Pardon my inexperience. How would I do that without providing a URL?

Bad caps should not make the supply voltages read high.
Getting the correct power supply voltages should be your first step.  You might be lucky enough that it solves all your problems.

I can't access dropbox files.  Could someone please attach the power amp and power supply schematic from the first post?

DrGonz78: If I play soft there is no sound at all, have to strum hard to get any sound, and then after 15 seconds or so of the amp being switched on, it starts to make a screeching/crackling sound and gets louder and higher over time.  Also, if I strum hard after the noise starts, you can still hear the guitar, but it is still very distorted and cannot be manipulated by the volume knobs.  Reverb seems to work though.  And switching from the clean to distortion channels with the footswitch seems to function, but they are still both distorted and very midrange-e, flat, no power.

Tassieviking:  When I plug the guitar into the return jack (Line-in) on the back, I get the same result.  I do not have another amp with a line-in/line-out.  Also, when I disconnect (unplug) the preamp board from the power amp board, the screeching noise basically stays the same.  Thinking problem is in the power amp board?

If I recall correctly, I think I was getting higher voltages (like 54V) at the +-37V and (approx. 24V) at the +-15V.  I suppose I could try and replace the caps?  Or could it be the RCA 3792 and/or 3716 power transistors?  Also, I wonder if I should be checking all solder joints on the preamp board?

One other thought, when last working (a long time ago) the amp speaker used to thump when I turned it off,  Now it just makes a weak cracking-like noise from the speaker, (no speaker thump like it used to do).  Even when connected to the KMD speaker cabinet, it does the same thing.  Not sure where to go next....

JMF Spectra 125 Amp and KMD 4x10 Cabinet

JMF Spectra 125 Amp Back

JMF Spectra 125 Amp Back Closeup

The reverb level is at the output of the first amplifier after the reverb output.
Pull out the reverb tank inputs and see if the hum exists.
If no, the drive to the tank is the culprit.
If yes, pull out the output from reverb tank and see if the hum exists.
If yes, the first amp after the tank output need to be checked.
If no, see if there are any other transformer nearby.
The tank will make hum if kept nearer to other transformers.

Regards.

Do you have access to another Amp?
If you do try connecting the send to the other amps return to see if the pre-amp is good, and then connect the other amps send into this amps return to see if the problem is in the power amp.
You could try the guitar into the return jack but it would be very low output, that would test the power amp by itself.
The best place to start checking might be the power rails, the +-15V and the +-37V.
Sometimes the main filter capacitors can dry out when not used for a long time.

Just a quick reply but perhaps study up on crossover distortion in a solid state amp. The symptoms you describe are similar. Play soft and it sounds fine. A hard attack signal will reveal the crossover distortion. It might be biased cold but that might not be what is going on. Just something to consider.

Hey, Psabin8951,
I'm no expert, but a couple of things popped in my mind.
Which fuses does it blow - primary or secondary?
As I understand Mesa Dual Rectifiers, it should have a rectifier selector switch. Does the amp do that with solid-state rectifier selected?
Could it be a short on the primary side of the power transformer? 🤔
Like DrGonz78 said, a schematic and voltage readings would help.

You need to be super careful and you must know what you're doing if you ever decide to tinker with it yourself. As you probably know, there are lethal voltages inside that thing. If uncertain, take it to a tech.

Quote from: mr.death on May 22, 2024, 04:11:09 PM

Quote from: DrGonz78 on May 19, 2024, 12:19:47 AMWhat method did you use to make the conclusion that the transformer is shorted?

I watched a dlab vid. He shows how to check the transformer. I did his test and it failed. Also when I turned the amp on I could hear the transformer frying, bubling sound. Its toast.

Okay just had to ask or we might have let someone trash a power transformer. I like D-Lab a lot and he is a solid provider of good services. I never heard a bad transformer make bubbling noises. I mean I have heard a buzzing sound from old worn out transformers. But most bad transformers and we're talking fuses blowing when turned on.

Quote from: Umlaut on May 22, 2024, 04:18:37 PMOh, and ref. biasing, nothing wrong with using 2 resistors in series to get to the right bias either.

The 337's I tried today were very consistent, or maybe that's just my limited experience. Out of the 15-ish pieces I tested, one biased at 4.44 V, the five I selected are between 4.51 and 4.55 V, and the rest gave a reading of around 4.6 V. All that with 1k emitter resistor, 22k collector resistor, and the 4.7M / 1N4148 / 1M network connected to the base, as per Joe Davisson's schematic.

Yeah, I meant the last stage, the emitter follower. I'll leave the load resistor at 10k then.
Speaking of the 3-stage BJT version, I kinda know how that thing sounds because there is a video of it on YouTube and, indeed, it does display the quality you described earlier. Still, I'd like to (and am going to) try the 4-stage preamp because I'm curious what it can do. There are no clips that I was able to find so I might become the first one to upload some
I started putting the circuit together on my breadboard, but I needed to take it apart halfway through because a lot of the connectors were overly loose, so I figured I'd retension them. Should have checked that first...
I might have some time to get back to working on it this weekend.