|HNKTNK||10/8/2017 6:12 AM|
|Question about test points on a PCB|
I'm having trouble locating the test points on a circuit board. I see them on the schematic, but can't for the life of me find them on the actual board. My question is; Can a TP in the circuit not always be a pad/terminal and instead be a lead from a component or something else entirely?
|Justin Thomas||10/8/2017 8:25 AM|
|My gut says yes, but, what are you working on, and can you put up the schematic and test point layout, etc? You'll get more info if you give more info...|
|Tom Phillips||10/8/2017 8:46 AM|
|HNKTNK||10/8/2017 8:47 AM|
|Enzo||10/8/2017 8:47 AM|
|Test points are places in the schematic. Only rarely is there an actual "test point" on the circuit board. The schematic test points are what they appear to be: the end of a resistor, the emitter of a transistor, the cathode of a zener, etc etc.|
Once in a while for setting bias, you might find a power amp with two little posts sticking up to measure a voltage - rather than getting right on an emitter resistor.
|HNKTNK||10/8/2017 8:56 AM|
|Thanks for the replies Tom and Enzo. So on page 3 of the schematic i posted, would tp11 be on the r52 and tp12 be on the output jack?|
|Enzo||10/8/2017 8:59 AM|
I was wanting to remove it and then jump tp11 and tp12 to see if I get any output out of this thing
First, listen to the relay or put your finger on it. Does it click on after a few seconds at power sup? If not, there is the basic issue. But there will be a reason the relay does not click on, you want to find that out before cross connecting the output. Is ther DC voltage sitting at TP11? Is there signal at TP11?
If the relay does click on, then look for signal at TP11. Also do the basics, plug a guitar or other signal into the power amp in jack, get sound? Likewise plug a cord from preamp out to power amp in and plug guitar into the regular input jack. Sound?
If the relay does not click on, it COULD be a bad relay, but far more common for the relay to not be turning on electrically, the relay doing its job in other words.
|Enzo||10/8/2017 9:02 AM|
|TP11 would be that resistor or any of R47,48,50,37,39,41.|
|HNKTNK||10/8/2017 9:07 AM|
|HNKTNK||10/8/2017 9:08 AM|
|Thanks for the insight, Enzo. I've been a long time lurker and gotten a lot of valuable information from you over the years.|
|Enzo||10/8/2017 9:21 AM|
|No relay click:|
Then we need to isolate the problem to either the relay itself, the power supply to the relay, or the circuit that controls the relay.
You want to find if the relay coil has power. The terminals are under it and hard to get to, but diode D19 is parallel the relay coil, so measure there. DO you see about 60v on the cathode? Note TP18 is the line controlling the relay. Is it OK? All those little transistors along the bottom of the page are the relay control, I see also TP17. All that has to work.
|HNKTNK||10/8/2017 9:28 AM|
|Jazz P Bass||10/8/2017 10:02 AM|
|If you have a blown output section there will be abnormal amounts of dc voltage on TP11.|
THAT will keep the relay in an off state.
|HNKTNK||10/9/2017 11:52 AM|
First off, I'm an idiot. I'm not sure how or why I ended up with the b2r schematic, because the schematic I needed was for the b4r (b4r and b5r share the same power amp schematic). This alleviated all of my confusion, because well, now everything is matching up from the schematic to the circuit itself. I was able to remove the cover on the relay, which enabled me to get a reading on the terminal. 15vdc like it should be. I checked all the test points in that circuit and got readings that pretty much aligned with what they're supposed to be. So does this pretty much point to the relay being bad?
|Jazz P Bass||10/9/2017 4:08 PM|
|How did you measure that 15Vdc?|
That circuit is a little different than the B2R.
This one employs +15Vdc on the coil at all times.
When it is engaged, Q120 will provide -15Vdc to the coil.
Thus the -11.5 Vdc of TP12.
You really should check that TP11 does NOT have gross amounts of volts dc on it.
|HNKTNK||10/9/2017 11:15 PM|
|Jazz P Bass||10/10/2017 12:34 AM|
|Surprising that you cannot hear it click as the circuit appears to be working.|
It may very well be that the contacts are not closing.
I would put a new relay in.
|HNKTNK||10/10/2017 1:19 PM|
|HNKTNK||10/12/2017 4:32 PM|
|Well I put the new relay in, and while that is working and I'm now getting signal to the speaker, the amp sounds very quiet and distorted. I'm not really sure where to go from here.|
|Enzo||10/12/2017 5:22 PM|
|You start as if the amp just now came in. Forget the problem you just fixed. Isolate the problem. use the power amp in jacks to decide if the low level is due to the power amp or preamp circuits.|
You spent time and energy finding the bad relay and replacing it, but we cannot assume all the things we did to cure the relay will impact this issue. It may or may not be related, so we clear our minds of baggage and just approach the problem by itself, not in the context of the relay problem.
|HNKTNK||10/12/2017 6:11 PM|
|J M Fahey||10/12/2017 9:29 PM|
, the amp sounds very quiet and distorted. I'm not really sure where to go from here.
Now letīs check what instruments tell:
Crate/Ampeg schematics are the best in the Industry, period, because they are chock full of Test Points, showing voltage and (incredibly) waveform at each one.
Look at schematic Page 1 and 2, TP tables show you that by feeding 0.68V RMS @ 1kHz at power amp direct input jack, you will reach 40VAC at the speaker out into a 4 or 8 ohm load.
Just for testing, work without load, simply to avoid overheating it (if you use a resistor) or becoming deaf (if you use the speakers).
If you can get clean 40V there, then we will try the same with a load.
Ok, do you?
Start building a load capable of dissipating 400 or 500W RMS without catching fire, not an easy task.
Search for water heater resistors which are close to speaker impedances, drop one or two inside a water bucket (not kidding) and wire them to a plug or speakon to load the amp head.
For example, a 120V 2000W water heater resistor, price between 10 and 20 bucks, is nominally 7.2 ohms and can be considered 8 ohms for all practical ends.
|HNKTNK||10/16/2017 11:54 AM|
|Ok, so I had more time to work on this thing today. I made a test probe that runs to a little battery powered amp and started testing. as soon as I get to zener diode d110 at tp9, I start getting really distorted signal, and everything after either sounds quiet and or distorted. I took some readings at tp9 and this is what I got: -11.2 vdc on the negative side of d110 and on the positive side it started at -10vdc and gradually kept going down. On r135 I was reading 67.1vdc on one side and -9.71vdc on the other. Any thoughts?|
|HNKTNK||10/16/2017 11:55 AM|
|This was all with no load|
|HNKTNK||10/16/2017 1:35 PM|
|I should also state that both aforementioned components test fine out of the circuit with my dmm|
|J M Fahey||10/16/2017 2:42 PM|
1) when talking about a diode, use "anode" and "cathode" rather than "the positive side" which can be understood in different ways.
And D110, a 1N914, is not a zener diode but a regular one, although somewhat smaller and lower powered.
2) a test probe wonīt help you much here, we are not searching "where" signal stops, but "what" is happening, so use a scope if possible or at least a multimeter.
The test point table shows whatīs to be expected at TP9 , under different conditions.
You quote some 10/11V negative, but I guess itīs with signal, not what the TP table asks for.
They ask for a DC voltage at idle (no signal) *or* 40VAC when fully driven, so please search for that.
In fact, Iīd start with DC readings which I presume bad.
Also measure and post what you have on TP10 - TP11 - and to have the full meal, also TP12.
For now: no signal and DC only.
EDIT: Voltages on power transistor boiard TP9 - TP10 actually come from the main/driver board, and we might hava a cable/connector problem here, so follow the wiring (on schematic and physically inside the amp) and measure same on the other end, we *might* have a connection problem here.
I mean [B_lo] and [B_hi] and also J45 and J46 on page 1 , so I expect 4 DC voltages, besides TP9 -TP10 ones.
I guess we are getting close to the core of the problem.
|HNKTNK||10/16/2017 2:47 PM|
|ah, yes I was measuring with signal. Ok, I'll check what you mentioned and get back to you.|
|HNKTNK||10/16/2017 3:36 PM|
|Awesome, I'll check that. In the meantime, here are my readings:|
TP9- d110 is getting -14.5vdc on the cathode, which drops steadily abd -13vdc on the anode which reacts the same way. R135 has -11.35vdc on one side that steadily drops and 67.6vdc on the other side.
TP10- D113 has -5.93 on the cathode and -5.15 on the anode, both measurements steadily drop. R136 has -10.83vdc that drops on one side and a steady -67.4 on the other.
TP12- just about -12vdc on outside terminals and -12.68 on the center terminal.
I noticed that the readings changed after the fan kicked on. Is it necessary to note those as well?
|HNKTNK||10/16/2017 3:57 PM|
|Also, I'm working on a b5r, but using the b4r schem via LOUDs advice. I thought that power amp "b" wasn't in this amp I have because it's not an amp with 2 power sections|
|g1||10/16/2017 6:07 PM|
|When giving voltages at test points, please give only that voltage. Describe other voltages elsewhere.|
For example, TP9 is exactly the anode of D110. Not the cathode. TP12 is the emitter of Q120, not inner or outer terminal.
You should be able to mention any test point and we will know the exact point.
When you get DC voltages that seem to change, or drop steadily, check that you are connected to the proper ground for the board you are working on, or use the ground at the main filter caps.
|HNKTNK||10/16/2017 7:24 PM|
|HNKTNK||10/16/2017 7:27 PM|
|And thanks for the advice on the dropping values. That was confusing me. I'm obviously no amp wiz. I've done some basic work on tube amps that I have, and now I'm just trying to get this one working, which is pretty over my head|
|g1||10/16/2017 10:41 PM|
|Ok. For example, TP9 has an arrow that touches a particular line. Everything in a straight line to that point is a direct connection.|
So D110 anode, D114 cathode, so on down that straight line to R162, all are points that you could call TP9, including anything directly connected to "A_HI" shown on the left of D110. All would measure zero ohms to each other.
Try to get some stable DC readings on the TP's JM mentioned in post #26, and see his edit about the connector. It could be the cause of your dropping meter readings.
|J M Fahey||10/16/2017 11:12 PM|
problem is we are "somewhere else"; be it 1 floor (or 1 room ) or 10000 miles away is the same, *we* are not there and "you and only you" are our "eyes", so we must all agree very well on what are we talking about.
To make it clear to you Iīll tell you what *one* TP means:
But ... but ... you are showing me one long zigzagging track, two diodes, one connector, one capacitor and 4 resistors!!!! and 8 solder pads !!!!!
Now I am more confused than before!!!
I put the black meter probe on ground or chassis, but WHERE do I put the red one?
ANYWHERE which is painted green (I forgot to paint one C115 leg).
Voltage is exactly the same anywhere green.
Notice that on every component I painted green *one* leg but none of the others.
Technically the track or wire joining various component legs is called a "Net" and is an essential element of PCB design, it shows "whoīs connected to who", here they call certain Net a TP because itīs useful for diagnostic.
Apply this knowledge to all other TPs, because the idea behind it is the same.
NOTE: some PCBs (notably Fender but many others) add an extra pad in the PCB, put some turret or connector pin and call it TP*** so people goes straight to them to measure, they might even neatly label it TP*** on the silkscreen, but as you saw that is not essential.
I remember one computer magazine, (many moons ago) which among the small ads offered small key sized "Any Key" stickers.
Point is, on many software packages often they include "press any key" ... some users panicked and emailed the Service Dept with the question: "I canīt find the Any Key anywhere on my keyboard
and the answer "just press any key, any d*mned key !!!!" didnīt help, so some entreprising genius sold them with a nice picture showing where could they be applied.
|HNKTNK||10/17/2017 9:24 AM|
|Man, you guys have been really helpful. This has really cleared up a ton of the confusion I was having!|