VT-Phasor Twin Tremolo

[Helmholtz]

Tremolo is not working currently, that's what I'm trying to fix.

The manual seems to distinguish between tremolo/vibrato/chorus. Maybe referring to different amp models.

[g1]

I would think 1.45V between E and B of TR8 should be sufficient to turn TR8 on.
Maybe the R36 adjustment or TR8 emitter voltage Helmholtz referenced in service manual will give some insight.

[The Dude]

Edit: Deleted post. I was looking at the wrong TR8.

[bobloblaws]

With intensity pot turned all the way down, what is voltage between B and E of TR8?

My bad, turns out I had the base and collector switched (currently connected with alligator clip wires and I swapped the new and old one a couple of times). It's actually 1.1V across E and B rather than 1.45V. I would think it needs 1.5V to turn the LED on, that's what I get when checking with the diode tester on my DMM but if I apply 1.5V-2V to it with a DC power supply I'm not seeing any light through the pinholes on the top of the dome.

Does the tremolo work?

I've been calling the effect I'm trying to get going "tremolo" but the marking on the front face is actually labeled "vibrato". I had mistakenly assumed it was actually a tremolo effect, probably because it is a more common effect to find built into into amps.There is also a switch for "chorus effect" which is at the bottom of the schematic on page 13. Indeed "Tremolo" could be exclusive to another model. Sorry for any confusion caused there.

Also check transistor voltages acc. to chart: Reverb/Vibrato Section 2.0.

The expected voltage at TR8 emitter is 0.35V, I'm measuring 0.39V to ground.

See Service Manual: Adjustment B.

Sorry, I don't know how to check LDR resistance or unity gain. The stock value of R36 in this particular amp is 470 ohm and I tried a few different values in the range described (330-750 ohms) which had only a marginal effect on the E and the E/B voltages.

[g1]

turns out I had the base and collector switched (currently connected with alligator clip wires and I swapped the new and old one a couple of times). It's actually 1.1V across E and B rather than 1.45V. I would think it needs 1.5V to turn the LED on, that's what I get when checking with the diode tester on my DMM but if I apply 1.5V-2V to it with a DC power supply I'm not seeing any light through the pinholes on the top of the dome.

I'm not clear where you are talking about the transistor and where you are talking about the LED as far as voltages go. And if you were applying voltage to the transistor or to the LED.
Also, how did you verify the LED was good, and did it light up under that condition?

[Helmholtz]

The expected voltage at TR8 emitter is 0.35V, I'm measuring 0.39V to ground.

That's good and means that there's enough current through the LED to make it light up.
If it doesn't there's something wrong with the LED.
Might be an unsuitable one which requires a much higher current than the original one.

[bobloblaws]

I'm not clear where you are talking about the transistor and where you are talking about the LED as far as voltages go. And if you were applying voltage to the transistor or to the LED.
Also, how did you verify the LED was good, and did it light up under that condition?

Right, I did get mixed up, I think at some point I started equating the voltage that you said was sufficient to turn TR8 on with the voltage required to light the LED.
In any case, if we can start over, you said you thought 1.4V across E and B of TR8 would be sufficient to turn on TR8. Now that I found I had the connection wrong and that it is actually 1.1V, does that change the evaluation?

I want to think the LED is OK since it seems to check out with DMM diode check. Forward voltage of 1.5V and O.L. if I reverse the polarity of the DMM probes. However, I was not able to detect any light from it when I applied a voltage through R35 (I don't know if it was necessary, but I disconnected TR8 when doing this test). If I'm correct to think that the test I did with the DMM indicates the LED is good, I would have expected to detect light from the LED. I don't fully understand all this stuff, but with another LED the DMM check indicated 1.8V and when I applied my voltage source, it started to light up right around 1.8V. So that's why I hoped I could do the same thing with the LED in the amp circuit to verify the status of the LED, in this case I expected 1.5V to be the threshold.

I had jotted down the LED voltages I measured in circuit initially, it was 11.8 on the cathode and 10.3V on the anode, to ground, so exactly 1.5V across the anode and cathode. I'm not sure what to make of that. As I said, I don't fully understand how this is supposed to work, still got some learning to do.

[bobloblaws]

That's good and means that there's enough current through the LED to make it light up.
If it doesn't there's something wrong with the LED.
Might be an unsuitable one which requires a much higher current than the original one.

So if I were to jumper C to E on TR8 and the LED does not light up, would I be able to say for certain that it is faulty?

[Helmholtz]

So if I were to jumper C to E on TR8 and the LED does not light up, would I be able to say for certain that it is faulty?

I'd say yes.
But that would mean a very high LED current of maybe 15mA.
Do you have a datasheet?

[bobloblaws]

I'd say yes.
But that would mean a very high LED current of maybe 15mA.
Do you have a datasheet?

Maybe this one?

DSA-302050.pdf

[Helmholtz]

Great, so found an original NSL5020?
This LED has a rated current of 20mA, so no risk when shorting C and E.
It seems that the LED is meant to only faintly light at "idle".

[bobloblaws]

Great, so found an original NSL5020?
This LED has a rated current of 20mA, so no risk when shorting C and E.
It seems that the LED is meant to only faintly light at "idle".

I searched for the NSL5023 datasheet based on post #9. The service manual says NSL100.

[Helmholtz]

I searched for the NSL5023 datasheet based on post #9. The service manual says NSL100.

The vibrato section wants an NSL5023.
What is the LED you have?

[bobloblaws]

The vibrato section wants an NSL5023.
What is the LED you have?

Oops, my bad, was looking at the list in the tone section.

[bobloblaws]

It seems that the LED is meant to only faintly light at "idle".

But it should be light up brightly with 10-11 volts? (which is what I reckon would be the case if TR8 is wide open/jumped)