You apparently only looked at the upper 1/2 of IC-1.
. . .
Where does the -15v come from?
Thank you, Crutschow!
You're absolutely right. I did initially overlook the 2nd half of IC-1, but being kind of a newbie I still didn't understand why it blew the transistor. In analyzing it after, I didn't see it as part of the feedback loop. I'm learning a lot by trying to fix this amp. I put in a sacrificial output transistor and it blew right away on re-power-up, as I half expected it might. I had tested all the nearby diodes and transistors and had found no shorts. But the bias transistor looked a bit funky testing it in circuit, so I will pull that later and test it out of circuit.
Interestingly, I found Q15 with two cracked socket pins while looking things over. The socket had been loose, I noticed, in earlier tests, but I wonder if the over-voltage finally cracked those pins or if it's just coincidence. Oddly, the transistor itself reads fine -- hfe is within range, etc.
As for the -15vdc supply -- it's generated in the Q11 circuit with its associated Zener. I can "trick" the voltage up to -15 with three 1n400x diodes in series with the Zener and the amp behaves much better, Q11 and its power resistor run much cooler. Nice to know I can do that, but my main reason for working on this old amp it to learn electronics. So I removed the 3 1n400x diodes I had put there and went searching for the real problem a few days ago. That was when I committed the folly of removing IC and now causing problems on my PA board. I think I'll have the PA board back to where it was soon, but I'm still curious about what might be dragging down the -15vdc supply. Disconnecting the preamp board has no effect, so it's nothing there. That's why my eye landed on IC-1 (well, HALF of it!)
I have a LM1458 I can put it its place once I get the PA board back to where it was. Then I can test that -15 supply again and see if it's where it's supposed to be. I also wonder if the cracked socket of Q15 was causing it to read low?
But first I have to trace out why Q19 keeps blowing. I'll update as I go along. In the meantime I'm open to all thoughts/ideas/suggestions. I'm trying to learn!
Peace,
Bud