Gibson GA-79RV Troubles

[earache]

Randall and 52 Bill
Thanks so much for your replies, you have given me plenty to work from!
Not only is this amp pretty complicated as it is stereo, the schematic leaves a lot to be desired as far as legibility goes. The values are hard to read and it would be much better if there were more voltages listed.

[DrGonz78]

I think this version of the schematic is a touch better than the one posted on the thread. Hope it helps a little, good luck.


This might help read it even better, maybe not...?

[earache]

Dr Gonz78...I really appreciate your efforts and also your Hunter Thompson references!

[earache]

Are the filter caps original or new? You need to be able to recognize what kind of hum you have to help you fix it. 60Hz? 120Hz? The oscillations and hum lead me to think bad e-caps.

Audiopete - I soldered similar caps over the existing filter caps in the separate power chassis - didn't change the hum.
I also have swapped in all new tubes one at a time save for the 7199 (which I don't have a replacement for) and the hum is still there.

[Jazz P Bass]

Pay special attention to where the reverb return is grounded.
Try moving it.

[earache]

I found where I made a mistake with a resistor substitution, and before I replace it could some one comment on whether it would cause my hum problem? Look on the schematic which is posted a few times in the thread. Just below the "V7" caption there is a 47K resistor... I put in a 4.7k

I will replace the resistor anyway, just wondering aloud..

[earache]

Pay special attention to where the reverb return is grounded.
Try moving it.

Is the ground at pin 6 at V4 (which is a 7199 tube)? I have a hard time reading the Gibson amp schematic.

[52 Bill]

Is the ground at pin 6 at V4 (which is a 7199 tube)? I have a hard time reading the Gibson amp schematic.

[ATTACH=CONFIG]31560[/ATTACH]

Yes, the reverb return circuit uses the second half of the tube, so pin 6 is the cathode. What's more important is where the shielded cable coming from the tank grounds and if the tank's RCA connectors are grounded or not.

Is the tank original or a replacement?

[earache]

Yes, the reverb return circuit uses the second half of the tube, so pin 6 is the cathode. What's more important is where the shielded cable coming from the tank grounds and if the tank's RCA connectors are grounded or not.

Is the tank original or a replacement?

The tank is original and isn't hooked up right now. I have to take the chassis out of this amp to work on it so the reverb leads are not hooked up right now. Are you saying that if a good tank was in place and hooked up there wouldn't be any hum? What if I just pulled V4 the reverb tube?

[earache]

Nope, pulling the tube didn't change anything.

[earache]

I have found a much better schematic of what I think is the exact same amp - a Maestro GA-78RV

Maestro GA-78RV.pdf

I am sure that this will make diagnosing this amp's problems much easier.

[Jack Hester]

I have the Maestro GA-78RV, that I have not looked at/listened to in several years. The amp worked great, but always had this low level hum, as well. I remember studying the schematic and tracing the harness from the power supply to the amp chassis. I came to the conclusion that running filament (AC) wires and line input (AC) wires in the same harness as the B+, was bound to induce hum. Figured I'd live with it, as in a room with a few people, you couldn't hear it. You'd think that the PS filter caps would eliminate this. Only in a quiet room was it a bother. I no longer drag this heavy beast out to Enjoy it. I may now do so over the holidays.

And, you may force me to try and de-hum it, as well.

Jack

[earache]

Hello All

For the sake of closure let me tell y'all how I finally found and fixed the source of the hum.

I had replaced some filter caps in the main chassis that consisted of (2) 20uf caps in a single can. The way this old cap was constructed, it had 2 wires coming from the bottom for the two positive leads, and 1 single lead for the ground. It had the usual metal flange along the bottom with two holes for mounting. When I removed it, it was mounted on a metal standoff with a screw which also completed the ground circuit for part of the amp. When I replaced it with two discrete caps, I attached their positive leads to the same place as where the can cap positive leads were attached, and both negative leads of the new caps went to where the negative wire from the can went.
This did not account for the ground connection that was created when the cap can was attached at the standoff.

Once I found where some of the amp had no ground reference, I ran a lead to ground and now all works! I only realized the problem by going thru picture I had taken of the amp guts when I originally received it.

Thanks to everyone who offered help and advice, this is a great community.