TSL100 signal passing at zero volume and gain

[Sowhat]

Got a TSL100 in that I worked on about 3 years ago. It originally had the dreaded bias drift problem and I fixed it up by hacking the main board apart and relocating the bias components on a separate custom etched high quality G10 board. Since then the amp has served it's owner well for quite a number of years and he uses it constantly in live performances and recording... he just loves the thing... crazy eh? well.. each to their own.

He brought it back to my shop earlier today with a strange complaint which was: The crunch and lead channels sound a bit subdued and he can't shut the reverb off on either of them but the clean channel works O.K. He said he had a little mind lapse and put a 16 ohm load on a 4 ohm setting and felt that he damaged the amp that way. I assured him that he probably did not hurt the power section with that little mistake and anyway this channel dependent problem has nothing to do with the output section.

I put it on the amp bench, hooked it up and did a little examination of the power section and everything is fine there as I expected. I started looking for his problem and I found one but not what he stated. With the volumes and gains all at zero and a test signal injected into the input I get a little output from the clean channel... I don't think that's right. When I switch to the crunch channel I get no signal as I would expect but when I switch to the lead channel I get a considerable amount of output. At this point I stopped my preliminary examination (because it's getting late) and figured I would throw this out to the brain trust and see if anyone has experienced this behavior and has any ideas or theories. I have not got around to checking this reverb problem because this initial finding needs to be sorted out first and is most probably directly related to it.

[g1]

Depending on whether the pots go completely to zero ohms can make a difference for signal bleeding through with volumes down.
Some of the switching on these is done with the 5201 switching op amps. That's usually the first place I look for switching problems in these amps. The M5201's are no longer available but modern subs are NJM2120D or JRC2120D (or 2121).

[gbono]

Start with the simple stuff. I worked on one of these recently with a similar issue and found multiple bad solder joints on the ground lugs W1,W14, etc. and else where.

[Sowhat]

Well folks... it all boiled down to a broken wire in the PEDL00021 footswitch cable. That thing is 25' (8 meters) long and there was a break in the yellow wire somewhere along it's length. The customer was fine with a shorter cable so I cut it in half and used the good half to rebuild the assembly. Still don't know exactly where the wire was broken but it's somewhere in the discarded 12 foot (4 meter) piece. After examining the switching circuit on that amplifier a small insignificant problem like that can manifest in the whackiest symptoms, many of which don't even seem related... that thing is too complicated for it's own good.