Twin Reverb tremolo trouble shoot

[spicoli]

Hi - I'm trying to get the trem circuit working on an early master volume (non-pp) twin reverb. All electrolytics have been replaced
but the feedback caps have not. The bug flashes and all voltages are about right on V5. Intensity pot resistance checks out, speed pot affects the bug properly. Everything else about the channel seems fine just no sign of tremolo (other than ticking)

The voltage varies from about 300v-150v on pin 1 of v5 with the trem jack grounded. Can I assume that the feedback caps are good if this is the case? The resistors check out except for the 2.2 meg at the bias supply - that measures about 1.3 megs (in the circuit)

I have very little experience with these circuits - where else should I be looking for problems?


Thanks.

[tboy]

If the lamp in the bug is flashing, then the oscillator is working. I'd be checking the LDR portion of the bug.

[spicoli]

Do I just measure the resistance of the LDR while it is flashing? - Looks like 1.6 meg with the light out and then it fluctuates when flashing.

[Raybob]

Do I just measure the resistance of the LDR while it is flashing? - Looks like 1.6 meg with the light out and then it fluctuates when flashing.

The light side of the bug is flashing, then the relay side is where to check. Be sure one leg is to ground, the other leg has continuity to intensity pot. Check all connections at intensity pot to their respective points, ground and "Y". If all connections are good, possibly a bad bug. If there a 'tick' sound, add a .01 cap from 10 meg resistor to ground point of bug.

[spicoli]

The ground is good, the intensity pot checks out. I read in another thread to short the legs of the resistor side of the bug and see if that kills sound going through the amp. It does not which would indicate that the bug is ok? What about measuring the fluctuating resistance on the relay side? would that indicate that the bug is ok? Is replacing the bug the only way to determine that the relay side is bad?

[jpfamps]

In my experience there are two causes of the symptoms you describe:

1) The LDR in the optocoupler has expired.

2) The inside of the neon has blackened to the point that not enough light falls on the LDR. This is due to small amounts of metal evaporating on the neon on striking, and condensing of the inside of the glass. Over time this can accumulate enough to stop enough light hitting the LDR..

You can repair the optocoupler with a replacement LDR (and or neon) and some heat shrink, or alternatively get a replacement.

[spicoli]

Thanks for all the help - I'll order up a new optocoupler - that simply has to be the problem.

[MikeH]

Just to cover all bases; is this a design where the tremolo won't operate without the footswitch connected? IIRC, my silverface fender is like that...

[52 Bill]

This amp does need a foot switch to turn on the oscillator, but the original post stated that the lamp was flashing, so it is already been used.

Replacing the optocoupler will probably fix it, but try your test of shorting the two legs of the ldr again, but this time be sure that the intensity control is fully turned on. If the pot/ground/wiring is all ok, shorting out the ldr should kill the signal.

[spicoli]

Yes - with intensity full up shorting the LDR does kill the signal.