dhart67 6/12/2017 9:44 PM
Standel Studio amp no sound
working on a 1971 Standel Studio amp model S24G. amp has no output to speaker. getting good pre-amp signal to power amp board. get +&- 42dcv on power feeds as well. check traces on back side of pcb found dc power feeds are not connected to anything? amp was worked on before I got it, looks like wiring error. can't locate correct schematic but using one off Standel site 1971-IC005 drawing, that seems close. when connecting power feeds up per that drawing voltage drops to 32 & 22. unplug my 8ohm dummy load voltages return to +&-42vdc. also getting 42dcv on speaker lead and several of the transistor base leads. what a mess. have repaired several cracked solder joints, 2 open traces and replaced an output transistor with a shorted E-C junction. looking for correct schematic and if anyone has a photo of this board(it's the pcb attached to large heat shink). does anyone know where to connect +&- 42dcv leads? thanks for any help ya'll can offer. I have a little electronic training but I'm no expert at this. have a signal generator and a scope.[ATTACH=CONFIG]43780[/ATTACH][ATTACH=CONFIG]43781[/ATTACH]
 
g1 6/13/2017 12:57 PM
The Standel site has quite a few schematics, have you looked through them, regardless of year?
Standel | Schematics
 
dhart67 6/13/2017 2:02 PM
yes, went to Standel site that's where I got the schematic i'm using, 1971-IC005.
 
olddawg 6/13/2017 3:32 PM
Is this close? https://www.google.com/search?q=stan...9jgPSxzKUaT0M:

Try google images. Search for Standel board. You never know.
 
Enzo 6/13/2017 5:47 PM
His board lacks the drive transformer, so not that model.
 
dhart67 6/13/2017 11:28 PM
searched Standel boards found 1971-IC005 seems close. I posted a picture of it in my first post.
 
dhart67 6/15/2017 10:43 PM
think i found the problem. previous repair attempt soldered in two transistors backwards. on schematic they are Q1 & Q2, MOT 2N5219 on drawing but had been replaced previously with 2 NTE123AP. drawing shows both emitters going to R24 variable resistor(pot), found both collectors were. in photo lower right corner right above variable resistor and yellow capacitor are the two transistors.
 
dhart67 6/15/2017 10:44 PM
standel amp
think i found the problem. previous repair attempt soldered in two transistors backwards. on schematic they are Q1 & Q2, MOT 2N5219 on drawing but had been replaced previously with 2 NTE123AP. drawing shows both emitters going to R24 variable resistor(pot), found both collectors were. in photo lower right corner right above variable resistor and yellow capacitor are the two transistors.
 
dhart67 6/17/2017 9:11 PM
amp plays great, clean output. cleaned all pots. only problem left is reverb, loud squeal when turning knob up. can't get to 1 on reverb control without squeal. will check this out tomorrow. will update.
 
dhart67 6/18/2017 8:18 PM
reverb fixed. corrosion on reverb tank rca plug & socket.
 
dhart67 6/19/2017 12:57 PM
anyone have any info on thermal protection disk located on left side of pcb photo? it's to the left of the lower ceramic resistor. two leads soldered to board and attached to heat sink with thermal grease.
would like to replace it. thanks
 
J M Fahey 6/19/2017 2:55 PM
Quote Originally Posted by dhart67 View Post
anyone have any info on thermal protection disk located on left side of pcb photo? it's to the left of the lower ceramic resistor. two leads soldered to board and attached to heat sink with thermal grease.
would like to replace it. thanks
This one should do
2x Temperature Switch Thermostat KSD301 thermal protection 35°-160°C NC 250V/10A | eBay
typical value is normally closed, 85C temperature
[IMG]http://i.ebayimg.com/images/g/dssAAOxybLpRgHJU/s-l300.jpg[/IMG]

If it goes to the board, it probably disconnects speaker hot lead from amplifier, follow it just in case to confirm.
 
dhart67 6/20/2017 9:58 AM
J M Fahey, thermal disk solders between two zenier diodes, you just see them in photo in my OP. thanks for your suggestion I'll check it out on ebay.
 
g1 6/20/2017 11:29 AM
Without an accurate schematic, I'm wondering why you decided it was a 'protection' device? My worry is that it is some form of thermal compensation for the bias circuit.
I'm not doubting JM's response, but he may have been going by your description of a protection device.
In the schematic you posted, the only zeners are near the input, surely that is not where this device is located?
Otherwise I do see 2 diodes in the bias string.
 
J M Fahey 6/20/2017 1:07 PM
Quote Originally Posted by dhart67 View Post
J M Fahey, thermal disk solders between two zenier diodes, you just see them in photo in my OP. thanks for your suggestion I'll check it out on ebay.
Sorry, that picture edge is fuzzy on my netbook so I donīt really "see" it, just some thermal paste smeared greyish indistinct shape with two wires attached, so Iīm going by your verbal description.
Thatīs why I suggested you check "from where to where" do they actually go.
G1 has a sharp eye and of course he may be right.
That "disk" might be, say, an aluminum disk or a piece of PCB epoxied to heat sink (which might be the chassis itself) , covering D1/D2 so they are in close contact with it, OR it might be a Normal Open thermal switch, which shorts across Zeners Z1/Z2 muting the amp when hot or ... or ... or ....
So first check where/how is it conncted and show it in the known IC005 schematic.

There might be minor variations, if at all, but basically that schematic matches whatīs seen on your PCB picture which also matches one of the published layouts.

As a rough test: with amp on, measure end to end across those "disk terminals", if arond 1.4V those are the bias diodes, if 0V or almost, it might either open speaker line or short amp input when hot.
 
dhart67 6/20/2017 3:08 PM
sorry about confusion. will post better photos, maybe that will help. though schematic doesn't show it seems part in question is located between D1 & D2 diodes. I believe this is in bias circuit. amp is back together so can't read voltages, but will post soon as I can. thanks for your help with this.[ATTACH=CONFIG]43846[/ATTACH][ATTACH=CONFIG]43847[/ATTACH]
 
J M Fahey 6/20/2017 4:27 PM
NOW that I can enlarge your picture, the earlier one was too small, plus I am using a small 7" screen , I apparently see "a black small "ceramic capacitor" under a blob of epoxy" .
Such black ceramic capacitor looking thingies are usually NTC Thermistors, probably 50 or 100 ohm value cold (25C) and going down as it heats up, just what the Doctor orderd for thermal compensation.
[IMG]https://4.imimg.com/data4/KY/OD/MY-9323543/ntc-thermistors-250x250.jpg[/IMG]
So simply [do not mess with them] [TM]
And yes, standard practice was to have a low value one in series with bias diodes to enhance their effect or a somewhat larger one (say 500 ohms) in parallel with them to "clamp" bias voltage if heat sink becomes dangerously hot.

Excellent parts, sadly they were once very popular, specially in the 60`s or early 70īs but then somehow fell out of favour and today are hard to find, only in a few values, and relatively expensive.

So I wasted, say, a Month at the University learning to compensate amps using them ... and then they dissappeared.

I feel as if I had wasted a Month learning how to grease wagon wheels with rendered suet and then all those fancy Oil lubricants appeared on the market.

Oh well, wonīt even comment on the wasted Architecture classes where they teached us how to orient the Pyramids pointing exactly to the Orion star so the Pharaoh soul would fly straight there.
Sometimes I feel a Mummy
 
dhart67 6/20/2017 9:18 PM
J M Fahey, ok, I did measure 71ohms across the leads of disk when I removed it from pcb. wanted to replace it with correct value since leads seemed in poor condition. there's a tinned coating on both sides of disk, a lot of top side coating was pulled off but had enough to resolder that lead.
 
dhart67 6/20/2017 9:43 PM
i'm attaching a Randall schematic for a RG-90 & RB-90 amplifier dated 1-4-73 which seems very close to Standel i'm working on. notice to right of R29 there is a symbol which I believe is the disk in question. doesn't give any values for it. it's the circle with a resistor symbol and a T inside it.[ATTACH=CONFIG]43855[/ATTACH]
 
Enzo 6/20/2017 10:40 PM
Thermistor.
 
J M Fahey 6/21/2017 12:18 AM
Quote Originally Posted by dhart67 View Post
J M Fahey, ok, I did measure 71ohms across the leads of disk when I removed it from pcb. wanted to replace it with correct value since leads seemed in poor condition. there's a tinned coating on both sides of disk, a lot of top side coating was pulled off but had enough to resolder that lead.
Ok, if you want to know its value as to replace it, you must measure it at 25C temperature. Not kidding.
Get a glass of water and a thermometer, also solder a couple 4"/10cm wires to the thermistor so you are not touching (heating) it with your fingers, get any combination of hot/cold water inside that glass so thermometer shows 25°C (77°F)
, submerge it for a minute or two and only then measure resistance.

Reason is that thermistors CHANGE resistance with any change in temperature, thatīs their job, so published value must not only be a resistance but also at what temperature.
Standard is 25C , 77F
 
g1 6/21/2017 11:59 AM
And note that JM specified NTC thermistor. They are pretty much the default but there are PTC (positive temperature coefficient) types and you do not want to use that here.
 
dhart67 6/21/2017 2:40 PM
ok here what I found. in 77f glass of water got 66ohms. tried putting disk close to soldering iron to change temperature got 74f=72ohms, 77f=58ohms, 81f=39ohms, 87f=12ohms. of course these are close estimates cause disk cools rapidly. you can see as temperature goes up resistance goes down so seems disk is actually still working even at 46years old. reading voltage drop across leads of disk got .682dcv with volume off and speaker unplugged, no load.
 
J M Fahey 6/21/2017 6:07 PM
Cool
So you now confirmed it is an NTC Thermistor.
What we expected, but itīs nice (safe) to check.

So if you want to replace it, get a nominal 66 ohms NTC at Mouser or similar, or closest one, and re-epoxy it to heatsink.

As usual, after messing with bias elements, start with amp plugged into a series lmp or start it slowly with a Variac, just in case.
If diode (+ thermistor) string opens, power transistors will be turned both full on at the same time, passing all current possible (say 10 Amperes) while standing full supply voltage at the same time.

End Of Life estimated in milliseconds

So play it safe
 
dhart67 6/21/2017 10:46 PM
JA Fahey, thanks again for your help. been playing through amp past few days reverb was working fine but was noisy and scratchy today. will check out tomorrow, may need to re-clean RCA connectors again.