VT-Phasor Twin Tremolo

[bobloblaws]

I'm working on a circa 1975 SS amp called a Phasor Twin built in Canada by a company called Vibration Technology. Specific model # is 2G12A. I have everything working except for the tremolo effect. I suspect that the 50 year old electrolytic caps in that section might be a factor, which I will address, but my question specifically is the LED shown in the schematic between R35 and TR8. It is actually missing, it looks like it has been snipped out for some reason. What is the function of the LED? In my experience a typical tremolo circuit would have resistors and caps and in some cases a photocell arrangement, but I don't recall seeing one with an LED. Attaching the service manual, the drawing in question is on page 13.

VT 2G12A Service Manual.pdf

[glebert]

The photocell tremolos typically use a light source and an LDR (light dependent resistor) in one four lead assembly (often called a roach). The classic Fenders use a neon bulb in the roach, but I have built my own with an LED. Since the LED on the VT is where I would expect the light source side of the roach to be, my guess is that is the approach they used. Then the question would be where is the LDR?

[x-pro]

then the question would be where is the ldr?

here R54, R59, R65, R71

[Helmholtz]

This LED is used for vibrato/tremolo modulation together with the LDRs.
Any optocoupler (photocell isn't a correct term) needs a light source.

Edit: Missed posts above.

[Helmholtz]

I'd expect the LED to be closely packed together with the LDRs for good optical coupling and well screened from ambient light.

[bobloblaws]

I'd expect the LED to be closely packed together with the LDRs for good optical coupling and well screened from ambient light.

Ahh, OK, that's starting to make sense. The black plastic component that has R54, R59, R65, R71 is right beside where the snipped LED leads are, but I thought it was for the chorus effect due to the proximity of the S1 switch on the drawing. The component has two pinholes on top, presumably for the light from the LED to get through. I'll work on getting something rigged up. I also have some "roaches' that I could swap in if needs be.

[Helmholtz]

A typical LED roach will control only one LDR.
Also the LDR resistance might not be suitable for the circuit.
I'd connect a red LED (observe correct polarity) and see if it flashes.

[bobloblaws]

A typical LED roach will control only one LDR.
Also the LDR resistance might not be suitable for the circuit.
I'd connect a red LED (observe correct polarity) and see if it flashes.

No illumination or flashing happening with the red LED. I do know there is some oscillation happening and the pot is increasing/decreasing the rate (I've heard it as a clicking sound when I've been probing around).

[x-pro]

LED NSL5023. The lens is a dilfused red.
https://media.searchelec.com/specshe...k1979_text.pdf

[Helmholtz]

No illumination or flashing happening with the red LED. I do know there is some oscillation happening and the pot is increasing/decreasing the rate (I've heard it as a clicking sound when I've been probing around).

Did you try reversing the LED polarity?

[bobloblaws]

Did you try reversing the LED polarity?

Yes, I did try that

[Helmholtz]

Yes, I did try that

Then check C12 and the TR8 darlington.

[bobloblaws]

Then check C12 and the TR8 darlington.

Sure thing, will do when time permits. Thanks!

[g1]

Can you post a picture of the LDR device? The parts list calls for RPY58A but those are single units, not 4 in a pack. What you describe should have 8 leads.
It seems a bit odd that it would just have 2 pin holes to allow the light. I would expect a cutaway of the size that would allow the LED to be inserted.

[bobloblaws]

Can you post a picture of the LDR device? The parts list calls for RPY58A but those are single units, not 4 in a pack. What you describe should have 8 leads.
It seems a bit odd that it would just have 2 pin holes to allow the light. I would expect a cutaway of the size that would allow the LED to be inserted.

Here's a few pics. On the top views you can see the snipped leads where the LED should be attached. For the bottom one I couldn't
get a good angle, but there are multiple traces leading to it which can also be seen in one of the top view shots. Funny about the pinholes, when I look down through them all I see is bare circuit board.