Help with Reverb/Tremolo wiring Gibson GA15-RVT

[devilsindetails]

Ok so my voltages at the tubes check out yes?

My tubes check out...

That leads me to conclude that my tremolo troubles have to be somewhere in that dammed birds nest of a tag board...

What I've run up against time and time again as a relative neophyte is that the numerical markings on the tag board have absolutely no relation to the schematic itself...does anyone out there have any kind of legend regarding this Gibson tag board? My suspicions are that the numerical markings were more than likely simply there for the poor devils constructing these amps on the manufacturing line.

When I compare this board to what I've found even in the budget Ampegs, Harmonys and Silvertones (which, I don't believe are known for HQ assembly) these Gibsons boards make troubleshooting a real pain.

Sorry for the semi-rant but this tremolo problem really has me scratching my head...as background a friend brought this to me with only one concern, that the reverb switch on the footpedal would not shut down the reverb...and that he was forced to use the reverb pot to remove reverb from the output.

At that time the trem footswitch and circuit was working fine. I looked at the footswitch wiring and saw a mess, which is how this thread started.

So I had an amp where trem and reverb circuits were working but a wonky footswitch that was only able to activate trem.

After the rewire i could activate reverb with the footswitch but had somehow impossibly lost tremolo whether switch was plugged in or as a test grounded out.

After a successful test on a known good working Ga-15RVT amp I know the footswitch is not the problem. I have also taken the working footswitch from the perfectly working GA-15RVT and it also is unable to activate the trem circuit on the defective GA-15RVT.

Which brings me back to the components on the tag board..I have checked all resistors in the entire amp and replaced three that were out of spec..with no change.

I have replaced three caps in the trem circuit, with no change to the lack of trem.

I will, of course make the required tests of the caps as suggested here...

I just thought that some further background of the history of this problem might provide the more experienced here with some additional context and insight...perhaps?

Thanks to all, Leon

[Firestorm]

Yes, it certainly seems odd that rewiring the footswitch into a working one would make the tremolo stop working, but perhaps it's coincidence.

There are two possibilities: either the oscillator is not oscillatiing; or it actually is oscillating and the signal is not making it to the center tap of the PI transformer. After making sure the frequency pot is solidly grounded (as it would be with the footswitch closed) check to see if you are getting a low-frequency AC signal on the cold side of C17. I've only ever tried to do this with an analog meter (you get to watch the needle swing in time to the oscillation) but you should be able to tell something with a decent digital meter.

This way you can isolate the problem to either the oscillator itself or to the connections to the PI.

[devilsindetails]

Nope..checked the cold side of C17 with analog MM and can see there is a AC present but no oscillation...

[devilsindetails]

Also checked C16, C17, C19, 20, 21 no leakage....any ideas where I look now?

[Firestorm]

Nope..checked the cold side of C17 with analog MM and can see there is a AC present but no oscillation...

IIRC, you have to set the analog meter to DC to see the needle move as the voltage rises and falls. Check to see if the same AC is present at the center tap of the PI tranny.

[devilsindetails]

Nope..checked the cold side of C17 with analog MM and can see there is a AC present but no oscillation...

IIRC, you have to set the analog meter to DC to see the needle move as the voltage rises and falls. Check to see if the same AC is present at the center tap of the PI tranny.

Ok, will do.

[tubeswell]

If you use a VDCmeter, then run the oscillator slowly. If you use the VACmeter, you will need to run the oscillator at maximum speed to get a reading.

[devilsindetails]

If you use a VDCmeter, then run the oscillator slowly. If you use the VACmeter, you will need to run the oscillator at maximum speed to get a reading.

Tnaks for the heads-up "tubeswell" will report my findings this afternoon.

[devilsindetails]

Been a way for a while but wanted to report to all who helped out on this subject that the Tremolo has been restored...on a hunch I replaced the new caps I had installed in that part of the circuit (they were NOS) with some new oil in paper of the same value and voila! Tremolo restored! I never was able to get the oscillating reading using the voltmeter tricks so I suspected these NOS caps were kaput.

Question? The schematics show these caps are non-polarized so does it make any difference how they are installed when using polarized caps? My understanding is that it doesn't.

Thanks to all for their help again. Much appreciated. Leon

[ampdoc1]

I just picked up on this forum today. I've been doing some vintage repairs lately and the first was a Gibson.

I had a shop for 10 years in the 80s-90s, but I had forgotten that virtually every Gibson I ever worked on in the past, NEVER was constructed per the Gibson Master Service Book.

I'm sure you"ll eventually find a fix, but it's a hunt and peck process.

Good luck.

a'doc1

[Structo]

Question? The schematics show these caps are non-polarized so does it make any difference how they are installed when using polarized caps? My understanding is that it doesn't.

Thanks to all for their help again. Much appreciated. Leon

Usually the foil end should go to the lower impedance side of the circuit.
Supposed to be quieter this way but many times it is hard to figure out which side is lower.

In the case of this amp towards ground would be the proper orientation.

[devilsindetails]

Yes. Gibson Amps are absolutely dreadful to work on...the chassis layout sucks and schematics to real examples vary widely. Luckily with the help here and some individual stick-to-it-ness I managed to sort both the footswitch and tremolo problem out.

[devilsindetails]

Question? The schematics show these caps are non-polarized so does it make any difference how they are installed when using polarized caps? My understanding is that it doesn't.

Thanks to all for their help again. Much appreciated. Leon

Usually the foil end should go to the lower impedance side of the circuit.
Supposed to be quieter this way but many times it is hard to figure out which side is lower.

In the case of this amp towards ground would be the proper orientation.

Thanks structo...it's working now without any noise so I'm not going to bother changing unless the noise problem starts to rear its ugly head.

[Phil_S]

Gibson amps are only dreadful to work on if you are doing it for money. If you are doing it for a hobby, they provide a high level challenge and hour after hour of amusement.

Believe it or not, because this is strictly a hobby for me, I got a huge sense of reward for fixing my Minuteman. If I had to pay someone, I probably could have bought several amps with the money as it took unreasonable amounts of patience and countless hours.

So, it's a matter of perspective. In either case, don't expect yours to conform 100% to any published schematic. I'm sure it happens, but just as likely, you have the amp du jour on your bench!

[Structo]

Yes I think a lot of amp manufacturers substituted parts when they ran out of the specified component.

Fender is famous for this and sometimes happy accidents happened making a certain amp exceptional.
Leo didn't like stopping production for any reason so he would tell them to use a different value if they ran out.
Even when spec'd parts are used, the tolerances were so loose that you could have a dog or a very sweet sounding amp.