marshall jcm2000 tsl122 problem-o

[acorkos]

hello....

this amp has low, distorted output (even on the clean channel), and when you hit the guitar strings hard, there is some compressing and cutting out of the sound. i replaced all tubes with known good tubes...no change. i checked all the resistors on the power tube sockets, and the plate resistors on the preamp tubes and PI...all seemingly check out with the correct values. nothing looks burned. i resoldered all the power tube socket connections and most of the other joints in the neighborhood, and cleaned and tightened all the tube sockets. again no change.

Enzo recommended to me that i clip the 22pF cap across pins 3 and 4 of the end power tube socket...again no change. any hints on where i should look next?

oh, yeah...one thing that may be of significance...the bias adjust pots on the back of the chassis seem to be acting funny. i don't know exactly how they function, but when i'm measuring/adjusting the voltage on the left side, i can turn the right side pot and it'll effect the readings on the left side, and vice-versa. is this normal?

thanks for your help,
aris

ps: i've never seen 3 stacked PCBs before

[Enzo]

Yes it is normal for the left side bias control to work the right side tubes. I have always wondered why the two controls were "backwards," since the littel control board is at the end of a cable.

Try this. Plug the guitar into the effects return. Does it still sound crummy? This tests the power amp wuthout the preamp. Likewise, while you play the amp going in the input jack on friont, run a cord from the effects send to another amp and see what it sounds like over there. This tests the signal leaving the preamp. Hopefully we then find which half of the amp is bad.

[acorkos]

Plug the guitar into the effects return. Does it still sound crummy? This tests the power amp wuthout the preamp. Likewise, while you play the amp going in the input jack on friont, run a cord from the effects send to another amp and see what it sounds like over there. This tests the signal leaving the preamp. Hopefully we then find which half of the amp is bad.

plugging into effects return yields same crummy sound. output muted, effects send into second amp...normal sound, no cutting out.

something is up in the power amp. some ideas of what to check?

thanks

[Amp Kat]

Even though the pots are kinda screwed up they still bias each side accordingly so revert back to the correct settings or bias it up correctly and make sure your not getting crossover distortion. A scope would be a pretty good tool here to check where all hell is happening and heres something that happens with these later Marshalls and is a widespread problem with JCM-900'S and JTM series is the heaters come off the power tubes then go thru this bridge rectifier and go DC to the preamp tubes. This rectifier circuit is not designed properly and takes on way to much heat and you can usually see burn marks are a dark spot where that rectumfier sits. Sometimes it will be flakey and changing it with a higher rated recto solves the problem. All of the grounds are tied into this point so all kinda freaky stuff happens. You could also have a bad filter cap messing with the plate voltage as I've seen that too on theese amps.

[Andy]

it's also possible that your screen grids are not getting any voltage or low voltage. with normal bias on common output tubes, this could effectively make it a class-C amp, and would require a very strong input signal to the power tubes to get any output at all, thus causing the cutting out, and the output would be very weak, because low screen voltage results in low power output. not saying this is the case, but just another thing to look at.

[JerryP]

I've replaced a few output transformers in the DSL/TSL Marshalls. Low, distorted output is one symptom of a bad OT. Those amps run the ground side of the 4/8 ohms speaker jacks thru the switch contacts of the 16 ohm jack. I've seen several amps with melted 16 ohm jacks and bad OT's.
Jerry

[acorkos]

I've replaced a few output transformers in the DSL/TSL Marshalls. Low, distorted output is one symptom of a bad OT.

where can i find a replacemet OT? thanks...

[JerryP]

Marshallparts.com

Mercury Magnetics sells upgarded transformers

Antique Electronics might have one.

Jerry

[Mark Black]

For whatever its worth I have had some luck fixing certain Marshall OT's (the lay-down type with solder-terminals, not sure if that's what is in your amp). There is a place under the tape near the terminals, on the terminal side, where the very fine primary wires cross over each other with only one layer of tape between as they cross the windings. The ones I have fixed had arced through the tape and I was able to (VERY carefully) separate the wires & re-insulate that area. It's really easy to cut or break the wires though so think "surgeon" if attempting this kind of fix.

Good Luck!

[MWAMP]

hello....

this amp has low, distorted output (even on the clean channel), and when you hit the guitar strings hard, there is some compressing and cutting out of the sound. i replaced all tubes with known good tubes...no change. i checked all the resistors on the power tube sockets, and the plate resistors on the preamp tubes and PI...all seemingly check out with the correct values. nothing looks burned. i resoldered all the power tube socket connections and most of the other joints in the neighborhood, and cleaned and tightened all the tube sockets. again no change.

Enzo recommended to me that i clip the 22pF cap across pins 3 and 4 of the end power tube socket...again no change. any hints on where i should look next?

oh, yeah...one thing that may be of significance...the bias adjust pots on the back of the chassis seem to be acting funny. i don't know exactly how they function, but when i'm measuring/adjusting the voltage on the left side, i can turn the right side pot and it'll effect the readings on the left side, and vice-versa. is this normal?

thanks for your help,
aris

ps: i've never seen 3 stacked PCBs before

Re TSL122 problems and in particular the thermal runaway, I posted a diagnosis in the TSL122 section of JC Maillet's website. The 22pF capacitor is a red herring - if shorted it simply converts one half of the output stage to triode operation. The PC board material is rubbish and behaves like a NTC resistor.

Malcolm