Play a steady signal into the FX return so we are only feeding the power amp. Use a signal generator or my old trick was a small cheap keyboard with a piece of paper wedged between two keys to hold one down and used the signal from that, But even just a feed from a CD player or Stereo receiver. Leaves your hands free.
Now while it plays, use a wooden chop[stick or other insulated probe and push on the things between that jack and the speaker output. That includes the ribbon to the power tube board, the connections to the jack. Well, jacks. Components on the board. Oh, 5150, so the phase inverter is on the preamp board, so you'll be poking at that too, and its ribbons.
Now while it plays, use a wooden chop[stick or other insulated probe and push on the things between that jack and the speaker output. That includes the ribbon to the power tube board, the connections to the jack. Well, jacks. Components on the board. Oh, 5150, so the phase inverter is on the preamp board, so you'll be poking at that too, and its ribbons.
Ran 440 Hz signal into effects return jack and poked around inside with a chopstick. Tapped and prodded all ribbon connections, tubes, jacks, connectors, exposed components, etc. Couldn't reproduce the error.
Ran same test signal into "normal gain" input and performed same chopstick test. Still couldn't reproduce the error.
Ran same test signal into "high gain" input and performed chopstick test. Still couldn't reproduce the error.
I noticed that tapping on the preamp tube closest to the end of the chassis caused a louder thump through the speaker than the other preamp tubes. Replaced preamp tube, but no volume change in tapping was observed. Reinstalled original preamp tube.
The only setup differences (apart from the signal generator vs guitar + cord) since the problem started were disassembly of the head and a change of power outlets.
I'll periodically perform the test again, but it's hard to troubleshoot if I can't reproduce the issue.
Any suggestions?
Ran same test signal into "normal gain" input and performed same chopstick test. Still couldn't reproduce the error.
Ran same test signal into "high gain" input and performed chopstick test. Still couldn't reproduce the error.
I noticed that tapping on the preamp tube closest to the end of the chassis caused a louder thump through the speaker than the other preamp tubes. Replaced preamp tube, but no volume change in tapping was observed. Reinstalled original preamp tube.
The only setup differences (apart from the signal generator vs guitar + cord) since the problem started were disassembly of the head and a change of power outlets.
I'll periodically perform the test again, but it's hard to troubleshoot if I can't reproduce the issue.
Any suggestions?
Well, you have succinctly described why intermittent problems are the hardest to solve.
That first preamp tube is usually the input stage and will naturally be more microphonic than the other tubes.
Poking around revealed nothing, so ball up your fist an whack the end of the chassis hard. Or use a rubber mallet. Does that trigger anything?
other weapons are a heat gun and freeze spray, those sometimes reveal sensitive parts.
And I am always suspicious of that muting JFET, Q7.
Just received my new tube package, so I replaced all preamp tubes with fresh JJ 12AX7s. Still didn't hear any issues on the rhythm channel, but after switching over to the lead channel I heard some crackling/volume fluctuations when manipulating the post gain knob.
It's likely these pots have never been replaced and are now ~25 years old. Is this within their service life? Is it a good idea to replace all the pots while I have it opened up?
Is it also worth replacing Q7 (J174) if it can cause intermittent issues?
My next step is to reflow all solder joints on both boards, but I'm shooting in the dark now.
It's likely these pots have never been replaced and are now ~25 years old. Is this within their service life? Is it a good idea to replace all the pots while I have it opened up?
Is it also worth replacing Q7 (J174) if it can cause intermittent issues?
My next step is to reflow all solder joints on both boards, but I'm shooting in the dark now.
My test is to simply remove it and see if things improve.
Its function is a momentary mute as the amp switches channels. It thus prevents unwanted noises at the moment of transfer. Without it, an individual amp may or may not make some godawful noises, we never know. SO if it does, ignore that, we just want to know if it is involved with the original problem.
Its function is a momentary mute as the amp switches channels. It thus prevents unwanted noises at the moment of transfer. Without it, an individual amp may or may not make some godawful noises, we never know. SO if it does, ignore that, we just want to know if it is involved with the original problem.
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