Sherwood S-7050 bad thermistors causing buzz?

[rbatterton]

I just inherited an S-7050 and I'm getting a buzz from the speakers, irrespective of the source plugged in or the speaker terminal used. In all the troubleshooting I've researched it sounds like an issue with a capacitor or resister since it's systemic across the device and the buzz doesn't increase/decrease when the volume knob is changed.

There doesn't appear to be any cap or resistor that's leaking/swollen, etc. However, on the riser I found two thermistors that were laying against the metal framing, and they've both created a rusty looking outline, leading me to believe they are the culprit. Pictures are below but I don't have a schematic of the S-7050. From an S-7000 schematic they're labeled as "Thermistor D-22A". On the S-7000 schematic and my S-7050 board, they're both labeled TH601a, TH601b. Both termistors are jet black with no discernable writing.

Could these two be responsible for the constant buzzing or should I be looking elsewhere? Thanks.

 

[cademan]

Thermistors are for limiting current and are not going to be the cause of your hum. Bad filter capacitors almost always contribute to or are the sole cause of hum aside from ground loops.

Just because a capacitor doesn't look swollen or leaky doesn't mean its good. They get electrically leaky not leaky with physical fluid. Venting fluid is rare but does happen if the capacitor is shorted or near shorted.

Most if not all hum that is volume independent is caused by bad power supply capacitors. Get your trusty meter out and do some basic troubleshooting but don't guess.

 

[OMGCat!]

I agree with cademan. I've only seen a handful of physically deformed/leaking capacitors (aside from old, old paper/wax which is a different story) but many that tested bad electrically.
If it's a 120 cycle hum then it's almost definitely your filters. If you have an oscilloscope you can put it on the B+/B- and see how much ripple is present. I'd bet it is excessive.