Gibson GA-80 Restoration Help

[Helmholtz]

All tube plates, cathode DCVs are either spot on or close.

So you checked all 18 voltages?

The input signal sine wave looks perfect until the volume wipers. Then it looks crazy. I am wondering if I should bypass the volume pot and clip in a new volume pot to test the scope signal, or is that a waste of time? Should I normally be seeing a clean sine wave after the volume pot?

A pot doesn't change signal shape by itself.
Pull the 5879 and see how signal changes at the pot wipers.

[misterc57]

So you checked all 18 voltages?

Yes all Vs on the chart. 20 of them.

A pot doesn't change signal shape by itself.
Pull the 5879 and see how signal changes at the pot wipers.

You are right. pulled the 5879s and clean signal at the wiper. I put the 5879s back in and scoped the plates of both output tubes and I have a very clean looking signal there also.

So I have been chasing a none issue which has been a good learning experience, but which now brings me back to my original problem.

I will try swapping output tubes but I do not think that will be the issue since channel one sounds fine on the low notes.

[misterc57]

I have been scoping a 1K hz signal. Perhaps a lower signal (100 hz) may show some anomaly.

[Helmholtz]

You are right. pulled the 5879s and clean signal at the wiper. I put the 5879s back in and scoped the plates of both output tubes and I have a very clean looking signal there also.
.

Seems you had a poor socket/pin contact.

Test signal frequency shouldn't matter.

[misterc57]

The problem is a rattling sound only on channel two that comes and goes and is more prevalent with hard strumming or picking.

I think I have isolated the problem to the channel two tone circuit. I bypassed the tone pot and tone selector switches coupling from V1 to the volume pot. I have yet to hear the noise since doing that.

I had a feeling that the tone switching system was going to present issues, it being so mechanical and 60+ years old.

My plan is to take the whole switching section out of circuit. The effect from each switch was barely noticeable anyway. I am debating taking it out of the amp completely to make other components more accessible.

Not sure if any of this will hurt the value of the amp.

[misterc57]

I was wrong. Problem is still there. Could use some fresh ideas on what to try next.

Channel 1 is fine. Channel 2 has a rattle noise. Can hear it after plucking the low E string. Rattling follows the note for as long as the string is vibrating.

I have swapped every tube. Different speaker. Cleaned all controls and tube sockets. Tried a different coupling cap from V1B.

Looking at the schematic channel 2 passes through the switch button tone selector which I also bypassed in testing. Channel 2 also has the tremolo circuit which sounds great.

Schematic shows one input to the Ch 2 volume pot coming from a coil (coil resistance is @ 800 ohms). However I see a second input to CH 2 volume pot, a wire coming from the tone pot. Wonder if it is suppose to be there, I have not tried temp removing it yet.

Ch 2 has a tone pot and 6 tone switches. First switch position is suppose to engage the tone pot only, the other 5 switches is suppose to use a preset tone.

Other observations; When the amp is first turned on and warming up, I can hear some slight metallic sounding ticks coming from the speaker. Also when tapping any part of the chassis the taps travel through to the speaker.

Thank you!

In the picture below the top row is channel 2. Left to right are; 2 inputs, vol pot, tone pot, tone selector switches.

[misterc57]

Can anyone tell me the purpose of the coil in the channel 2 tone switching circuit?

Thank you

[Enzo]

Same purpose it would have in a crossover in a speaker cab. It is part of an LC circuit. The switching combines it with various capacitances which make various resonant frequencies.

[misterc57]

Same purpose it would have in a crossover in a speaker cab. It is part of an LC circuit. The switching combines it with various capacitances which make various resonant frequencies.

I am back to suspecting this circuit to be generating noise. I will be replacing all the caps in the switch panel. I did find one noise maker at the tone pot connection of the 4th leg, may have just needed a reflow but I installed a new .01 cap to the pot.

These pots have a fourth leg on the opposite side of the standard 3 legs. In this case the first position of the 6 button switch connects to this 4th leg via the .01 cap I replaced. In this position the tone pot controls the tone. In the other 5 positions the tone is fixed by the selections capacitor and coil network. Not sure how this type of pot with 4 legs is designed to operate, I could not find any info online about it.

I am suspicious of those caps associated with each button position and plan to replace each of them. Can caps or a coil generate noise? I am more suspicious of the caps than the coil as I have had the coil out of circuit and have had noise.

[Helmholtz]

I think I have isolated the problem to the channel two tone circuit. I bypassed the tone pot and tone selector switches coupling from V1 to the volume pot. I have yet to hear the noise since doing that.

Is that result still valid?

When doing this test, did you disconnect the coupling cap and the vol. pot completely from the tone circuit?

[misterc57]

Is that result still valid?

When doing this test, did you disconnect the coupling cap and the vol. pot completely from the tone circuit?

I have been through a bunch of various testing. Seems to be no noise when I go directly from V1 via coupling cap to the volume pot. Tone circuit and coil all disconnected.

As soon as I add elements of the tone circuit back in I get noise most prevalent with low notes or hard strumming.

[Helmholtz]

Could be a bad ceramic cap, corroded switch contacts or a broken solder joint.
Inductor is less likely unless it's loose.

[misterc57]

Replaced all the caps at the tone selector switches. Everything else reconnected. So far no noise!