Ampeg Gemini I photocell weirdness

[drewl]

So, the trem photocell of this amp has a 5th wire which is a shield to the resistor side that is tied to the ground side of the lamp.

The Y side of the photoresistor is shorted to that shield (measures 68 ohms) so when you turn the Intensity pot up it kills the signal obviously.

Here's the weirdness, if you remove the Y connection to the pot, the tremolo works great as long as the Intensity pot is turned all the way down.

Well, maybe not too weird, I just thought it was interesting, resistance of the photoresistor side is still changing with the oscillator.

[Helmholtz]

Here's the weirdness, if you remove the Y connection to the pot, the tremolo works great as long as the Intensity pot is turned all the way down.

Yes, that's a consequence of the Y side being essentially grounded.
But that's not how the trem is supposed to work - at least from the schematic.
If that's the original roach, the shield should not make contact to the Y terminal. So I assume an internal short.
What happens if you disconnect the shield?
I'd expect normal working trem but clicking noise.

[drewl]

It's not how the trem is supposed to work, but it does!
It is weird.
If I remove the shield connection to ground the trem is very weak, probably bad oscillator caps , oscillator signal at roach is only around 25v peak to peak

[x-pro]

X Y points are exactly connected to the Intersity resistor ?

[drewl]

The intensity pot?
Yes.
So you have an oscillating resistance between the X wire and ground.
The Y wire is not connected
So it has trem with the Intensity pot turned down

[x-pro]

Here's the weirdness, if you remove the Y connection to the pot, the tremolo works great as long as the Intensity pot is turned all the way down.

Of course it will work. Because otherwise the Y point of the potentiometer is the main signal to be amplified. And it doesn't have to be on a common wire, as Helmholtz pointed out