Gibson GA-20RVT trem LDR replacement

[Randall]

Trem works, but reads various resistances when off and is dragging the reverb channel down. Can I use a standard Fender type roach for Z1 in this situation? If so, just pop it in or would I need to make an adjustment, I'm guessing on the neon side?

[Jazz P Bass]

The Gibson lamp is shown as an incandescent type.

The Fender 'roach' uses a neon lamp so it not work.

[Randall]

if that is the case, what to do?

[1ampman]

Try pulling it apart and replacing the bulb or use an LED. Not a fun project, with some experiments it'll work.

[Jazz P Bass]

Find out what is bad.
The lamp or the resistive element.

Fellow member Tom has success with looking at the lamp wires.
http://music-electronics-forum.com/t23156/
Sometimes they can be repaired.

[52 Bill]

Have you tried adjusting the lamp driver bias control? If the lamp stays on too much when the trem is off, the signal will be attenuated all of the time.

[Randall]

I adjusted it to 2 volts, it had been 3.6v. I'll try tweaking it.

[Randall]

Still scratching my head over this. Powered up with the trem switched off, the resistor side of the roach sits at 10 - 12K. Powered down, it slowly falls to infinite resistance. The bulb side measures ~200ohms, out of circuit. I can get significant resistance measurement changes by shining a light in the bulb side, as well as connecting to a 9v battery, so I think the roach is working. But why is it sitting at 10 - 12K when the trem is off? I adjusted the pot for 2v on pin 8 of V5B. When the resistive side is at 10 - 12K, the bulb reads 12v across it's leads, so it seems it is not turning off. I tried swapping out V5, no luck. The V's on the tubes in question look close enough.

What am I missing?

[Jazz P Bass]

If the Tremolo works & it's dragging down the Reverb, have you tried replacing C29?

[52 Bill]

Still scratching my head over this. Powered up with the trem switched off, the resistor side of the roach sits at 10 - 12K. Powered down, it slowly falls to infinite resistance. The bulb side measures ~200ohms, out of circuit. I can get significant resistance measurement changes by shining a light in the bulb side, as well as connecting to a 9v battery, so I think the roach is working. But why is it sitting at 10 - 12K when the trem is off? I adjusted the pot for 2v on pin 8 of V5B. When the resistive side is at 10 - 12K, the bulb reads 12v across it's leads, so it seems it is not turning off. I tried swapping out V5, no luck. The V's on the tubes in question look close enough.

What am I missing?

You aren't missing anything. The photocell is staying at 10K-12K because the light bulb is staying lit enough even when the trem is off to keep the resistance low. If the lamp was turning off completely, the cell resistance would increase to the point where it didn't reduce the signal volume.

Can you watch the lamp while it is in circuit? Does it turn off or is it always lit just a little?

[Randall]

"If the Tremolo works & it's dragging down the Reverb, have you tried replacing C29?"

It's dragging down the Reverb channel, not the Reverb itself, but yes I did try swapping out C29 to no avail.

I put a dc supply on the bulb side and lowered the voltage to just where it did not lower the signal and it was 5v. Also, I can raise or lower the signal by raising and lowering the supply, so my guess is the lamp is staying on a little. But, it seems like that wouldn't have changed would it? It looks like the schematic shows a 10v drop across the bulb, which is about what it is now.Can the resistive part have changed over time? Is it possible that it conducts more with less light than it used to?

[g1]

Is it back in the cab, or are you covering the chassis while testing?

[Randall]

It's covered on my bench. I noticed when powered down ambient light effects the resistance readings. I also notice that if I crank the cathode bias pot full it makes it better, but still is dragging down the signal enough to be an issue.

[g1]

I only ask because you didn't mention it, but you've tried other 12AU7's ?

Edit: I see you did mention it, even after re-reading I still missed it. Doh!

[Randall]

I did mention it actually, but yes I did try a new 12AU7, no dice.