Heathkit TA-17: power-stage resistor keeps blowing

[Mark Hammer]

The 330 ohm resistor labelled R313 in the upper right hand corner here: http://harmony.demont.net/documents/...hkit_TA_17.gif keeps blowing.

Any ideas what might be the source of that?

I picked up this amplifier last autumn and haven't really had a chance to use it. That resistor was blown when I got it, so I replaced it and power up, and the same resistor blew again. I did replace several of the largest electrolytic caps with newer units having a suitable (and safer) voltage rating, but that didn't help.

[Jazz P Bass]

My first suspect would be Q12.
Collector short to ground.

[Mark Hammer]

Thanks. I'll check that out.

[g1]

The most likely candidate is as JazzP mentioned, Q12.
Should it turn out to be ok, it looks like the double diode (D601,D602) is mounted off the board somewhere, probably on the heatsink (points G & H). If any of the leads short to the heatsink it could fry R313.

[km6xz]

Which of the two resistors R312 or R317 looks like it is overheated. If R312, look at Q12 or if R317 looks more heat discolored then look at Q14 for a short.

[J M Fahey]

Measure and post voltages, otherwise it's just guessing.