starcitysongs

I have a friend who brought me two different Marshall heads with the same problem. I'm the resident tinkerer in this neck of the woods and have built a couple amps from kits, done some repairs and the such but have not had much experience with circuit board based amps. The two heads are a TSL 100 dual reverb and a JCM 900 50 watt. Both play fine for a few minutes but then pop or crackle and drop down to maybe 1/8th volume if that much. In other words practically inaudible. I checked the power tubes first and replaced them with known good tubes I had at my shop and the same thing happened. I ran a line out of the effects loop to another power amp and had no problems. Even when the problem was happening the send was giving proper signal so the problem is isolated to the power section. I also read in another post that the 16 ohm jack is notorious for burning out and causing problems. I checked them both and saw no signs of damage but went ahead and replaced one of them just in case it was hidden damage but that did not change anything. I checked the boards and saw no visual signs of arcing or burning and all the components around the power tubes test accurately with what is on the schematic. Any thoughts, ideas?
-thanks
Aaron

5er driver

Aaron, I had that symptom on my SL-X. The cause was a dirty effect return jack. Switching jacks seem to become problematic, they get dirty and/or malfunction. I cleaned all the jacks on the front and the back panels, the speaker outputs as well. I douched each one with a good shot of pot cleaner/lube and then rammed a guitar plug in & out about 20 times. Cured my problem, I hope it's a simple solution for your situation.

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The 5er Driver

starcitysongs

Thanks for the suggestion. I gave that a shot today and it didn't remedy the problem. What are the possibilities of this being a bad OT in both amps? Is there a safe way to test an OT? Also anyone have any other suggestions?

PaoloJM

You can check for DC resistance when it is powered down/
You should read 40-100ohm on the primary winding,
.5 - 1.5 on the secondary,
and at least 10k ohm between the primary windings and both the secondary and the chassis.
This is just an indicator, it could pass these tests and still be faulty.
If you have either a variac of signal generator you could also (when powered down) inject about .5Vac signal into the secondary and check the voltage on the primary. Anything much outside the turns ratio you would expect or no signal will indicate a problem.

I think you should start checking the preamp tubes with known good ones. At the same time check their plate resistors for signs of damage and that they are reading correct values. If that fails to solve the problem it's time to start checking DC resistances on the pre-amp stage coupling caps. The problem does not appear to be with the power section from what you've said so it's not likley but double check the filter caps anyway for any signs of bubling and DC resistance to ground.

It's unlikley that the OT would be gone in both amps as it is rare enough to go in one amp assuming the correct HT fuse is installed. Also when they go, they go. They wouldn't normally work OK for a while and then go. I found a 5 amp in a amp recently and that was the first time I've seen a failed OT.

RogerAF

I suggest checking connections around the power tubes and the output stage. If the effects send continues to work, then your preamp stage is likely OK. Look for cold solder joints. Run a signal through the amp and wiggle wires etc. Also check the tube sockets, sometimes they have internal fractures that aren't apparent. It could be something that only shows up once a certain temperature is reached. The way you describe it as working and then going down after a period of time makes it sound like it could be happening once the amp warms up. You could try spraying Freeze-It on various components and seeing what happens.
Good Luck & Good Hunting.

PaoloJM

You're spot on, I mis-read it that he had checked the power section with a different pre-amp, my bad.