gibson ga-40

[trem]

This is the Les Paul version.
http://el34world.com/charts/Schemati...PAUL_GA-40.pdf

Does this foot-switch turn the Tremolo on when it is closed.?
The schem shows it in Open position.
I have tried it both ways and the Trem does not work so i have other problems, but right now (working or not) i am wondering about the theory behind the foot-switch operation.
I was assuming that the ground connection needs to be made.?
Thank You

[52 Bill]

Does this foot-switch turn the Tremolo on when it is closed.?
The schem shows it in Open position.
I have tried it both ways and the Trem does not work so i have other problems, but right now (working or not) i am wondering about the theory behind the foot-switch operation.

The tremolo oscillator is typical phase shift design. In order for it to run, the resistor that is a combination of 500K speed pot and the 47K fixed resistor must be grounded. So the switch must be closed for the tremolo to work.

The usual problems with that type of circuit are off value feedback caps (the three 0.05uF) and off value resistors. Of course a bad tube or footswitch would also cause the oscillator not to run.

[trem]

right...thank you

[rockinrob]

Many of those have yellow astron caps that, although great sounding, are known to be leaky. My GA20T has had 3 of them bad when I restored it, and I lost another a couple years ago.



It really sucks in that amp that the caps are on the backside of the circuit board - it gets tiring taking all the pots and input jacks off!

[Enzo]

If cap is hidden behind, I clip its leads where they come around, and push it off the rear and fish it out. Then I install the new cap on the facing side. No removing stuf.

[trem]

Yeah...good idea, but in my case there are (i flipped the board over as mush as i could) about 15 caps on the other side.
Problem #1. 5-6 of these caps are those Yellow Astron tubes. My limited experience (10-15 old Fenders) is that about 70% of the ones i have seen leaked DC pretty bad. But in a Fender, you can easily lift a leg (i clipped on a 5Meg resistor to ground) and get at the caps.
However (Somebody(s) has worked on this amp previously...i hate this scenario).....there are 3 of the white Mallory 150 under this board. Who knows what they might or might not have done. It DOES seem to be wired properly though.
I think it would be best to amplify the "Enzo Protocl" and just get ALL those caps out of there, and get them up top with new caps.
Did i post a schem.?.......i did.
I get the 3x RC for the phase shift, but what is going on with the 5x 005 1M circuit.? What is that doing exactly, or what is that called.?
Thank You

[rockinrob]

This vibrato is a strange circuit...

The string of resistors is to keep the trem from oscillating, and this circuit is unstable without it.

I don't have hard feelings about replacing the caps in the trem circuit, but I do not recommend shotgunning the signal caps. You have to man up and prepare to go in there a couple times!

You can rig a way up to run the amp with the board half supported so you can test the caps - but please be safe with this. It isn't the safest this way, but at least you can test the caps.