Fender Custom Vibrolux Reverb - Vibrato dead...

[dporto]

The Vibrato on my CVR has just gone away. I've already determined that the footswitch is ok, and even without the switch, the vibrato isn't happening. Anybody have any similar experiences with this amp?

[Jerry]

It's probably the roach that's bad.
Sometimes it's a cap or two in the oscillator.
Rarely is it one of the pots.

[dporto]

Ah, the roach... Uh yhea, I think I smoked it ...Really though, where might the "roach" be found? (please don't say the ashtray). FWIW, I did check the neg. voltage at TP32 and it was correct (-45vdc). I also poked around with the ol' chopstick and nothing obvious happened. Where exactly is the oscilator circuit located? Thanks

[52 Bill]

That amp doesn't have a roach. It modulates the output bias, more like a Princeton Reverb.

The oscillator is built around tube V5. Try replacing that tube first. Check to see if all of the test point voltages at TP28-TP31 come close to the schematic values.

If the oscillator is working, but not getting to the output stage (at TP32), check R56, C28, C29 and the Intensity pot R57.

[dporto]

Thanks for the info Bill. I did a tube swap on V5 with a working 12ax7 - no change. I haven't checked tp's 28 - 31 yet so that'll be my next step. I'll let you know the outcome.

[dporto]

allrighty then, here are the facts: all values are Vdc

TP 28 = 0
TP 29 = .87
TP 30 = .9
TP 31 = .002
TP 32 = -47.1

I tried several different tubes in V5 (4th tube from the left including the 6l6's, or the 5th tube from the right if you're looking at the amp from the rear) with no change in sound or voltages so i assume the tube is ok. Any ideas on what the problem is or where I should start replacing things? Thanks in advance

[MWJB]

First thing I'd check is voltages at either end of R64. There's a lot of B+ at the "Z" end of R64 for a 1/2W resistor, I wouldn't be surprised if it has burned out. If so replace with 1W or better.

[TD_Madden]

does this help?

[dporto]

Thanks TD, but I've got the schematic as well as the layout with the test points (see my voltages above).
MWJB, what should the voltage be @ the Z side of R64? What will it be if the resistor is bad (same at both sides? 0?). It seems like this resistor is at the "tail end" of the circuit with a bunch of improper voltages in "front" of it...Or, am I looking at the circuit backwards? Once again, any help is much appreciated!

[MWJB]

Point "Z" on the B+ rail (hot side of R64) should have 436v -ish. As TP30 has only 0.9v on your amp, this suggests that something is causing massive resistance at R64.

[dporto]

Ok, thanks. I'll give that a shot. I know I have found several points in that immediate area that do have that sort of voltage - I just don't rmember off hand exactly where they were. So you're saying that if there is high voltage on the z side and little or none on the other side, the resistor is bad and needs to be replaced? Going with a 1 watt resistor of equal value is ok, or should I double the value as well ( obviously my physics/electronics knowledge is lacking, but I like to learn...). Thanks

[MWJB]

"So you're saying that if there is high voltage on the z side and little or none on the other side, the resistor is bad and needs to be replaced?" That's the idea. Keep the same value in ohms. That 1W rating is a minimum, have a look at the board, if it will fit a 2W/3W wouldn't hurt.

[52 Bill]

MWJB is right about R64, but I wouldn't go more than 1 watt, as I doubt that the board will allow it to fit in.

Also be sure to check C37, because if it has shorted out, it may be the reason the resistor opened up in the first place.

[dporto]

Ok thanks guys, how should I check C37 ?

[MWJB]

Only one end should be grounded, check both ends to ground with a continuity/diode test (amp unplugged).