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80's JMF Spectra 125 SS Combo Amp Screeches and Minimal Distorted Volume

Started by jbarrie88@sbcglobal.net, May 22, 2024, 05:24:20 PM

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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


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.
"A person who never made a mistake never tried anything new." -Albert Einstein


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.
There are no stupid questions.
There are only stupid mistakes.


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


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?


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


Thanks Jazz.   :)

Quote from: jbarrie88@sbcglobal.net on May 23, 2024, 05:44:09 PMPardon my inexperience. How would I do that without providing a URL?
When you are in the 'quick reply' area click on the 'preview' button.  There will be an option to directly attach your files here.


I finally got to taking some voltage readings.

g1: Mentioned getting the voltages might uncover issues.  Below is an image of voltages taken at various points.  Would anyone have suggestions based on this?

Jazz P Bass

What is important is the follwing 4 voltages.
These are the rectified dc voltages that the amp needs to function properly.
Please list them.


Jazz P. Bass:  I included the voltages on the .jpg and .pdf attached here this time (I must have attached them incorrectly last post) and exactly where the measurements were taken.

At the +-22VDC point, the measured voltage was +-20.3VDC.
At the +-37VDC point, the measured voltage was +-68.2VDC.

Upstream of that, the transformer secondary windings were as follows:
Lower voltage taps were 17.4VAC and 34.7VAC.
Higher voltage taps were 27.5VAC and 54.6VAC.

I'm guessing that the 68.2V is an issue but am not experienced in how to troubleshoot further.  Thoughts???

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Those Voltages are correct, you just measured them in the wrong place.
68.2 divided by 2 = 34.1 which is close enough to spec.
You just measured across both + and - Voltage rails, forgot the common center tap.
Same with low supply, 29.4 is the potential across + and- rails.
Both Preamp and poweramp supplies use a split supply where you have 3 wires from transformer the center tap is the common or Zero point.
Sadly none of this helps much as the amp is faulty.
There looks to be a mute circuit hung off pin 6 of U1b consisting of Q1 & Q2 which may have failed causing very low output.
Other here will know more.


Phatt:  Thanks for the tip about the center tap, and yes, voltages are as you stated.  Even when I plug the guitar into the Line In, however, bypassing the preamp, I get the same output, and the amp hums instantly (since reflowing some solder points) and screeches after being on for 10-15 seconds or so as mentioned earlier.  Getting worse instead of better.  Going to attempt posting a video later on to show some sounds.


Well if the Amp hums Instantly then set meter to DC and measure how much DC is on the output.
It should read very low, in the millivolt range.
If there is large DC voltage at that point then the power amp has failed.
It's Best to disconnect the speaker otherwise you can burn out the speaker coil.
Better minds here maybe able to help you test some voltages in the poweramp section.


Apologies Phil, but where would I check for the output DC?  Are you suggesting check DC voltage directly at the output plug for the speaker?  Thank you.
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