Peavey Backstage 30 - Loud Constant Hum

[Jrm]

Thanks again, Bill. Here are the newest readings:

Q9 3624 (out of circuit w/diode setting on meter):
Positive base, negative emitter: .000v
Negative base, positive collecter: .001v
Negative base, positive emitter: .000v
Positive emitter, negative collector: .001v
Negative emitter, positive collector: .001v

With Q7 and Q9 out of circuit, here are the newest readings on Q10, Q11, Q13, and Q14. I determined their position on the board by their common collector connection to the collector of Q12, which also helped me determine the pinout of the transistors as 1B 2C 3E. Is that correct? Also, I did NOT lift any legs from the transistors before taking these measurements, as your edit suggested:

Q10:
Positive base, negative emitter: .860v (climbed at a trickle pace and this is where it really started to slow down)
Negative base, positive collector: .505v
Negative base, positive emitter: .517v
Positive emitter, negative collector: .890v (also climbed at a trickle pace)
Negative emitter, positive collector: .908v

Q11:
Positive base, negative emitter: .939v
Negative base, positive collector: .505v
Negative base, positive emitter: .517v
Positive emitter, negative collector: .890v (trickle)
Negative emitter, positive collector: .909v

Q13:
Positive base, negative emitter: .487v
Negative base, positive collector: OL
Negative base, positive emitter: .998v
Positive emitter, negative collector: .835v
Negative emitter, positive collector: .836v (trickle)

Q14:
Positive base, negative emitter: .486v
Negative base, positive collector: OL
Negative base, positive emitter: .997v
Positive emitter, negative collector: .834v
Negative emitter, positive collector: .835v (trickle)

I've included a photo of the bottom of Q7, but you said it's fine, correct?

Thanks again. Glad we're able to make some progress.

[52 Bill]

Well, Q9 is toast and needs to be replaced. It seems that all 4 outputs are okay, but you are missing a few readings. There should be 6 sets of readings for all of your tests, you are missing positive to base and neg to collector on most of them.

For this set of tests, I don't think that it will matter, as the shorted tests all have gone away with the removal of Q9. You didn't post a retest of Q8 C-E.

Time to find a replacement for Q9.

As for Q7, I don't know what that is on the bottom of the case. It does look like smoke residue, as it looks like it is on the leads as well. I would try and clean it off with a little alcohol on a cotton swab. I don't think that it is original to the amp.

[Jrm]

Sorry for the readings I missed. Things got a little bleary there for a minute, but here they are:

Q8: negative C, positive E: .693v
positive C, negative E: OL

Q10: positive B, negative C: OL
Q11: positive B, negative C: OL
Q13: positive B, negative C: .480v
Q14: positive B, negative C: .480v

Let me know if this fills out the missing data and I'll start looking for a replacement for Q9.
Anything else I can do at this point?

Thank you again, so very much.

[52 Bill]

Those figures are great. What you are calling Q13 and Q14 are actually Q10 and Q11 and vice versa.

I will suggest that you use the 2N3053 as the replacement for Q9 and that you go to your local Radio Shack and pick up a new 2N3904 to replace Q7. The 3053 can dissipate more power, so it would be a better choice for the Q9 driver position.

Edit: Hold everything. I'm confused about the transistor numbers and I think that you are too. Please list all of the transistors in the power amp by the number that is printed on them.

I know that the 2N3053 is an NPN and will work for the Q9 position, but I just want to be certain that you're really talking about Q9.

[Jrm]

Ok. I wasn't completely confident in identifying those, so thank you for that. And I actually have a 2N3904 in my NPN bin, so I'll get right on this. Any other tips before I get down to business? I love that the 2N3053 can be repurposed here. I'll wait for the go-ahead from you before plugging these transistors into the board.

I guess what I'm asking is if I can just plug these in without worrying about conflicting pinouts. Both transistors are EBC, with the 3053 being a different package that goes EBC counter-clockwise from the tab, viewing from the top, correct?

[g1]

As for Q7, I don't know what that is on the bottom of the case. It does look like smoke residue, as it looks like it is on the leads as well.

I wouldn't worry too much, if it were burnt, the blue bits wouldn't be blue anymore.

[Jrm]

Bill, here are the transistors on the board, listed by their Q#. Some differ from what's on the schematic:
Q7: 2N3053 on the board, 3904 on the schematic
Q8: 690 on the board and schematic
Q9: 3642 on the board and schematic
Q10/11: RCA1C12 on the board, 430 on the schematic (going by your correction that I misidentified them)
Q13/14: RCA1C13 on the board, 431 on the schematic (ditto)
Q12: 3644 on the board, 3638 on the schematic

[52 Bill]

Bill, here are the transistors on the board, listed by their Q#. Some differ from what's on the schematic:
Q7: 2N3053 on the board, 3904 on the schematic
Q8: 690 on the board and schematic
Q9: 3642 on the board and schematic
Q10/11: RCA1C12 on the board, 430 on the schematic (going by your correction that I misidentified them)
Q13/14: RCA1C13 on the board, 431 on the schematic (ditto)
Q12: 3644 on the board, 3638 on the schematic

I think the only transistors that were mis-identified were the 4 outputs.

I think it will be safe to use the 2N3053 in place of the shorted Q9 and use a 2N3904 for Q7. The 2N3904 with the flat side facing you will be EBC. The 2N3053 is a TO-5 case and will have the emitter closest to the metal tab and the base in the center.

So install the two transistors and see what happens.

[Jrm]

52 Bill, there is a special place in heaven for you. It worked! I simply can not thank you enough or tell you how much it means to me to have had the guidance of you and other contributing members to this thread. Fixing this amplifier has been a bright spot in kind of a tough time for me right now, and these small victories go such a very long way. You have my most sincere and enthusiastic gratitude for taking the time to help me with this project. The amplifier belongs to a very dear friend who was about to throw it away and I knew that it could be salvaged. Now to give the case a good scrubbing and give it some DeoxIT G5 love.

Seriously, this makes my whole week. I can finally take off the magnifying goggles, turn off the soldering iron, stop staring at the schematic on my 40" screen, and have a DRINK!
I toast you all, but especially you, 52 Bill. If you're ever in the Austin area, I will absolutely buy you a beer or some BBQ. Or both.

A huge weight has been lifted. Thank you, thank you, thank you.

[TomCarlos]

Congrats Jrm and nicely done Bill !!!

You mentioned DeoxIT G5 ... you might consider picking up a can of DeoxIT F5. That is what you use to clean the pots. DeoxIT D5 is what you can use to clean your input jacks and anything with a contact.

[Jrm]

Noted. Thanks TomCarlos.
Something worth mentioning as I piece this thing back together after a proper cleaning; since I replaced all the axial electrolytics with radials, should I support them with a little hot glue to relieve stress?

[52 Bill]

Noted. Thanks TomCarlos.
Something worth mentioning as I piece this thing back together after a proper cleaning; since I replaced all the axial electrolytics with radials, should I support them with a little hot glue to relieve stress?

Congrats on getting it working again! It's been a long time since I visited Texas, but if I'm ever back there, I'll look you up.

A small dab of hot melt or silicon sealer will help to keep the radial caps from working loose from vibrations.