Boy, Tautech, you are 2 for 2 at guessing failed parts. I get the feeling you have done this a few times
No guess, Q6 is a linear regulator pass transistor and it was not doing it's job. It could be either conducting or open, in this case failed short. Remember the likes of pass transistors are doing some work, much like output stage transistors and they ocasionally fail, but why? Overcurrent?
Ok. I understand the basics of that. Not sure why the resulting voltage was more like +24V. I guess it was just taking the input voltage.
I assume it was over current that caused the failure. I think C7 may have been bad. I should have taken notes. I had replaced it.
So Q6 is bad... Measures as a 63 ohm resistor on the Transistor Tester, and the Micronta will not even read an hFE.
Q6 is a 2N3906.
I don't have any of those on hand, though I do have some on order (slow ship though). I do have a 2N2907 though. Do you think this would be a suitable replacement? Or should I really wait for the right part, which is coming from China?
A 2N2907 should do to get her running although there's enough differences in datasheet specs to not leave it there. Watch the pinouts, they are very different.
I installed the 2N907 (which had a measured beta of ~234 even though AllTransistors said it was 35 minimum). And now everything is working!
So Paul, keep that soldering iron cold for the moment.
I am still powering it off my bench supply hooked to the battery terminals. Doing this just to watch current flow, and not waste fuses. It is drawing about 110 mA, if I recall.
Voltage measurements to follow, but everything is within spec! None of the transistors are warm, either.
Thank you guys for all your help. Paul, I may consider some of your parts as spares if you are game.
Pics to follow too. I think I shall wait for the proper transistor to really calibrate this guy, but it is pretty close as is.