JCM2000 Unusual low volume on clean channel

[Fabster]

Clean Channel of my Marshall DSL JCM2000 has an unusual low volume compared to the overdriven channels. I have to turn the Master and Drive volumes almost to the highest levels to match the Overdrive Channels when set to 3 or 4 volume level. Took a look to see if I find any blown tube but at least from the outside they look ok.

Any idea what could be the problem? tks!

[wyomingrocks]

Just a guess, perhaps a bad preamp tube. Do you have any known good ones floating around, if so try substituting. Good luck

[Corben]

Hey i got the same problem i posted about it before i saw this one. Mines a tsl100. Its got r24 burning up allways, and r69 gots really hot. Though so far that r63 hasnt effected the output but i know its getting much much to warm, its sligtly brown and will burn to the touch in like 1-2 seconds.

When i change r24 it gets louder but i hear alot of random intermitten static poppy sounds and then the clean channel lowers in volume alot when it finelly burns out or unsolders itself from the heat. While the other 2 channels work fine.

Is yours doing anything similar or haveing the same parts failing?

[oc disorder]

If I can but in Corben something is drawing too much current from the HT supply post R24.
It could be something as simple as one of the first two tubes (V1,V2) or more likely one of the filter capacitors that go to ground after R24.
C20 10uF 450v , C22 10n 500v,
C10 10uF 450v , C11 10n 500v
C19 10uF 450v , C21 10n 500v
If any of the electrolytic capacitors look like they have got hot - bulging or swelling with the wrap or label shrinking ,its possible they are leaking and need to be replaced C20,C10 & C19.
Likewise it wouldn't be the first time ceramic caps have developed a fault and may get hot in the process C22,C11 & C21.
They are the yellow/brownish small caps next to the electrolytics.
First remove both V1 & V2 and see if R24 still gets excessively hot if not replace them one by one and see if one makes the resistor hot.
If not (ie the resistor gets hot without the tubes in) I would suspect one of the capacitors mentioned above.
I should point out there is aprox 380 to 430 volts dc in those areas so be careful please.

[Corben]

Please do bud in indeedily. I allready changed the tubes. The original preamp tubes all were insultingly weak. The output tubes were annoyingly weak to so it got a brand new happy set.

I shall def. swap the caps per your mighty demmads. I tested it for ESR and dc resistance and reg old capacitance with a low voltage cap meter. (As usual) The caps all test pretty good with those 3 test. I really should not rely on those test for telling me a cap is good at all any more, I guess those tests are only good for displaying indications pointing towards a bad cap. Sigh.. and i had such high hopes for ESR.
None of the caps had any buldges, deformities, or physical abominations on them and were pleasently cool to the touch.

I do have an ole' confangled sprauge high voltage cap tester that tests up to 600 volts ive rebuilt and calibrated, oh how i love seeing that eye tube glow!!!! but sadly its almost allways the case that by the time i have the part out to test i just might as well pop in a known good one. And how i do enjoy any opertunity to use test gear & playing with those test probes with 600 volts running across them as i can feel/fear the power surging through them.
I do use aligator ends on them most of the time makes it a touch safer i find.

[oc disorder]

Whoops I just looked again at the schematic and see that C11 and C10 are not fed from R24.
So those are not causing trouble.
A can of freeze spray with a thin nozzle on to the two possibly suspect ceramics C21 and C 22 may help isolate the problem. Although they may pop and splutter a bit anyway, if one is faulty
it will be a rather loud pop and splutter if it's faulty.
I'm assuming that the fault is still there in the other channels but because of their massive gain its not noticable.
Probably more to the point that section of the first 12AX7 the "AA" section may not always be in circuit in the Crunch and OD channels wheras what I usually see as the second half the "BA" section (pins 6,7 & 8) is in circuit all the time.
The BA half of the tube does not get its supply via the resistor R24 but the AA half (pins 1,2 & 3 ) does.
Pity it's such a major disassembly to get to the caps makes it hard to substitute quickly to see if thats the problem.
Normally an ESR reading as you say gives an indication of the condition of the cap although the real conditions (high HT etc) can bring out the gremlins.
The ceramics would be my first port of call.