Peavey Backstage 30 broken.

[served]

Hi.

I have a Peavey Backstage 30.
It has problem. Its too quiet for 30W amp.
I know that poweramp section is working, so lets leave it out.
I think there is something wrong with Q5 but what do you think?

You can see the chematic here.
http://www.instrukcia.ru/images/sm_peavey_backstage-30.jpg
(only schematic I found from google)

I also have a original pdf schematic for clear view. http://www.upload.ee/files/3092481/Backstage-30.pdf.html (you will have to download it)

I probed the preamp (feed the input with signa) and found that everything is ok until C12 (Q5 side).
If I touch the ground with my probe and connect it between C12 and Treble the amp starts to work. If I probe the Drain of Q4 and then touch again between C12 and Treble it will go quiet. Signal is okay all the time on the Q4 Drain.
Is it a bad transistor?

Just think with me theoretically. I can see how voltage varies on Q5 Drain and Source, so it opens if I ground touch the Gate and it closes after I touch it with loaded probe.
Its a really messy design on a PCB so some thinking has to be done before starting to change stuff.

[R.G.]

I think there is something wrong with Q5 but what do you think?

I probed the preamp (feed the input with signa) and found that everything is ok until C12 (Q5 side).
If I touch the ground with my probe and connect it between C12 and Treble the amp starts to work. If I probe the Drain of Q4 and then touch again between C12 and Treble it will go quiet. Signal is okay all the time on the Q4 Drain.
Is it a bad transistor?

Just think with me theoretically. I can see how voltage varies on Q5 Drain and Source, so it opens if I ground touch the Gate and it closes after I touch it with loaded probe.

First off, thank you and congratulations on your approach to the problem. Having the schematic available really helps with thinking about it. And you have good intuitions to think about the problem first.

I do not think it's a bad transistor. It might be, but that would not be my first guess. You say that signal is good at Q4 drain all the time. Then if you temporarily ground C12/treble point with a "ground loaded" probe tip, the amp works right, but goes away on the second touch. To me, that indicates that something about the voltage on the C12/treble point is making signal start/stop. If Q5 ever passes signal correctly, then it is probably OK. I think you have a bad connection between some component lead and ground somewhere between Q4 drain and Q5 gate.

You could test this theory by measuring the DC voltage on Q5 drain, and then doing the probe-touch turn on/off process. If that voltage changes materially, then something you're doing on the gate side of Q5 is changing the drain bias.

If I were fixing, I would first examine the solder joints and leads of R11-R15 and C9-C12, including any wire leads to the controls, and especially the connection between the treble and bass pots and R15. If the ground path through R15 was interrupted, it might cause exactly what you describe. If I did not see any problem, I would still re-melt and make sure of all the solder joints for those components. This may fix it entirely.

If it does not, reply here with the DC voltages on Q4 and Q5 pins for both cases of probe-touch to make it work and not work.

[served]

Hi.
Thank you for your thoughts.

Good thing is that I can recall this situation as much as I like. If I connect GND-loaded-probe (the cap loads its leg with gnd) then the amp starts to work and it works until 1) I turn the amp off 2) I connect a Vcc loaded probe. So it could be that some caps are leaking. I will go and take a new fresh look at the amp tomorrow!

[blue_tokai]

Not sure if this would apply to the 'Backstage 30', but with my 'Bandit 112' the problem was similar. What I ended up finding was oxidation on the jacks. I used contact cleaner on all the jacks, inserting a 1/4" plug into each several times after cleaning. I also contact cleaned the RCA jacks on the reverb tank. After researching alot, I discovered this was a common issue.
I took the time to spray clean all the pots too



**** Make sure you do this with the power cord unplugged!!!! *****

Good luck,

Russ

[R.G.]

Good thing is that I can recall this situation as much as I like. If I connect GND-loaded-probe (the cap loads its leg with gnd) then the amp starts to work and it works until 1) I turn the amp off 2) I connect a Vcc loaded probe. So it could be that some caps are leaking.

If the caps were leaking, it would not stay one way or the other, but would always leak to the working or not working.

An open connection to ground is more likely to leave it one way or the other, I think.

[PRR]

Clean or bypass the PreAmp Out jack.

If that's a cure, clean it. Not just the part the 1/4" plug touches: there's a switch finger under the tip-finger. That contact is hard to see. Or clean. Use strong coarse paper, with contact-cleaner if you have it, and work it through the contacts. You might also try to tension the switch finger to press harder on the tip-finger (be sure you still get good contact on a plug tip).

[served]

Hi.

As I mentioned before, signal gets stuck to the EQ and will not pass Q5.
Its the same way wheter I connect the pre amp directly to another amp or use its internal poweramp.
I also disconnected and hardwired the pre-output jack principle just to be sure. Problem stays. Going to loog for a bad solder joint or bad caps. This old school PCB is really something I am not used to work with.

[served]

Hi.

Problem solved. Bass-potentiometer didn't have a connection. So there was no DC connection between Gate and ground.