Took the board off, pulled the solder off R49 again, cleans the area (some brown residue), soldered it back again, took off C31 and it read fine. Re-flowed some questionable looking pads and it's working again. Have to try it for an extended period once my  8-month-old is up from his nap

I can't get any signal out of it now, seems like I left it on long enough for something to fry for good. All of the voltages checked against the schematic seem within reason 3.2 VDC where it says 3, 26.8 VDC where it says 25, 30.7 VDC where it says 27. Nothing too out of whack.

One odd thing is when I touch D7 it reads 13.8 VDC, but I get a big pop and radio starts coming through.

Q: What exactly does the amp call itself on the serial number plate?  Rage 158 TT?  Are there any other numbers there like '92.
A: TRANSTUBE RAGE

Q: Does this drawing set match your board better?
A: Yes, I think that's it. Thanks.

Here's the amp. The initial problem I fixed was the top, left-most solder joint on R49 had cracked out. Probably due to the board mounted jacks and minimal board support.

Q: Next time it kills the sound, leave it running, ball up your fist, and whack the top of the amp.  Does the amp come back on, even for a moment?
A: No.

Q: Does the amp react to your whack in ANY way, like a noise?
A: No.

Q: When it dies, listen CLOSELY to the speaker, is there any background hum or hiss? 
A: Yes.

Q: Turn up the controls to hear that better potentially.   If there is any such background, to the volume and tone controls affect it at all?
A: Low hum, not affected by volume or tone, buzzes louder if I short the signal cable with my thumb.

Q: Get the schematic from customer service at Peavey, and then see if your power supplies are remaining up or if they are going out.
A: 120VA into the transformer, 25VA out  in the no-signal state.

Q: With it running and signal applied, poke each part on the board with a wooden chopstick or similar.
A: No effect.

I tested the diodes, they all read good except for the two 1N4003's by the power transistor, they short both ways -- maybe that's expected? It also seems if I leave it on in the no-signal state for 5 minutes or so the power cycle no longer fixes the issue, I'll have to wait and see if it works again after being off for a while. That brings me back to heat, but nothing on the board seems overly hot (hit it with infrared thermometer).

Also, still trying to determine if that's the correct schematic. It's a Peavy Rage 158 w/ Transtube, but the component markings aren't all matching up. The initial sold solder I fixed was on an R49 which I can't seem to locate.

The amp is the same as the one pictured on this thread,

https://www.diystompboxes.com/smfforum/index.php?topic=97068.0

I whacked the cab a bunch after it dropped, no effect what-so-ever, even lifted 1/2" and dropped it, that sort of thing, nothing. Power cycle and it's back. Got it open again and I'll start taking some readings.

Update: Removed incorrect schematic.

Picked-up a Peavy Rage 158 that wasn't working right for another fellow. Found a cold/bad solder joint which fixed a board wiggling cut-out issue. Now everything works fine for a couple of minutes, then no sound. Cycling the amp power brings it back, just off long enough to hear the "turning off pop" then back on, so that tells me maybe not a heat issue as it wouldn't have cooled that much, but is perhaps after a cap discharge? Any ideas of where to start?

Anyone have (and willing to part with) or know where I can get a couple of these wacky 70's Yamaha knobs from a YTA-95? Or know some other amps that used them?

Quote from: Enzo on October 23, 2016, 02:58:00 PM
The black one is connected to ground, but in terms of the transformer, it is the center tap.  Whatever you get red to red, black to either red should be half of it.  In other words your 24v red to red ought to net you 12 from black to either red.  Or 17 there should make 34v red to red. Those readings don't really work.

Or you have some strange situation.  Pull the red wires off the board and measure them unconected to circuit.  Oh, only if you feel like pursuing it.  I don't know about under spec, but it could just be defective.

Thanks, good to know. I'll take those measurements again when I have it open.

Quote from: J M Fahey on October 23, 2016, 12:21:09 PM

QuoteI'm reading 22.3 VAC across the output of the transformer, so I guess it's a 110VAC to 24VAC, not sure what amp rating I should be looking for.

Mmmmhhh, I find a problem with your measurement.
To begin with,there is not "the" output and quoting a single number (22.3VAC) or

Quoteso I guess it's a 110VAC to 24VAC

, all of which imply a single winding, because that transformer has two secondary windings, sop'froper answer would be quoting two voltage values (even if identical).
Look at the transformer schematic,you will see that secondary has two windings, one connected from W3 to W4 and the other from W4 to W5.

I haven't done a lot of work with transformers, so I'll describe how I got that measurement. There are three leads coming off the secondary side of the transformer, two red and one black. I had thought the black was some sort of ground -- is that not the case? The amp is off the bench for now, but I would think the black was W4. Measuring VAC between the two red leads was 23.3VAC, measurement between either of the reds and the black was something in the neighbourhood of 17VAC.

When next I open the amp up I'll check the voltages you describe, after upgrading the caps and cleaning the jacks, I'm going to see if the problem is still there -- so far it hasn't occurred but I'll need some time to verify. Thanks, the sentiment on this thread seems to have changed for the better and I'm grateful for that.

Quote from: phatt on October 23, 2016, 07:29:41 AM
Or go read the data sheets, the chip draws 5Amps at full tilt so any transformer larger that what you have will likely improve it a tad.
Purchasing a new 4~5Amp 24VAC transformer is false economy for such a small amp.

(...)

Arrh huh,, this amp has relays and a dedicated 5volt rail for the digi FX setup.

If there is some sag they might not work right,, so in that case a larger Xformer might inadvertently fix it.

So just maybe the bigger Xformer kinda fixes things but if one had time to dig deep you may find it has more to do with the 5volt rail which runs the FX setup.

OR,,, the preamp rails are not able to regulate under load,, Just some quick calc on the voltage you gave earlier,,
24 VAC x 1.4 = 33.6 VDC /2 = 16.8VDC...
So ZD1 and ZD2 are 15 Volt zener and they need at least 2~3 volts ABOVE their rating to work.
even a small sag in supply will make the preamp supply fall out of regulation.

simply by replacing those two Zener's with 12 Volt units would fix the issue,,,Far cheaper than a new transformer me thinks.

of course it's all up to you.  8|
Phil.

Yes I agree with  Enzo who just posted. I could add many similar stories like his.

Thanks, I hadn't thought of checking the chip sheets. I'm okay with a little bit of false economy since I got the amp for free. If it keeps exhibiting the problem I'll have a look at ZD1/ZD2.

Quote from: Enzo on October 22, 2016, 05:43:56 PM
If it cannot provide enough current, it will drop voltage on its own rather than harm the circuits.  The power amp IC will work less hard.  Many of these transformers have thermal fuses inside them, but those are open or not, they don;t reset themselves.

And I apologize if you feel the directness is hostility.  I for one haven;t any hostile feelings.  Just as you don;t want to spend hours troubleshooting, when I offer advice, I don;t want to spend a lot of time beating around the bush, and otherwise "softening the blow" so to speak.  I just say what I think.  The reason Juan and I react to transformers as diagnosis is that for some reason that is the first thing the novice jumps to when it ought to be the last thing on the list.  Not calling you a novice, just that the transformer hopped to the top of this repair too quickly, from our experiential point of view.

The JC120 is a complex - overly so - stereo amp, and is really not remotely like working on this little amp.

I don't mind directness, there have been overtly inflammatory statements made that I consider hostile, not by you. The largest barrier to troubleshooting this amp is that the problem is intermittent, and I'm not talking about wiggle a plug intermittent, I'm talking about unable to ascertain the circumstance intermittent. Using the amp for 20 minutes straight the issue may not arise, other times, right away. I'm sure you can understand that makes things especially difficult from the get go.

If you read the thread that I referenced it's a very similar problem, a number of owners on the thread with the same issue, some talk of jacks and others talking about the power circuit. I'm familiar with the switching jacks, have some spares around, but the jacks would be a pain to swap out and support the circuit board in this amp so swapping for anything other than the exact same board mounted jacks (which I don't have) may cause complications, I cleaned them, maybe that will fix it, maybe not, I will monitor the jacks if the problem arises again. While I had it open I swapped the filter caps for double the uf and double the voltage rating, maybe that will fix it, maybe it won't -- if I'd gotten information regarding a suitable transformer upgrade I probably would have swapped that out too and had a fair chance of fixing the issue, without having to delve too deep.

As it is, I've done all that save the transformer and the issue may be gone, I'll have to continue to use and monitor.

I've got very limited time to dedicate to this sort of thing, so frankly it's either risk the quick fix or do nothing, especially on this amp which many consider disposable -- due to issues such as this.

Quote from: DrGonz78 on October 22, 2016, 04:44:23 PM
How does the amp sound when you plug in headphones and play it that way? Enzo is right about it might only be a $2 part or cleaning the headphones jack. Actually all the advice given so far is correct and nobody here will agree with misleading advice. So you ask for help locating a PT? Well measure some voltages to test your theory of a bad transformer. PT's don't go weak and only just go bad, there is really no in between here. To offer advice to spec and select another PT we would want to know what B+ is on the amp. Hostility??  I don't see anywhere here that someone was being hostile with their advice. Straight to the point honest, but not hostile.

I never suggested it was a bad transformer, or a bad power circuit, just that it was under spec'ed for this amp, as a result the amp is prone to overheating and that causes a variety of components to flip out, the IC's specifically, and eventually fry.

I don't believe anything is fried on my amp (yet) since it works perfectly fine, just every so often the volume drops to almost nothing and odd things start to happen.

In that referenced thread, not just one, but a number of folks detail similar issues and also relate that upgrading the filter caps and transformer alleviate the issue. Not sure why everyone here is so against trying that first if it's worked for others with the same amp, experiencing the same issues.

I swapped the 2200uf 25v filter caps with 4700uf 50v, ran the amp for the last 30 minutes and no sign of the issue, that's not to say it's fixed, or fixed in the best way it could be, but it may be fixed enough for me not to bother any further with an amp I could buy fully functional right now for under $100. I appreciate these forums, and got a lot of help recently trouble shooting and fixing an '81 JC-120, but that took a lot of time and effort, which this particular amp isn't worth.

Quote from: J M Fahey on October 22, 2016, 03:45:11 PM
Hope you read this before leaving:
you do not have a power transformer problem.

Neither did anybody at the Marshall Forum.

Not sure what the motivation is for all this hostility, if anyone wanted to help they could've offered advice on how to spec and select a transformer. Everyone in this forum has tried a fix that they weren't 100% sure would work because it had a fair chance and was quicker and easier than the next troubleshooting step -- that's all I'm trying to do. None of the pontificating in this thread has been useful, the quoted reply included.

Quote from: nosaj on October 22, 2016, 03:00:06 PM
"quote"I picked up an MG30DFX, for free actually, because it had intermittent issues for the previous owner and they got fed up. I've read it's most likely an weak power supply.

This thread folks recommend replacing the power caps and the transformer.

http://www.marshallforum.com/index.php?threads/marshall-mg30dfx-problem-volume-fade-instability.36882/#post-1346279"quote"

This is how your guessing.   Goober over at this other site said doing this fixed his amp what can I buy to fix my amp ?

Get voltages at your rails to really see if you have a power supply issue.  The transformer and caps are only a part of the power supply, just as oil and water and gas are only parts of a car engine.

People come here to learn and to hopefully pass it on. Maybe their cost benefit isn't worth your time.  Ask for help be ready to supply some answers, if its not worth your time why be here?
It's one of those days so excuse me
nosaj

Folks at other forums aren't by default Goobers. It's not that any component in the power circuit is bad, the theory is that they're just not providing enough power, a bunch of stuff get's overheated and things start going wonky. Testing the power circuit won't tell me that.

Anyhow, I didn't post to get lectured on how to troubleshoot an amp, I asked a simple question where my experience is lacking and it's not looking like I'm going to get an answer.

Browbeating me for looking for the quickest solution based on other's experiences instead of using the scientific method isn't going to change the fact that I don't have the time to troubleshoot this amp component by component. I've already replaced the filter caps, I'll look elsewhere to answer my transformer inquiries.