Pignose HummmmPOP

[Densman]

Hello! I'm trying to fix a friends amp. It's a Pignose 30/60. He bought it about 6 months ago and worked fine, and sounded great. Perfect for band practice. He went to turn it on a couple days ago and it made a loud noise for about 2 seconds and tripped the circuit breaker. It doesnt have a fuse. I can't find a schematic. It's not one of the battery powered amps, and I'm guessing its from the 70's. I'm not a total newb, I've built a few tube amps, and modded a few pedals, but I don't know much about SS stuff or how to troubleshoot. Does anyone have an idea what it could be?  Thanks,  Denny

[J M Fahey]

It always interested me but I never could find the schematic.
Post a couple pictures of its guts, maybe we can suggest something.
The power amp must be shorted, build a series lamp limiter to be able to measure something instead of tripping that breaker all the time.

[Densman]

I actually have one of those light bulb limiter things. I'll dig my camera out and try to post some pictures.

[Densman]

OK,here's a pic

[Densman]

I think that picture was too small. I'll try again.

[J M Fahey]

Good picture.
Start by plugging the amp into the lamp limited socket, inhook the speaker, turn it on.
Does the lamp
1) flash and then become orange, or
2) it stays brightly lit?
If 1), locate the PS filter capacitors, they must have 1 leg grounded and 1 which is either the + or - rail; what do you measure on the rails?
Hint: look at the rectifier bridge at left: the upper legs of both upper diodes must be +V, the + rail; the lower legs of the lower 2 should be the -V rail.
3) Measure the DC voltage on the wire/connector which goes to the + leg on the speaker, what do you find there? Less than 100mV is good; any voltage from 2 to +V or -V is not.
4) Take another picture from above, showing the output transistors and what you have behind the pots, which is now obscured by them.
Post the inscriptions on ICs and transistors.
Love those Motorola aluminum case 2N3055 and 2955.
5) If (2) we move to Plan B.
Anyway measure +V and -V . Finding a couple volts there is not the same as finding almost 0.
Good luck.

[Densman]

The bulb stays brightly lit.

[phatt]

While you have the glowing bulb,,,,
Measure the Voltage on the speaker terminals.

And Both voltage rails while you are there.
Your in good hands with Mr Fahey :tu:
Phil.

[Densman]

Speaker terminals:  -60mv  -20mv  Rail: +205mv  -3.7v

[J M Fahey]

OK, that's a *real* short you have there.
I've been looking at your picture, I see the output transistor collectors/cases are grounded, without micas or other insulators.
I've been searching around, and although these amps are quite obscure, they seem to have been designed by QSC, which love to ground the power amp "hot" rail, and take the audio signal from the supply capacitors center point, which is *not* grounded. Oh well.
In the long run it ends up being the same, but everything is upside down.
Back to your amp:
1) let's discard that it's a PSU problem.
2) check one by one the PSU diodes, meter on "diode check", black to band leg, red to the other leg, you should see something around 500 to 650, depends on your particular meter. Inverting probes should read either open or a much higher value, over 1000, usually around 1800 or so.
A reading close to "0" means a shorted diode, which just by itself would cause the symptoms shown.
To confirm unsolder and lift one leg and measure "in the air"
3) repeat measurements both ways across the big power capacitors.(which I don't see on that picture).
When red to positive leg, black to negative leg, you should see initially a low value which goes up, as the capacitor charges. Final value?
Close to 0 means a shorted capacitor, to confirm unsolder one leg so it's free (or just pull the capacitor from the board) and re-measure.
4) To measure the transistors, I'll need that picture showing the part of the board hidden by the pots.
I see 2 medium sized transistors (Q1 and Q2) which *might* be the drivers ... or not.
The same picture taken from below, showing the solder side will help.
Try to take both from about the same distance, just flipping the board, so it's easier to match the parts I see on top and the connections below.
As I see it, you have one or both output transistors shorted, which may have killed the drivers.
Worst case, replacing all 4 should be the ticket, plus maybe some other small parts, but I don't like to "shotgun" if possible.
Be careful when desoldering, pamper the PCB.
Good luck.

[Densman]

I'm back. I checked the PSU diodes and my meter reads .500 & if I switch probes 1.000 thats without pluging in the light bulb limiter. On the power caps with light bulb limiter plugged in I get 3.4v on one and less than a volt on the other. I also tried my ESR meter and it reads .1 ohms on both of them. I have more pictures too.

[Densman]

Pic

[Densman]

Pic

[J M Fahey]

Good! Excellent pics.
So that's where they hid the PSU caps!
Very clever packaging, loved the bent chassis lip which doubles as heat sink and PCB support.
The circuit looks *too* simple, has far less transistors than usual, I see only *one* DIP8 IC, probably a dual Op Amp , plus what looks like a DIP4 ¿optocoupler?
I only see those medium power transistors, labeled Q1 and Q2, please identify them, plus the output TO3 ones.
Is there no other transistor? Amazing!!
Please repost the first picture with a little text pasted over each IC and transistor, with what the PCB calls them (Q1 ... CI1 ... etc.) and post what's written on each of them.
Also post another, say 45º angle picture, which shows the tracks without being covered by the blue capacitors.
Besides that, diode test the power transistors, which very probably are shorted, and the drivers (Q1/2) which may be bad too.
You should get: base to emitter and to collector, arround 600 one way, much higher or open the other; collector to emitter high or open both ways.
To give you pinouts need to know what they are.
I'd order a 2N2955, 2N3055, and both drivers anyway; you'll save time and parts are cheap. Order the ICs too, same considerations.
Good luck.
EDIT= checked that DIP4, the VM08 comes out as a miniature bridge rectifier, probably supplies clipping as the W008 in JCM900.

[Densman]

To check the transistors I have to take them out of the circuit, correct?