Just joined here, total noob, and mechanical engineer, so electronics is an art that I'm just trying to begin to learn.  I've done rudimentary soldering and should be able to replace any component that is available today.  I'm at a loss now after some troubleshooting.  Looks like a lot of good minds on this forum, so here goes.

I have a 1980's JMF Spectra 125 SS Combo Amp (pre-Dean Markley) that is no longer producing clean sound and volume (on the Clean or Distortion channels).  It sat in the basement for years and years (probably corroding) up until now.  I've cleaned the pots (no crackling), replaced two corroded ceramic resistors in the power amp PCB (.33 Ohm and 33 Ohm in between the Power Amp board 3792 and 3716 RCA transistors), brushed the boards somewhat clean, and reflowed one solder joint.  The amplifier produces some lower volume, distorted, flat sound (midrange-e only) but only when I strum kind of hard.  Otherwise, the amp doesn't pick up the sound from the guitar pickups on light strumming.  Also, the volume knobs basically don't do much no matter what volume/master volume I turn. The reverb knob changes the shape of the sound slightly.  Using an extension speaker gives same sound, so it's not a blown speaker.  It also starts screeching/rumbling after 30 seconds or so of being switched on, but hard strumming overpowers the screeching while playing.  Plugging guitar into the Line In/Power Amp In also gives same result.  Also, plugging a short cable into the Line-In and Line-Out together does nothing.  I have attempted to check resistor resistances (the two ceramics were bad), capacitors, diodes, transistors with a decent Fluke multimeter, but can't see anything apparent.  Note, I did not remove all components from the board.

I don't have an oscilloscope, nor do I know how to really use one anymore, but I don't really want to have to hand it over to an electronics repair shop (repair will likely cost way more than it's worth).  Wonder if anyone has any suggestions/questions/comments?  Hope the links work.

Power Amp Board

Power Amp Schematic

Preamp Schematic

If you are talking about the last bjt, emitter follower indeed, I wouldn't go above 10k unless I wanted to purposefully introduce "cathode follower"-ish distortion.
FWIW I breadboarded the 3-stage version of this circuit, not the 4-stage one, and there was plenty of gain and still some wiggle room to push it with a TS without turning into white noise. YMMV
Oh, and ref. biasing, nothing wrong with using 2 resistors in series to get to the right bias either.

Located in Toronto Canada. Thanks thus far...

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.

Hi,

I have a Marshall Mosfet Lead 100 guitar amp (3210), and when I switch to the boost channel while playing, there's an audible pop (also if connected directly to a mixer from the DI in the back). The other way (boost->normal) there's no pop if I switch while playing. Also, normal->boost there's no pop if I'm not playing.

What I want to ask is, is this expected behaviour?

So, I did some breadboarding today to find collector resistors for my BC337's. I've found a set of 4 that bias very close to 4.5 VDC with a 22k in the collector.
I didn't bother to look for the 5th one because it's an emitter follower. Should I do it? Also, should I increase the value of the buffer stage's emitter resistor from 10k to 22k?
Next step is to put the whole thing together on the breadboard.

Just for the record, I'm attaching the datasheet for the TIS58.  It's pinout does not match the interfet 2N5484, nor the on-semi or centralab 2N5484.  So that's 3 possible pinouts for the Randall Fet's.   

From the voltages you listed, it would seem that T4 collector is connected to T6 base, as you have the same voltage listed for them.
But T4 collector should go to T6 emitter, so once again the voltages listed are questionable.

Thanks so much Loudthud!  I swapped those JFET leads and we now have audio!  Thanks to everyone who contributed to this thread!

Quote from: Umlaut on May 21, 2024, 03:18:58 AMGK schematics are an extremely useful resource on how to bias and design jFet gain stages, btw. Most if not all of their guitar amps from the late 70s on (and a lot of their bass amps) feature jfets in one form or another.

So much to learn, so little time...