Tascam M-308 Mixer with distortion/overload in buss section

[gbkd80]

I have an M308 that is getting a squelch/distortion at any volume only on BUSS cards 3/7 and 4/8 when going through the main output, monitor, headphones, etc... this also affects the tape return path that resides on those same cards. All the actual channels 1-8 function fine on their own. This issue is presented when I assign a channel to the buss, use the sub-in PGM of that channel, or use the tape return. HOWEVER. If I go directly out from the affected PGM buss, the signal is clean. I've recapped the Monitor PCB as a preventative measure and a hope that it might help solve the problem but it did not.

My main issue is trying to read that schematic and figure out what to measure or scope. I'm technical but an idiot when it comes to using a scope. Everything I probe seems to show the same results. Not sure how to figure this out. I'm not sure this will be seen but my main reason for asking is if anyone has ever seen or experienced this type of problem before.

 

[SkywaveTDR]

Since the problem seems to be in the Buss section then I would put a 1KHz sine wave into the buss ins and then go looking at where the signal gets distorted. A 1 KHz sine wave should be able to show the distortion pretty fast. I suspect a power supply section or a single IC chip that might handle the busses you are having trouble from. i have worked on a lot of the Teac mixers and have always been successful in the repair- I guess that is why they made me Lead Technician when I was there. Regardless, signal tracing with a scope should be easy- just stay focused and start at either input or output and trace back or forward. Also know that distortion can be cause by the wrong things plugged into the output thus providing an improper load but at this stage anything is possible including a bad Electrolytic cap.