Trouble diagnosing bad power transistor GFX212, please advise

[jimbrady57]

P.S. I just checked the voltage on Q9, from base(n) to both emitter(p) and collector(p) I got 4vdc, it would start at like 4.9 but then go down to 4 over about 20 sec. I also got .02vdc between emitter and collector. no a/c.

[1ampman]

It seems you need the equipment to follow the signal thru the amp. Osilloscope & signal generator or the repair is more difficult than expected... Borrow some equipment & study the schematic..... I think you've done everything your capable of.....

[nickb]

P.S. I just checked the voltage on Q9, from base(n) to both emitter(p) and collector(p) I got 4vdc, it would start at like 4.9 but then go down to 4 over about 20 sec. I also got .02vdc between emitter and collector. no a/c.

Q9 is a JFET and its purpose to reduce a thump when turning on. The gate to source voltage should not be varying like that and in any case should be nearer 9V. My first suspect would be that C28 is leaky. You can remove it as a test (check for 9V) and then replace it in due course.

If that doesn't help remove Q9 and test the gate/source voltage.

The drain to source DCV is correct, close to zero.

[jimbrady57]

Okay guys, I'm still with it. Just might be working a lot. Believe me, my attempts to be stingy and do this myself (replying heavily on your help, lol) will not be easily defeated.

1ampman - what really sucks is I used to have an awesome oscilloscope, but I threw it out because I never used it.

[jimbrady57]

Possible accidental Breakthrough!

I was farting around checking Q9 and C28 again,

https://drive.google.com/open?id=0B9...nZqaXY2MlM5cDA

when suddenly I saw/heard a loud electrical ark out of the corner of my eye, by instinct I felt like i saw it come from R89 (the right one of the three), but I felt all three of them, and R90 (the left one with the screwdriver on it) was very hot. Both R88 and R89 were warm but when I tuned the amp off they quickly cooled down and R90 was still hot.

https://drive.google.com/open?id=0B9...FFKMHc4VERlSUU

So do you guys think it's safe to assume that is my problem, I do.

P.S. I'm quite sure that I did not cause the short as I was being very careful and just testing for voltage. I think that it arced because whatever signal was making it hum had no where to go and I had the volume turn all the way up. (just philosophizing, no idea).

[The Dude]

A resistor gets hot because something is drawing too much current through it. Your problem isn't the resistor. It's something that the resistor supplies. Often those larger power resistors do run warm. If you saw an arc, also check the solder connections in that area.

[jimbrady57]

((Solved))

It was definitely the resistors solder connections (R90).

I've been going so slow because I've been working 84 hour weeks for the past 2.5 months and it's been a 2-2.5 hour drive each way to work. Either way, at one point about a month ago I finally broke down and brought the amp to a repair shop. The guy said it was cold solder connections and charged me $90. I just played it a minute ago and it worked fine for about 3 minutes then the problem stared again. (cutting out, no volume).

I was just about to bring it back and complain but decided to check around for loose solder connections myself, and wouldn't you know, it plays fine when I wiggle that resistor. I'm gonna re-solder it sometime, but the lesson I learned is to check for poorly soldered components via a wiggle test sooner rather than later next time. Thank you all for the help though. Just goes to show that while I've been pulling my hair out this whole time it was just something stupid that I didn't think of testing for.

P.S. I just noticed that you told me to look for that (The Dude), so another problem that compounded this was my apparent inability to read.