Gibson G20 Main Filter Cap Replacement Problem

[Dave H]

I think the bridge rectifier wiring is wrong. Check that its negative terminal connects to ground and the negative terminals of both capacitors. Bridge negative is its left terminal looking at the picture. In the last picture it looks like bridge negative connects to the new cap positive.

[g1]

Well, you have the schematic.

Maybe. I got the impression that the amp model is just a guess.

[Perkinsman]

I’ve never been sure of the model, many of these type Gibson models looked very similar. After the last message, I brought it to a music store amp dept & they told me that it’s definitely a G50! This makes more sense since the cap cans are 2000uf each as opposed to 2000 & 150.

[Helmholtz]

I’ve never been sure of the model, many of these type Gibson models looked very similar. After the last message, I brought it to a music store amp dept & they told me that it’s definitely a G50! This makes more sense since the cap cans are 2000uf each as opposed to 2000 & 150.

Now that makes a huge difference, as this amp has bipolar supplies. Only one of the caps (C39) is connected with its negative end to ground, the other has positive to ground.

You might have DCV at speaker terminals, so remove speaker until the +/- 35V power supply voltages are fine and DCV at the output is zero.

Find out where your cap wires connect.

[Perkinsman]

I have 5 wires that were disconnected from the old cap,(C37?). 2 of the five are shrink wrapped together. Of the two wrapped wires (orange & red), red one is directly from the negative side of the rectifier, the other attaches directly to the circuit board. A black wire attaches directly to the negative side of the other cap (C38?) A white wire attached directly to the ground side of the foot pedal socket. The last wire (another black) attaches directly to the circuit board.

[Dave H]

Now that makes a huge difference, as this amp has bipolar supplies.

Yikes! it sure does. Disregard post #16
I think the new cap is reversed in post #11. It should be as the picture in the first post.

[drewl]

Yes!
fwiw I have one of these that lives in my van as a backup amp/speaker can.

Got it free, works great.

[DrGonz78]

Here is a screenshot of a Gibson G60 that was worked on by Dlab. It shows very similar wiring and it might prove to be a good way to analyze the before picture you forgot to take.

[DrGonz78]

Around 19 minutes in he takes voltages and displays the polarity of bi-polar supply.

https://m.youtube.com/watch?v=xrlpciTBaYA

[Perkinsman]

I’m trying to learn to read schematics so I’m taking baby steps to reattach these 5 wires to C38...actually, more like 4 wires since 2 are shrink wrapped together & obviously get attached together.
What I can see here is that the + side of C37, the existing cap that hasn’t been altered, is connected to
the + side of the rectifier. If I’m reading the schematic correctly, the - side of the rectifier connects to the - side of C38, correct?

[The Dude]

Yes. That's correct, if we are using the schematic in post #18.

[Perkinsman]

Yes, I’m using the G50 schematic. Ok, that takes care of two wires. I see that the + side of C38 goes to ground. Does that mean that I can attach the + side of C38 to any wire that is to ground? If so, the white wire here is connected to ground at the rear footswitch jack. Is that a good choice?

[The Dude]

I'd use the same ground as C37 minus side. Probably on that terminal strip.

[Perkinsman]

C37 negative side has two wires, one directly from the transformer, I believe the other was connected to C38, is it possible that C38 positive is connected to the C37 negative side? Then another wire was from C38 positive side to the jack switch ground?

[The Dude]

My bet is that all of those grounds (C37 -, C38+, transformer center tap, and jack ground) were soldered to a ground lug on the terminal strip. I wouldn't count on jack ground for a solid chassis ground. They may have also used a ground lug or lugs on one of the old can caps.