Fender Vibro-King CSR4 tremolo speed issue...

[pecorporation]

This amp will only engage the tremolo effect once the speed control is set to 10 and wait one second. From there, you can back the speed control down to 7 and still have the tremolo effect. As you turn the speed control from 7 to zero, you loose the trem effect and the neon bulb just stays lit on the roach.

I have -68VDC going into the speed circuit & 475VDC "Z" voltage. I've replaced the V6 trem tube, the LDR roach, all three RC circuit caps & resistors, and both cathode bypass caps. The footswitch jack is shorting to ground and both speed & intensity controls are 5Meg verified.

Anyone run across this before or have any ideas?

Thanks in advance,

Steve

[Enzo]

With the circuit running, or at least set up to run, that -68v ought to be gone within the circuit.

Isolate the problem. If the neon stays steadily lit, then the roach is not the problem. The circuit has an oscillator stage and a drive stage. If the oscillator won;t run, then the drive stage has nothing to do. If the oscillator is running OK< then something about the drive has failed.

Your schematic has three revision levels, which one suits your amp?

[pecorporation]

The bias supply (brown wires) suits REV A while the oscillator stage suits REV E.
The -68V drops after R58, 2.2M.

[Enzo]

OK, so the -68 is grounded out, good. Now how about those other things? Is the oscillator still running when the drive stocks steady on? Or is the drive faithfully doing what the defective oscillator tells it?

[pecorporation]

It appears the drive is faithfully doing what the oscillator tells it. I put a scope on the output of the oscillator and when the wave form was on the scope, the neon was flashing. When the wave form went to zero (SPEED set at 7), the neon was steady on.

[pecorporation]

Well, I looked at some other Fender tremolo circuits and found most of the speed controls are 1meg to 3meg linear pots. I replaced the 5meg pot, per spec, with a 1meg pot and changed 270K R53 to 150K and the tremolo circuit came to life and now works as it should. Hmmm.