JCM 800 2203 weird problem

[tubby]

hi all,

after 15mins of playing, the amp starts to 'treble boost' by itself every 20 secs or so. the amp gets louder, noise level and hiss increases, tone becomes very trebly and bass and mid controls don't do anything. this goes on for, say 15secs, then the amp goes back to normal and the EQ is working. then it's 'treble boost' again and on and on....

this all happens by itself, regardless of pot positions and signal levels applied.

i've never experienced this before, if someone could point in a general direction. thank you!

[Gtr_tech]

Had this problem with a reissue Vox AC100 that was in a coupla months ago. It ended up being the resistor that feeds the bass/mid controls. Usually this will be a 33k off the stage 4 (cathode follower) cathode in the JCMs. I'd bet my lunch money thats yer issue.

Keep in mind this resistor checked good out of the circuit. It was breaking down under working voltage.

[Gtr_tech]

Also check the gnd connection at the mid pot. Verify that the pot has not gone intermittant at any of the lugs. Lifting the gnd at the tone stack effectively takes it out of the circuit explaining the signal jump.

[tubby]

thanks gtr_tech!

followed your advice, replaced the slope resistor in the tone stack.
i also checked and resoldered the ground connections on all pots. sorry to say all of this didn't change anything.

finally i noticed that when the amp is in 'self-treble-boost-mode' and MV on zero, there's still a rather loud signal coming through. i immediately
checked ground connections on the MV pot again. they were all fine.
so the wiper of the mv pot is at ground potential and there's still a signal? there's no other path to the power amp, except through the MV pot. i don't get it....

well, anyhow, lacking any other ideas i swapped the pre-amp tubes
and the problem was gone. at least for tonight. i'm gonna check again tomorrow, cause i'm totally not convinced i've got the culprit. yet.

[tubby]

update: ok, the problem reappeared with different pre tubes.
thought so. next thing that comes to my mind is changing the MV pot.

it just doesn't seem to make any sense, since the problem seems heat related (at least 10 min warm-up time) and the MV doesn't even get warm....

[JerryP]

Is it an original 800? Have the filter caps ever been changed?
Jerry

[tubby]

hey jerry,

no, it's a russian clone which is about 10 years old.
do you think the problem could be related to the filter caps?

[Gtr_tech]

I really doubt it. The thin sound points to a component opening up and all you get thru it is tinny high end from capacitive coupling. Here's where you'd trace the signal with a scope when the problem happens and find out where the problem first occurs.

Possibly a cracked resistor or cracked solder somewhere. Run the chassis upside down and tap everything on the board with a plastic pen or similar "spudger" stick. If its mechanical in nature the whacking on components will find it. If its a resistor/cap breaking down under working voltage you need a scope.

[jmaf]

finally i noticed that when the amp is in 'self-treble-boost-mode' and MV on zero, there's still a rather loud signal coming through. i immediately
checked ground connections on the MV pot again. they were all fine.
so the wiper of the mv pot is at ground potential and there's still a signal? there's no other path to the power amp, except through the MV pot. i don't get it....

That's odd... There should be no signal in that case. Take a piece of hookup wire and short the wiper output to the chassi touching them temporarily, without soldering. See if all signal dies. If it doesn't then part of your problem is from the inverter onwards. If it all noise does die then that pot is a candidate for substitution.

[Mars Amp Repair]

Hello folks,
sound bleeding through with the volume all the way down can ideed be leaky decoupling filter caps right there in the early stages. So, you might try jumping the cap with a known good one to eliminate that possibility before it drives you completely nutz! glen

[tubby]

thanks for the input so far guys!

i tried jumpering the MV pot to ground, no effect. so it's definitely not the MV Pot. turning the MV pot (unjumpered) has no effect when the weird stuff is happening. the signal doesn't even get louder on MV 10.

next thing i'll try is subbing the pre-amp filter caps ony by one as suggested by Glen.

Meanwhile I'd really like to learn and understand what's going on here. If the PI input from the MV is at ground potential, where does the signal that gets amplified come from? and what's the role of leaky filter caps in all this?