JCM 2000 TSL 602 What to replace first????

[brydon2]

This amp has a problem with a switching component apparently, the clean signal is fine but the crunch and lead are not reaching the output.

I tested all tube voltages and they are good, the relays also tested good.

I figure just start replacing silicon based on order of failure rate, I can not trace the signal in this amp, I figure there is 30 or so jfets and etc that are cheap and I could try.

I would like to know what parts fail most often?

[Enzo]

The first thing to replace is the notion that all problems are parts needing replacement. The second thing to replace is the idea that things fail according to lists.

How did you test the relays? The fact they click on and off tells us nothing about the contacts inside. You would have to take resistance readings across each set of contacts in both on and off conditions. A bad relay contact can certainly open your signal path.

And by all tube voltages, may I assume that means none of the cathodes are at zero volts.

Isolate the problem.

APply a signal to the input, set the amp up for the bad channels to play. Trace the signal through the amp. FInd out where it is and is not. Use a scope, or lacking that, an AC voltmeter. Why can't you trace the signal?

Easter egg hunts are a terrible way to fix something. WHat if you replace ALL 30 of those JFETS you mention and it still doesn;t work because it was cracked solder on a cap? And a cap or resistor could be open. I don;t think silicon first in a symptom like this. Oh it could well be a bad JFET, but that wouldn;t be the most common failure, in my experience.

[Enzo]

And can we please not start two threads for the same discussion?

[brydon2]

misc.

Enzo, I understand your frustrations with my shortcutting, Its only out of

desperation.My electrical knowledge is somewhat limited and this circuit is way

out of my league.This is a friends amp and the best tech in town looked at it and was unable to fix it.I thought "maybe" with the help of some knowledgable forum guys we would have a shot.

Anyway, should I be able to "see" AC signal with a $20.00 meter with 200v and 600v settings, I wasn't aware that I could but I will try.

I thought the "other" thread was exhausted as I did not hear anything new for a few days and I wanted to head in a new direction......Regards!

[brydon2]

AC signal

Enzo, your right, I can "see" ac current with my meter, I cannot see any from R100 through to the output using the crunch or lead channel.

OCD was kind enough to trace the circuit from R100 to output but it looks as if the problem is prior to R100, some progress anyway.I can see the clean signal with the meter though....Thanks!

[Enzo]

Brydon, I am not frustrated. I am not even upset. I just want to make an effort to keep things efficient. CAn you imagine calling a music store to see if they had 6L6 tubes in stock and they told you the answer, and then you drove down there and asked them in person too?

[brydon2]

My apologies for being hardheaded and impatient, the good news is I think I am now making progress.

At this point I have all 3 signals at R87/C100, I'll start again in the a.m.