Gibson GA-35RVT Tremolo problem

[T9c]

I am repairing a Gibson Falcon (GA-35RVT) and have just the tremolo to fix now. There is oscillation I see on my DMM, but there is no tremolo mixing with audio output. It looks like the LDR has been replaced with one that reads "948-003636 NSL Canada" on it. Voltages compared to schematic are close to right on. I've swapped the two 6EU7's & 12AU7's, tried a new 12AX7, and cannot get any tremolo on the output. I am close to ordering a 6EU7 and 12AU7 to try, but I'm wondering if anyone had any insight to lend before I go down that path.

Here's a pic of the board:

[Enzo]

I really doubt the problem is a tube.

The relevant 6EU7 is V3, and if you have oscillation, it is working.

The oscillation, LFO is easier to type, the LFO comes out the depth pot wiper to the grid of V5b. 12Au7 in the socket? If we have the LFO< we then need the drive circuit to interface with the signal path. Pin 6 of V5b, got LFO there? That trim pot in the cathode leg is to set the threshold of the opto lamp. Have you swept that through its range?

We need to verify the V5b is indeed driving whatever lamp is inside the opto.

Then we cross the divide into the photoresistor side. LDR. The LDR is just a variable resistor acrosss the signal path at the grid of V4b. C32 is added in series so it doesn't mess with the DC levels. You need to find out if the LDR is reacting to the lamp. The LDR could be not responding to the lamp, or the lamp may be inappropriately driven, so having no effect. SO set your meter to ohms and measure the resistance of the LDR while things run. Is the resistance varying as the LFO runs?

Ideally a scope would be the thing to watch with though.

[T9c]

Thanks Enzo.
I do have a scope, and I can see the LFO on the wiper and other places indicated.
One correction to my original post about the voltages, though. I am only getting around 10vdc on V5 pin 6, whereas the schem shows 130v there. Resistance across the lamp is 1.1megohm, though. I think whoever tried to repair the tremolo before me tried it with this LDR and wasn't successful b/c it's not within spec of the OEM LDR.

On other points, resistance on the LDR lamp is a steady 80ohms, with no change to increased signal gain or any other variable. Sweeping the trimmer didn't change much of anything , but I noticed that the voltage on it is low as well, reading less than 1vdc throughout the range.

Enzo, thanks again. Your stepped approach helped me sort out the confucious funk.

I really doubt the problem is a tube.

The relevant 6EU7 is V3, and if you have oscillation, it is working.

The oscillation, LFO is easier to type, the LFO comes out the depth pot wiper to the grid of V5b. 12Au7 in the socket? If we have the LFO< we then need the drive circuit to interface with the signal path. Pin 6 of V5b, got LFO there? That trim pot in the cathode leg is to set the threshold of the opto lamp. Have you swept that through its range?

We need to verify the V5b is indeed driving whatever lamp is inside the opto.

Then we cross the divide into the photoresistor side. LDR. The LDR is just a variable resistor acrosss the signal path at the grid of V4b. C32 is added in series so it doesn't mess with the DC levels. You need to find out if the LDR is reacting to the lamp. The LDR could be not responding to the lamp, or the lamp may be inappropriately driven, so having no effect. SO set your meter to ohms and measure the resistance of the LDR while things run. Is the resistance varying as the LFO runs?

Ideally a scope would be the thing to watch with though.

[T9c]

Well, I subbed the LDR for a Fender LDR and the tremolo is now working, but the depth range is pretty narrow, as only the final quarter turn has any effect on the strength of the trem signal. I think I'll find the old threads concerning modding the circuit for Fender LDR's.