethyl cyanoacrylate is Crazy Glue.[/QUOTE] Crazy Glue is *one* Cyanoacrylate adhesive. There are [B]many[/B] more out there, catering to specific needs ;)[/QUOTE] Thanks, Juan. I didn't know that you were a polymer chemist. :D That there are many derivatives is not the point. The point is that one specific formulation, *ETHYL* cyanoacrylate, is [I]Krazy Glue[/I]. It's also the exact same formulation that was linked to in post 5. MSDS: [url=http://www.krazyglue.com/docs/default-source/MSDS-Sheets/mkg0925.htm?sfvrsn=2]KG0925 KRAZY GLUE ALL PURPOSE BRUSH-ON[/url] That's relevant because you can buy generic ethylcyanoacrylate at hadware stores in the USA for 99-cents, or you can pay [URL="http://uk.rs-online.com/web/p/cyanoacrylate-adhesives/1987144/"]44 pounds[/URL] (!) for it in the UK if you need to have the Loc-Tite brand. [U]It's the exact same chemical at both prices[/U]. All that differs is the packaging and the price. Caveat emptor.[/QUOTE] I have bought several different brands and formations of CA glue, with most of them being very different. Gorilla Glue adds something to make it tougher (I get the impression that it is something rubbery mixed in) and Titebond makes a formulation that works better with wood. StewMac sells 3 different formulations - thin, medium and thick. With all of the talk about CA I am not sure it is the ideal solution here. Many luthiers use it to secure frets to the fretboard and the recommended way to loosen and remove those frets is to heat them up with a soldering iron. So I'm not sure how well it can stand up to soldering (or repeated soldering) not to mention the damned fumes when heated. One solution for lifted traces is to run the component leads in their place. In some cases I have used desolder braid with rosin. There are a lot of new adhesives available today that can be used to be secure wires to a circuit board. I have been a big fan of the As Seen on TV 5 Second Fix which is a light cured plastic welding agent which works better on some substrates than others. I like to use it to protect nicks in guitars as it can be easily removed later with a pen knife when you get ready to repair it properly. I ran across a new light cured adhesive a few months ago which includes CA for a double bond: where the UV light can't reach the CA can do the job. And the light cure adhesive holds everything in place as the CA cures. [url]https://www.amazon.com/dp/B016MB0JF6/[/url] [url=http://www.lightlockglue.com/product.php]Light Lock Glue[/url] Watch "SureHold's LIght Lock Glue" on YouTube (it sold me on the product!) [video=youtube;3Mge37rW-4A]https://www.youtube.com/watch?v=3Mge37rW-4A[/video] Here is YouTube video comparing 99 cent CA to $3-4 Loctite CA to Gorilla CA... they all pass equally well for the plastic used in models and miniatures but I would like to see tests with different materials... Watch "SUPER GLUE TEST! - CHEAP VS EXPENSIVE! -what is the best for miniature hobby?" on YouTube [url]https://youtu.be/4m4GFj6NfY4[/url] Steve A. P.S. I have to say one thing NOT to use on Fender eyelet boards is hot melt glue! After reading too many magazine articles I rebuilt my 1965 Pro Reverb as a fire-breathing channel switching monstrosity in the early 80's. What a mess!
 
Mick Bailey 3/22/2018 2:44 AM
One thing you certainly can't get from MSDS/SDS is the number of distillations used in the process; cheap stuff has a single distillation, more expensive uses multiple distillations. Maybe that's why primers and accelerators behave differently. Even the 'universal' accelerators such as 'Zip Kicker' has different outcomes on different adhesives.
 
Steve A. 3/22/2018 2:48 AM
[QUOTE=tedmich;483877] A decent analogy for this exists in the 90's bad capacitor plague, which saw umpteen Asian companies copy the general formulation of the water based electrolytic gels, perhaps to a level of resolution seen in their SDS listing, but omitting specific low level corrosion inhibitors to stop the water catalyzed corrosion of the aluminum metal, which creates gas and pops the caps. [/QUOTE] Does that also apply to compressor and condensor fan run caps used in HVAC equipment? I sure changed out a lot of them between 1990 and 2006, usually right after a hot spell. Most of the residential equipment used dual run caps for the compressor and fan so I had a plastic tackle box with all of the different value caps and would secure them together as needed with wire ties to get them back on-line in 30 minutes rather than having to come back later with the exact replacement. The dead giveaway was the top bulged out but when they exploded it was a REAL mess... Steve A.
 
bob p 3/22/2018 8:15 AM
> This thread like many others has been hijacked by you, Bob, I freely admit to hijacking. It's the norm around here, everyone does it. What strikes me as odd is that you're choosing to single me out, when there have been three people involved in this hijack: me, g1 and Mick. I sense bias. But that is nothing new.
 
bob p 3/22/2018 8:32 AM
tedmich, looking at the Loctite 382 additive list, I'd be interested in discussing the role of hydroquinone in the formulation. If you want we can take this to PM.
 
Jazz P Bass 3/22/2018 9:36 AM
From Wiki: "There are various other uses associated with its reducing power. As a polymerization inhibitor, hydroquinone prevents polymerization of acrylic acid, methyl methacrylate, cyanoacrylate, and other monomers that are susceptible to radical-initiated polymerization. This application exploits the antioxidant properties of hydroquinone."
 
bob p 3/22/2018 9:52 AM
Oh, I know why those 3 chemicals are present in the mixture. I'm really interested in discussing the reaction mechanisms. I seriously doubt that anyone reading this thread would be interested in watching us push around electrons.
 
Mick Bailey 3/22/2018 12:22 PM
[QUOTE=bob p;483905]> I sense bias.[/QUOTE] I wish I could do that. I have to resort to using a DMM and other artificial aids. My Aunt could sense a storm coming, but I don't think even she could sense bias. I'll put up my hands - guilty to hijacking. What's the punishment around here?
 
bob p 3/22/2018 1:14 PM
That was funny. :D for your penance, you have to help someone fix their amp. it's what we do around here. :thumbsup:
 
ralphonz 3/30/2018 11:32 AM
[QUOTE=Mick Bailey;483316]Ah. So that means the power amp is OK and probably what we're looking at is a fault that lies in the preamp. I'm thinking that 52 Bill maybe had the answer in post #6 and those caps should be examined in more detail. So that would be C13, C19, C30, C34, C37. These are in the feedback loops of the opamps and serve both to stabilize the circuits to prevent oscillation and also to prevent broadcast radio from being picked up and amplified. A good DMM should be able to 'read' oscillation - certainly my Fluke does this when set to AC volts mode (mine autoranges but manually you could begin with 200mV). You see on the schematic where those caps are located? Pins 1 or 7 respectively are the opamp outputs. If you insert a plug into the 'power amp in' socket you'll disconnect the power amp to prevent further damage. Ground the negative lead of your DMM and check with the +ve lead to see if there's any AC voltage on the outputs of those opamps. There's usually a little due to power supply ripple and noise, but should be no more than a few mV at most. Sometimes an oscillating opamp will get much hotter than the rest. Turning up the controls will give you the best chance of spotting anything wrong. The only caveat with this is the multimeter itself (or scope probe) can sometimes stop any oscillation.[/QUOTE] Ok hi everyone, I've finally got back to looking at this amp. I replaced the resistors and c49 as suggested. I plugged a jack into the power amp input and turned the amp on to test the opamp outputs. Within seconds the resistors in the zobel network started to cook and smoke. Does this give anything away? What could be the problem? Clearly it's a little worse than I thought.
 
g1 3/30/2018 6:54 PM
The amp has an oscillation problem, and right at start up. This is going to keep burning those resistors until it is corrected. It is not that simple of a power amp, and you said you have not worked on one before. So you are going to have to decide how much time & effort you want to put into it. You seem to be fine with measurements and soldering, so that's a good start. :) I'll make a couple suggestions, but hope others will jump in here also as oscillation can be a tricky beast. First, if those resistors aren't already open (measuring much higher resistance than their proper 47ohms), remove them or disconnect one side each. This will prevent them from burning and doing more damage to the board. Now you will want to get a baseline measurement for the oscillation at the output. Measure the AC voltage between the CP13 and CP14 connectors. Hopefully you will see a significant voltage there so you will know later if you get rid of it. Now I would suggest plugging a cable into the power amp in jack, and shorting the tip to sleeve. Again check the AC at the output (CP13 to CP14). If the oscillation is still there, then it is in the power amp. If it is gone, it must have been coming from the preamp. A couple things about grounding. Some amps are picky about everything being mounted and tight. Grounding can rely on pot connections to chassis, board mounting screws connecting traces to chassis, etc. Grounding issues can be involved in oscillation. Not sure if you checked that all the hardware for being tight, or if you are running it with everything back together or not, but it could be a factor.
 
ralphonz 3/31/2018 3:01 AM
Hi g1, thank you! I'm prepared to put hours in to fixing this, as a learning experience as much as anything else. I figured the amp definitely has an oscillation problem at start up, but with my limited experience I don't even know where to start, so thank you for your advice. The first time I replaced the zobel network resistors and tested the amp I'd put the whole thing back together. The second time I didn't bother, if I put it back together then there is no way to get the probes in to test voltages! I had read that some amps were picky about grounding, and that grounding issues could be involved in oscillation. On this particular amp the pots seem to be grounded to the chassis with a metal casing around them which is soldered to the board. I'm not sure if this is common but I've never seen it before. Other than that though I think everything is still grounded to the chassis when I have it open. I'll try out your suggestions today and see if I can narrow the oscillation down to pre or power amp. In this case I'll test it open and put back together as I'm testing CP13 and 14 which is possible with it all screwed into the chassis. Cheers!
 
ralphonz 4/1/2018 4:38 AM
So, I measured the AC voltage across CP13 and CP14. It measured around 2.8V both with the shorted jack cable pugged into power amp in and also without it, so I suppose this means the problem must be with the power amp and my original diagnosis that it "worked fine" playing through the power amp in must have been wrong. I dare not attempt to put any signal through it again to check until I can find the source of the oscillation.
 
Steve A. 4/1/2018 4:56 AM
FWIW the Fender Stage 112 se amps sell used for ~$125-150 in the US. Of course the shipping charges to the UK would be outrageous but someone might want to ship you just the printed circuit board. Just a thought... I love these 1990s Fender SS amps and have all of them besides the 160W Stage 112 SE. But I do rewire the preamps and tone control circuits... Steve A.
 
J M Fahey 4/1/2018 8:10 AM
Avoided adding to the hijack, but since it became personal, let me add a few lines to the mess it already is: [QUOTE]Quote Originally Posted by bob p View Post Quote Originally Posted by J M Fahey View Post Quote Originally Posted by bob p View Post LMAO. Ethyl cyanoacrylate is Crazy Glue. Crazy Glue is *one* Cyanoacrylate adhesive. There are many more out there, catering to specific needs Thanks, Juan. I didn't know that you were a polymer chemist. [/QUOTE] Typical despising Bob P answer, so no surprise here. But Bob P, *you* might be surprised: 1) I very much doubt you are a "polymer chemist" and if you claim to be one, please post Academic degree proving so. 2) surprise surprise, I studied 4 years Industrial Engineering which includes a fair amount of Chemistry, basically centered in Industrial processes (I wonder why ;) ) , before switching to EE (2 more years) and then to Business Administration (1 year), since there is not a *specific* University level career focused on "Musical Instrument Design and Manufacturing". Although not as much used as other skills, I still keep and use my Chemistry knowledge as needed for Production, a very important part of which is based in knowledge and use of Adhesives ... or else. And yes, I found different cyanoacrilate based adhesives to be different, specially into what concerns ma and Production. IMPORTANT parameters (specially in speaker building) are: adhesive strength (varies a lot), flexibility/elasticity (again varies a lot), viscosity (same) and temperature at which it starts losing mechanical properties (again). So do not dismiss me ironically as an ignorant. As a final note (surprise surprise) I have successfully run a **Chemical** based Industry :surprised: , Fotoquímicos Victoria, making Photography oriented products, mainly Developers, Fixers, Fog reducers, wetting agents , etc. . As a bonus: also ran the parallel plant dedicated to electrolytic recovery of metallic silver from used Photo fixer baths, which we got back from larger Photo Labs, printing Press/graphic arts ones, and mainly Hospital X Ray Labs. ------------------------------------------------------------------------------------------------------ back to the OP problem. IF you want to check the presence of oscillation in that amp output (and I guess we still have it) , regular meters wonīt show it or lie about its level, they are simply not designed for that. Best tool: an oscilloscope. Will not only show continuous oscillation but way harder to catch oscillation "bursts" which may not exist under idle (no signal) conditions, yet be triggered at certain points of the Audio wave when driving a speaker (which is a *complex* load) This one shows a not too strong oscillation , wonīt burn and maybe not even overheat the amp or speaker (might slightly brown the Zobel resistor) but makes amp soind dirt, hissy, specially acompanying the "S" letter ... and must be corrected of course. [IMG]https://i.stack.imgur.com/XD4E3.jpg[/IMG] this one is a horrible waveform produced by a (poorly) homemade LM1875 chipamp: [IMG]http://i40.tinypic.com/att180.jpg[/IMG] No plain multimeter will show that. Second best is to build the old Ham Radio amateurs, TV servicemen and a few others secret weapon: the RF probe. It turns unreadable RF voltage into very readable DC :thumbsup: [IMG]http://n5ese.com/rf_probe_schem.jpg[/IMG] You will have a hard time finding a Germanium 1N34 , 1N60 and similar diode, but since we donīt expect *real* RF here but high ultrasonic, a small signal diode such as 1N914 or 1N4148 will do. Heck, not for precise measurement but to simply show HF oscillation[B] presence[/B], even an 1N4''2 (or higher) should do . So build a probe, omit the 4M7 series resistor, connect it between speaker out and multimeter set to 20V DC scale. Blinking when connecting and then zero is whatīs expected; some steady voltage, from , say, 0.5V to full scale indicates a problem. If you get a reading, first disconnect power amp from preamp but leave its input open, to decide whether itīs internal oscillation or it comes from outside. Then short power amp input and repeat test. Results?
 
ralphonz 4/3/2018 4:35 AM
Ok, thanks for that. I was wondering, does anyone think it might be worth getting one of these? [url]https://www.ebay.co.uk/p/KKMOON-LCD-Digital-Storage-Oscilloscope-frequency-Meter-DIY-Kit-With-Professional-BNC-Probe-USB/2220836755?iid=322572481126&_trkparms=aid%3D555018%26algo%3DPL.SIM%26ao%3D2%26asc%3D49131%26meid%3Dabc9bca43acd414db3b0086497a51b05%26pid%3D100005%26rk%3D1%26rkt%3D6%26sd%3D222908155931%26itm%3D322572481126&_trksid=p2047675.c100005.m1851&thm=1000[/url] Or would that be a waste of Ģ50? I don't wan't to fork out a lot for an oscilloscope just yet but there are some old analog models floating around for a similar price on eBay.
 
g1 4/3/2018 11:05 PM
I think you would do much better with a cheap old working analog scope than that unit. Just from a quick read through, the language (poorly translated) indicates there would be problems with documentation and support, both for building and using that unit. And it looks like a very small display as well.
 
ralphonz 4/11/2018 3:24 PM
Hi Juan, Sorry I have been very busy, my computer motherboard decided to die! Anyway, I finally built the probe, with an actual 1N34A (bought some from eBay for a couple of pounds!), and omitted the resistor. I set my MM to 20V DC, clipped ground of the probe to chassis and test lead to speaker+. In all 3 scenarios the results of the test were the same. Turn amp on, MM reads 32 V initially which drops all the way to around 7.75V after a minute or less where it stabilises. The first time I carried out the test (with preamp connected) I hear the occasional crackle/pop in the speaker.
 
J M Fahey 4/11/2018 5:58 PM
[QUOTE=ralphonz;492722]In all 3 scenarios the results of the test were the same. Turn amp on, MM reads 32 V initially which drops all the way to around 7.75V after a minute or less where it stabilises. The first time I carried out the test (with preamp connected) I hear the occasional crackle/pop in the speaker.[/QUOTE] Then you definitely have oscillation there. a) I guess you are usin g a speaker load, please confirm. b)initial jump is normal, you are measuring turn-on Pop, no big deal c) *staying* at >7V DC is not normal at all, indicates presence of Audio at the speaker out. And since you hear nothing plus itīs stable, it means you have an ultrasonic oscillation. Please check and confirm results of: [QUOTE]If you get a reading, first disconnect power amp from preamp but leave its input open, to decide whether itīs internal oscillation or it comes from outside. Then short power amp input and repeat test. Results? [/QUOTE]
 
ralphonz 4/12/2018 6:10 AM
a) Yes, testing with a speaker connected b) Good to know :) c) Confirmed, I get the same reading >7VDC with pre amp disconnected and power amp input shorted. Double checked readings. Occasionally goes >8VDC but never <7VDC
 
Steve A. 4/12/2018 2:46 PM
[QUOTE=J M Fahey;485085]Avoided adding to the hijack, but since it became personal, let me add a few lines to the mess it already is...[/QUOTE] I don't think of threads here at MEF being hijacked, Juan... it's more like a side trip in our journey, some of them helpful and productive, but unfortunately some of them not (or they might just go on [b]way[/b] too long.) I'd guess that we all have a bit of libertarianism in our blood and that must be mine... Let threads go where they may and heaven help the OP! :spin: :spin: :spin: :spin: Steve A. P.S. Are there any oscilloscope programs/devices for a computer or tablet that you (or anyone else here) might recommend? Like something that would plug into a USB 2 or 3 port... It would have to be safe (no possible damage to computer.) IMO display resolution would not be as important as the display speed since oscillation above the audio frequencies can cause all sorts of problems.
 
J M Fahey 4/12/2018 3:40 PM
USB scopes can read very high frequencies, "draw" a nice drawing purportedly showing what "you would see if you had a real scope" and display that nice coloured picture on PC screen. Easy peasy. I published here and elsewhere full instructions and usage examples for a safe signal attenuator which matches whatever you are reading to a PC/Notebook *audio* (not USB) input, whether Line or Mic, usually a 1/8" jack. Signal is diode clipped above 600/700mVpp so 200mV tops , fullt safe for any PC input. Processing comes courtesy of the Sound Board so it displays waveforms faithfully with the following limitations: a) AC only since Audio boards are exactly that. b) bandwidth limited to somewhat above 20kHz by Audio Boars filtering. In any case you can usually see HF oscillation, whether continuous or bursts, just you canīt resolve its waveform , only its envelope and "shadow", which are still useful if all you want is to get rid of it. Not too different to whatīs seen on both screen captures shown on post #62
 
ralphonz 4/16/2018 3:39 PM
I just managed to pick up an old Philipps oscilloscope (PM3540) on eBay for pretty cheap. Now I just need to figure out how to use it :noob: [quote] If you get a reading, first disconnect power amp from preamp but leave its input open, to decide whether itīs internal oscillation or it comes from outside. Then short power amp input and repeat test. Results?[/quote] Haven't tested with the scope yet but using the probe I read >7VDC both with the input open and with the power amp input shorted.
 
glebert 4/16/2018 3:58 PM
[QUOTE=ralphonz;493170]I just managed to pick up an old Philipps oscilloscope (PM3540) on eBay for pretty cheap. Now I just need to figure out how to use it :noob: Haven't tested with the scope yet but using the probe I read >7VDC both with the input open and with the power amp input shorted.[/QUOTE] To get familiar with using your oscilloscope, I recommend getting a function generator app for your phone or tablet and hack up a cheap headset cord to have a "known" safe thing to measure and practice with.