Mid '60'S Gibson GA20 RVT tremolo not working

[Fred G.]

Hi all,

I'm working on a mid-60's Gibson GA-20 RVT Minuteman. The footswitch is missing. I managed to jumper the two lugs on the footswitch receptacle to get the tremolo circuit connected, but it is not working properly. It works, but it is a very subtle, faint effect, obviously not working properly. The depth and frequency pots do work and affect the signal. I tried adjusting the internal trim pot on the cathode of the 12AY7, to no effect.

I have read some suggestions on other Gibson amp tremolos which suggest replacing all the caps in the tremolo circuit (4 total) - as well as the cathode bypass cap on the power tubes ??. I've tested some of the circuit voltages, and they seem within spec of the schematic.

I'm not sure where to begin on this one! How does one troubleshoot the optocoupler? Should I do a shotgun replacement of the caps in the circuit (only 4 total)?

Help!

Thanks in advance,

Fred G.

[Helmholtz]

Please always post complete schematic.

[Fred G.]

[Fred G.]

[Helmholtz]

It looks as if the light source inside the optocoupler ("R38") is an incandescent lamp with a typical voltage of 12V.
Can you verify the 12V voltage drop?

[vintagekiki]

How does one troubleshoot the optocoupler?

Check (replace) V5b (12AU7) and V3b (6EU7)
If lamp poorly flashes, replace it.
Check that there is hermetic connection between photocell and lamp.
Photocell and lamp must be in complete darkness.
1)
https://reverb.com/item/30983627-reproduction-lamp-ldr-optocoupler-for-vintage-gibson-ga20-rvt-tube-amp-tremolo
Reproduction lamp-LDR optocoupler for vintage Gibson GA20-RVT tube amp tremolo

[Helmholtz]

According to schematic it's no neon lamp but an incandescent (filament) lamp.

[vintagekiki]

Where neon lamp is mentioned?

https://reverb.com/item/30983627-reproduction-lamp-ldr-optocoupler-for-vintage-gibson-ga20-rvt-tube-amp-tremolo
Reproduction lamp-LDR optocoupler for vintage Gibson GA20-RVT tube amp tremolo

About This Listing
Reproduction Sylvania LDR500 (PL8224) incandescent lamp-type optocoupler... ...

[Leo_Gnardo]

I have read some suggestions on other Gibson amp tremolos which suggest replacing all the caps in the tremolo circuit (4 total) - as well as the cathode bypass cap on the power tubes ??. I've tested some of the circuit voltages, and they seem within spec of the schematic.

I'm not sure where to begin on this one! How does one troubleshoot the optocoupler? Should I do a shotgun replacement of the caps in the circuit (only 4 total)?

I'd replace all those caps first. They may test OK but old caps often don't get the trem ball rolling. Do that first.

That you're getting a weak trem, tells me possibly the opto is OK. Even with a new/refurbished opto, if the caps aren't up to snuff your wobbulator won't wobble.

[The Dude]

It's simple enough to check an incandescent bulb with an ohm meter. I'd start there. Either the filament is open, or it's not.

[Helmholtz]

Where neon lamp is mentioned?

You did before your edit (still have a copy ).

[vintagekiki]

You did before your edit (still have a copy).

In translation minor errors are possible, and they are corrected on fly.
Since MEF is not quick move chess game, I usually comment post after 5-10 minutes, so that writer can possibly edit and correct post.
Anyway thanks for comment.

[g1]

Maybe the translate bot is not so perfect after all.

[Mick Bailey]

DC leakage in those feedback caps can reduce the trem or kill it completely. After checking the tube and optocoupler, a cap change can bring about wonderful results. Deterioration in the effect is usually so gradual as to go unnoticed for a good while, and the improvement a cap change makes seems to be far more dramatic as a result.

[vintagekiki]

Maybe the translate bot is not so perfect after all.

Greg, all languages globally have an average of 28 letters (characters) with only different schedule.
Different letters (characters) schedule is basic root of human notunderstanding.
In order for people to understand, they use simplified knowledge some foreign language or machine translation, because how learn some hundred or more different languages, dialects or slang.

The fact that someone keeps to himself because he know some language is relative, he knows only local speech slang, which is far from original literary language.
Each individual (same as computer) has own individual properties, i.e. ability to, or not understand with someone.
Greg, as can be noticed, only you have with vk notunderstanding, because you seem to be obsessed with vk nickname.
Others either understand what vk is writing about, or they do not show that they are superior because they know own language.
Your basic problem in communicating with vk is not problem in machine translation, your problem is in notunderstanding syntax because each language has its own syntax.

https://www.merriam-webster.com/dictionary/syntax
https://en.wikipedia.org/wiki/Syntax
Nobody forcing you to communicate with vk.
The solution your problem is very simple. Just read what it says in (last row) post#135

post #64 (Simplified English Introduction)

If administrator thinks that with this post violated MEF code of conduct, or injured integrity some individual, I would ask him if take decision to remove this post, feel free remove me (vk) from forum member list.

Machine translation is as it is. Who wants to understand, will understand, who will not I can not help him.
It can't get better than this.
vk

Thanks to everyone who separate time to read this post.