(http://i284.photobucket.com/albums/ll33/bonosurf/vbm1_zps6cd81f13.jpg)
I need some extra eyes (brain cells would help too) looking at this problem.....
What "we" know - the amp will power on and not blow fuse(s) - you can chase a signal from Q1 (NPN) to Q2 (PNP) but there is no signal at the collector of Q2. When checking the supply voltages I get a +15/+9 and -15/-3.5 - checking the 9V zener diode (ZD5) and it is okay. When I lift R63 on the -9V supply I get -9V at ZD5 - someting is loading down the supply - but what? I checked Q2 through Q5 with a DMM in circuit and found no issues. D1 and D2 (are these bias diodes helping the BJT turn on??) also test good and the ESR on all the bypass caps (C8,12,20) all test out okay. Check the voltages on Q2 - Q5 but they have to be wrong since the -9V supply line is pulled down. I hate to start removing parts without a strategy.......
Just a quick thought... Perhaps you can test Q2-Q5 and even IC1 for any of those components heating up? R23 could be pulled up one side to help split up the circuit and might tell you better where to look for the fault. '
Edit: I don't think IC1 has anything to do with the problem.
You might be right about IC1, but it calls to me for some reason. Simple enough to see if it has an output stuck to a rail.
As far as I can see, -9v powers four transistors. And three caps to ground. So isolate the problem. You have 3v ther. So what is up on C12, and C8? If we find zero on one of them, that might explain the 3 on the zener. You did ESR check? In circuit? So power off and measure simple resistance at each cap, any measuer short to ground?
What if the IC slammed over to -15 on its outputs, that would turn on Q4,5. That could be involved, though those 4.7k collector loads ought to limit current to a milliamp or two. Eh, prolly not.
Divide and conquer. I might lift C8,12,20 just to see. I also might lift R23 and see if the supply comes back up, the load is before or after that resistor. Likewise R14, same test. If i got the numbers wrong, then the 220 ohm and 330 ohm resistors. Lifting those resistors breaks it down to sections, and ought to reveal if the trouble is in th Q2 area, the Q3 area, or the Q4,5 area. Once we know where it is, we can usually see why it is.
Forget testing part by part, divide it in blocks and test functionality.
This list implies something: if something checks,prioceed to next one:
1) do you hear anything (pop/click/hiss/hum) on the speaker at turn on/off?
2) check one terminal is grounded, Ext Sp jack cuts ground, might be bad/weak/dirty.
3) check speaker continuity
4) check LM1875 +/-V rails (expect something like +/-20V or thereabouts)
5) check touching its input it hums, when Volume pot is halfway (not on 5 but when wiper up and wiper down resistance is roughly the same)
After that test leave volume pot on 10 .
6) check that touching any end of R51 or R52 you hear hum
7) check that +/= 15V and +/-9V exist and reach their destinations.
8) touch IC2B input (pin 5) .. hum?
and so on, go backwards until the input.
Find where sound stops.
As you see, I haven't suspected (yet) of any individual component, let alone pull one for measurement.
ESR measurements? ... fwiw I don't even own one .
Good hunting :)
Quote from: gbono on September 02, 2014, 07:12:16 PM
When I lift R63 on the -9V supply I get -9V at ZD5 - someting is loading down the supply - but what?
Try the same check with R14 and R23, should help isolate the problem a bit more.
yes C12 was the issue - DMM better than ESR test :grr
(http://i175.photobucket.com/albums/w156/vingaard2/applause.gif)
Quote from: Roly on September 03, 2014, 06:17:13 AM
(http://i175.photobucket.com/albums/w156/vingaard2/applause.gif)
THX Roly I needed that.......... ;)
Quote from: Enzo on September 02, 2014, 10:48:05 PM
You might be right about IC1, but it calls to me for some reason. Simple enough to see if it has an output stuck to a rail.
What if the IC slammed over to -15 on its outputs, that would turn on Q4,5. That could be involved, though those 4.7k collector loads ought to limit current to a milliamp or two. Eh, prolly not.
Yeah point well taken there Enzo. I was thinking that since IC1 was powered on a 15v rails and not the 9v rails that it was less likely affect the 9v power. I see your explanation and I understand better. Always learning more as we go... thanks. :dbtu:
I've recently acquired one of these amps and I noticed that it does a strange, noise-gate-y sound as a note decays or you turn down the guitar's volume while on the high gain setting.
I'll do some measurements soon and report them here, because I don't think that's supposed to happen, and if I can find the problem it'll be helpful to other people who might encounter the same issue.