JCM2000 TSL100 Head-aches

[cjm808]

Hi all-
New here posting but longtime 'lurker'-so much valuable info and help here its unreal! Have my shop Marshall TSL100 on bench and am stumped. Amp started losing volume and began breaking up on a show so I hung it up for several weeks and finally got around to opening it up. Noticed burned (but still reading ok) resistors R61, 62, 63 so pulled them and replaced (4.7k, 1k, 4.7k respectively), also found R76 open and replaced as well. Per past posts here I also checked C46, C14-in fact all caps (hopefully) for any shorts-no apparent ones. Put back together and fired-take off standby and in matter of 10 seconds R61-63 begin smoldering. Metered for brief moments-first reading across R63 350v and 250v (about), across R62 250v and 180v, across R61 180v and 0. Back and forth on standby so resistors don't burn up. Pulled EL34's then 12AX7's-resistors still heat up. With all tubes pulled only voltage reading now is on one leg of R63 (350v) with 0 on the other side and still wanting to 'burn up'. I have an 'inherited' bench with a variac and some test equipment and a 'little' knowledge (to be generous) and do basic servicing on equipment mainly on my own gear (am a sound provider and backline supplier) so be gentle-am not as 'super-knowledgeable' as most of you guys here. Any assistance or guidance would be most appreciated!!
Craig

[Twist]

You may have lost a tube when R76 burnt out. Check C33 for short in the B+ circuit. Others to check would be C11, C10, C31, C14, C32, C33. Another possibility is someone has serviced the amp and dropped some hot solder on the PC and it's causing problems.

[Enzo]

Yep, if those resistors are buning up, it almost has to be shorted C33, 32, or31/14. Or a mechanical short like Twist's solder blob.

You cannot measure the caps with a meter. Just because they don't show short on your meter - which yses maybe a volt to test - doesn't mean they don't leak like a seive at 300v.

About the only reason for R76 to burn is a bad tube.

[cjm808]

Thanks gentlemen for the quick replies-really appreciate it. Yes Enzo I did meter the 22uf/450's and they looked okay but.....
So-is there any other way to check these (probably stupid question) beside lifting them or should I (for future reference) just assume they may be bad and replace them?
Since I don't have any matching caps in stock I will go ahead and order replacements and swap them all out-I just like knowing what component was faulty for some peace of mind.
Anyway thanks for the help-you guys are the best!!

[Enzo]

The problem you are trying to solve is the B+ going away and burning up resistors. SO at the moment we don't care what the amp sounds like or if it hums. Your meter cannot test the high voltage response of the cap. So simply lift the cap out of circuit. If it stops the resistor fire, then you know the cap is bad. The fault will be downrange from the last burning resistor. Downvoltage? Down-something. After, how's that.

If you remove that cap in a working amp, then you lose decoupling and the amp might sound funny, make parasitics, oscillate, hum. But so what? Right now we need to stop the fires.

So unsolder on end of the cap and lift it from the circuit. Makes a difference or it doesn't.

If only R63 is burning, then C33 is suspect. If C33 shorts, then no B+ gets past it to damage anything else.

If R63 and R62 are burning, then C32 is suspect. And if all three resistors are burning, then the last cap is suspect - C31 and the parallel C14.

[cjm808]

Okay-
Got new 22uf 450v caps in and replaced C31,32,33 in the circuit. Brought up on standby-ok. Turned standby off and R63 begins to heat up-R62 and R61 are not burning. Flipped circuit board and brought up mains to 80 volts with standby off and metered voltage-B+ shows up on one leg of R63 but drops drastically on other leg-doesn't seem to be flowing to ground at C33-metered the leg there. Went and physically removed C33 to make sure-R63 is stil getting hot (it is a 4.7k 2 watt-checked again to make sure!). Any ideas.....
Thanks-Craig

[Twist]

Did R63 still heat after removing C33? I would remove R63 and take a resistance reading on the secondary side to ground. The driver tube uses the B+ there but with the resistance of the plate resistors you shouldn't be drawing a lot of current.

[cjm808]

Just jumped back on this critter after pushing it aside to clear out paying customer projects and generally clear my workload. Pulled all three caps (C31,32,33) and pulled all tubes-lit up to 90 volts line or so and since no current draw yet threw standby off and waited to meter R61-63 when all of a sudden I got little 'firecreacker' activity by the line of resistors R37-42. Shut unit down and checked these out-looks like arcing from R42 to somewhere and under all resistors there is this brownish crud. Made assumption that area got wet once upon a time and proceeded to lift and test all resistors and clean the board under these guys. Replaced R42 and fired up again-off standby and 5 seconds later another resistor starts arcing (the number at this point is irrelevent). Shut unit down again and now looking with both eyes open (am oriental so eyes tend to be a little 'narrow':>) notice that the arcing resistor is laying across a board trace that is not the most healthy specimen (the other row was also across a trace) in that I could see traces of copper exposed-duhhhh. Now again with a new outlook on life (or at least the amp) I notice that there are many corroded joints and traces probably due to the wonderful 80% humidity we live in out here in Hawaii. The point of all this now is does anyone have a line on getting a new amp board or an idea what one may cost-am weighing out the 'worthwileness' of getting this head back and running or parting it out (any takers?) or just shooting myself for not looking closer initially and possibly saving some stress levels. Thanks for listening and all the advice!
Craig

[Twist]

If the amp is yours then salvage it for parts (knobs, pots, transformers and switches..etc). I always welcome parts amps in my shop. You could piece it out and sell it on Ebay but if you have a repair shop it's worth more to use the parts for other repairs. Most of my customers don't care if I use old parts if it saves them money on the repair. Unless it's a hobby, time is money in the amp repair business. Two hours is the most I will spend on a repair. I can't afford to spend hours or days on a repair.

[cjm808]

Thanks Twist for the reality check-since the amp is one of mine and I am new at doing this I looked at it as a personal project to try and revive it-I'll still keep an open eye for a replacement board assembly 'just because'. You and Enzo have given me at least a glimmer of hope and also looking over hundereds of other posts/replies I have gained more insight on what to look for and as I always say-learn something new every day!
Thanks for the help here-will continue to post as problems come up. Also where/how do you make contributions to keep the forum running-I looked and didn't see where to do so as I would like to at least kick something in.
Thanks