Solid State Vox AC30 vibrato problem please help!!!

[sinnerboy]

Hi,
I have a Solid State Vox AC30. I've managed to repair most of its problems but I can't work out why the vibrato stopped working. Here's the schematic  http://schematicheaven.com/voxamps/ac30ss.pdf
The channel works fine but when you press the vibrato foot switch nothing changes but it still passes audio.  Im sure its not the foot switch at fault, its working fine.  I think the main reason I can't repair it is because im not sure how the vibrato circuit works.  If anyone could offer any guidance or advice on what I should look for it would be most appreciated.   

[teemuk]

The 2N3819 FET depicted on left is the tremolo oscillator amplifier. Randall Aiken's website has an article covering the operation of this type of phase-shift oscillator circuit. The article is written with tube amplifiers on mind but the theory is completely interchangeable.

http://www.aikenamps.com/PhaseShiftOscillators.html.

Basically, the amplifier incorporates a feedback loop with enough RC elements to phase shift the feedback signal and make the amplifier oscillate.

The output of the oscillator amp drives another 2N3819 FET. It is configured as a variable resistor that changes its resistance in proportion to drive signal. The variable resistive element is on the signal path of the tremolo channel.

If it doesn't work simply check out if you get any output signal from the oscillator or if the resistive element (FET and the surrounding circuitry) has failed.

[sinnerboy]

Hi,
Thanks for your help.  Pardon my ignorance but Im still struggling to understand whats happening.  Does the audio signal go through the RC network and the two 2N3819 because I can't see how it is connected to the rest of the circuit

[J M Fahey]

Hi. It doesn't exactly go "trough" the 2N3819's.
Signal goes from IC-1C to IC-2B through a network of resistors and capacitors. Not only that, part of the signal is fed to the inverting input and part to the non inverting one. I can't say I fully understand this, but it does strange things to phase and frequency response.
In fact it is similar to "one" phaser cell, a crude one.
It would be nice if somebody (hint hint) simulated this part of the circuit and posted the frequency response for various values of a "virtual resistor" replacing the rightmost 2N3819.
I'm not the simulator kind guy . (This sounds good, eh? Yet the girls don't believe me    )
Back on track: imagine the Fet I mentioned is a potentiometer of, say, 100K, wired as a variable resistor to ground.
You´ll say: what ground? 
It's wired to the +side of a 100uF capacitor, in parallel with a 9.1V Zener, and fed DC by a ¿2k7? resistor !!!!!
For  *audio* purposes, that point is ground.
Now you understand that "turning that virtual pot" back and forth will vary the resistance to (audio) ground and attenuate the signal rhytmically.
That's what the Fet does, it works as a variable resistance controlled by the voltage on its gate, which is a very low frequency wave generated by the left Fet. Read the link offered by Teemu.
To check your circuit, with the footswitch "off" (shorted)  inject signal into that channel, hear it on the speaker, and vary the bias preset setting, the audio should lower at some point. If not, either you don't have those 9.1 Volts (check why), your Fet is dead (replace it) or the preset is broken. Also check the area for broken tracks, cold solder, etc.
If it works, check that the oscillator works (a scope or a "needle" multimeter is better because you see the wave or the needle flickers) but if not, a Digital one on the 20VDC scale should show some erratic but rhytmically varying voltages.
Please post a few pictures (in and out) of that amp, I heave never seen its guts, nor the speajkers it uses, which I gess were not BVox Blues anymore.
PS: the available schematic is quite unreadable, if you have some spare time, write the actual values that you see on the board right by the unreadable ones (or all) and repost it, it will be *very* helpful.
Teemu has "reconstructed" one, but there are still some values missing.
Thanks.