GT10 problems

[zintere]

I bought a 1976 Ampeg GT10 from a guy who said it worked last time he used it. I plugged it in and it seemed to be working fine. I turned the volume up to about half and it quit . All I get now is a slight hum and some crackling sounds. No smoke , no smell just hum and crackle. I replaced all the electrolytic caps , still nothing but slight hum and crackle. I also tried a different speaker and got the same results. The hum and crackle do not increase if I turn up the volume. I would appreciate any help.

[Hangman]

Do you have a digital multi-meter? feel comfortable pulling the chassis out and taking a picture?

It could be something very simple.

what i might try if I were you is giving the amp a solid whack with your fist, see if it pops back on, that could indicate that there is a loose connection.

if that doesn't work, it is likely more involved, and would helpful if you could take measurements, or at the very least some pictures.

I bought a 1976 Ampeg GT10 from a guy who said it worked last time he used it. I plugged it in and it seemed to be working fine. I turned the volume up to about half and it quit . All I get now is a slight hum and some crackling sounds. No smoke , no smell just hum and crackle. I replaced all the electrolytic caps , still nothing but slight hum and crackle. I also tried a different speaker and got the same results. The hum and crackle do not increase if I turn up the volume. I would appreciate any help.

[zintere]

I have the chassis out. when it first quit I used a pencil and tapped every part and wire with no results. That's why I went ahead and replaced all the electrolytic caps. I used my multi meter to check all resistors. I went ahead and replaced all the sand block resistors and the 2 big 2w cc resistors.

[Hangman]

Sorry, did a quick read the first time and missed that in your post.

So do you have a DMM? The GT-10 is a really simple amp. shouldn't be to hard to figure out what is going on.
do either Q10 or Q11 get warm or hot?

I have the chassis out. when it first quit I used a pencil and tapped every part and wire with no results. That's why I went ahead and replaced all the electrolytic caps. I used my multi meter to check all resistors. I went ahead and replaced all the sand block resistors and the 2 big 2w cc resistors.

[zintere]

neither Q10 or Q11 get overly hot they check out voltage wise with the schematic. Yes as I said I am using a DMM. It seems like it could be a bad ground or possibly a bad cap. I hate to go chasing all those old Mullard caps. Some would probably be hard to find. All the E-caps were Nichicon so it wasn't to hard to find replacrements for them.

[Hangman]

ok, so do the DC voltages check out? there is a schematic in the docs section of this website. If all the DC voltages check out, then we need to figure out a way to trace the AC voltages through the circuit. do you have an oscilloscope or signal tracer?

[zintere]

Don't have either of those. I may have found part of the problem. There is a single diode next to C24 that reads the same voltage at either end. I pulled it and checked it and it tests bad., but I can't find one to purchase anywhere. The diode says "SI560" .

[Hangman]

I believe a 1n4003 diode would work there. if that diode is anything like ampeg's other solid state amps.

[zintere]

I emailed Ampeg and they sent me a parts list. It calls for a 1amp 800P.I.V. diode. I've got them ordered.