Polytone no Vibrato

[drewl]

Ugh...why do these amps even exist.....
they look like homemade junk

After chasing some bad grounding that was causing horrific noise from the reverb circuit, accidentally killing one of the 4793 chips dues to really stupid chassis (if you can even call it that) design...
i have the amp working fairly well except there is no vibrato.

as you can see below it uses a lovely Vactrol VTL9A5. I get no resistance reading across the pins, is there another way to test it, or I could possibly use a pair of other Vactrols to get the oscillator working.?

The two .47uf caps were just hanging off the board at one end, thought it would be an easy fix.

Of course trying to work on this annoying thing, ch1 is now fuzzy and low volume, hopefully I didn't kill another 4793, I think I have a couple spares from an old junk rusted out Sunn solid state mixer/amp.

[Jon Snell]

The Vactrol has no effect on the working of the oscillator.
Sort out the oscillator and it will probably be OK.
Pins 2 & 6 is the incandescent lamp; 10volt 22mA and should have a DC resistance accordingly.

[drewl]

Ah, so the oscillator should be oscillating on it's own?

I get no resistance reading across any of the vactrol pins, I'll double check.

[Jon Snell]

Yes, the oscillator will run on its own. The vibrato switch removes the link to ground for the lamp. The intensity control increases or decreases the current through the lamp and the speed control varies the reactance of the two amplifiers with the 0.47u capacitors.

[Enzo]

Well two of the pins are an LED, so that won't register on your ohmmeter. The other leads ought not read open, but they would show very high resistance until th LED is lit.

[drewl]

Hmm, the foot switch removes the link to ground?

It looks like it's connecting -16v to the lamp through the trim pot to me.

And I'm reading 65 ohms across the lamp, pins 2 and 6 of the vactrol.

[drewl]

Well, so far replaced weak u4 and I'm getting some warble when I rotate the speed pot, jumper a resistor across the 47k on u4 pin 6 and oscillator fires for a couple seconds.
usually replacing caps in a tremolo circuit does the trick but there really aren't any here.

[Jon Snell]

Well two of the pins are an LED, so that won't register on your ohmmeter. The other leads ought not read open, but they would show very high resistance until th LED is lit.

The VTL9A5 has an incondescent lamp. You are thinking of the VTL9/2, that has an LED.

[Jon Snell]

Hmm, the foot switch removes the link to ground?

It looks like it's connecting -16v to the lamp through the trim pot to me.

And I'm reading 65 ohms across the lamp, pins 2 and 6 of the vactrol.

Your Ohm reading means the lamp is OK.
The oscilator is a little different to the Fender type Colpitts Oscilator. It uses two inverting op amps. And phase shift is set with the 0.47u capacitors and the speed control.
If you do not know how it works, two inverting amplifiers equal an amplifier with 100% positive feedback (in amplifiers they oscillate uncontrollably, so if you slow the feedback down with one capacitor, you get a 90° phase shift, add another capacitor for another 90° shift and trim it a bit with a resistor to adjust the pasing off the 180° line and you have a controllable oscilator. The capacitors and resistors set the frequency.

[drewl]

Ha, I remember studying a Colpitt's in school, but forgot long ago.

Okay, if the amps are both working, why won't it run?

[drewl]

Okay, tried increasing the feedback resistors, nada

Decreased feedback resistor 47k and it runs.

Think I'll put a pot there to tune it.

[drewl]

Hmmm, oscillator runs too fast if I lower the 47k enough for it to run.

[Jon Snell]

Hmmm, oscillator runs too fast if I lower the 47k enough for it to run.

That is because the time constant, of the astable oscillator, is changes from 47k and 0.47u to whatever value you have inserted. The 0.47u capacitors tune the amplifier to the required frequency that has enough reactance to start oscillating. (Think of it as heavy negative feedback except at one frequency at which point it becomes possitive feedback due to the reactance of the 47k and 0.47u capacitors and oscillates).
As there are only a few small components and it is easier than spending lots of time trying to figure out any issue, I would firstly check the supply voltages and the reference voltages, (pins 8, 9 & 13 or U3 and pins 2, 3 & 6 of U4) they should all average around 0v.
If the SNE160 has a base emitter short, it will load the oscillator and stall it.
Before blanket changing the oscillator components, my money is on the 0.47u caps with the 1M across them or the +-16v supply is not correct.
Many years ago I built a flame detection amplifier for power stations. The original system had a sensor looking at the heat generated by the burning fuel. If the flame went out, the fuel continued to fill the firebox and the mess was awful, putting the furnace out of action for days.
I used a simple amplifier, almost the same as the schematic show, this showed a high gain at flame flicker frequency, ( your 0.47u and 47k does the same) and very low gain at other frequencies ... see where I am going with this.
If the gain was too high, the op amps became unstable and oscillated, like your amp should.
(Not saying I had to have that Bsc in Electronics to design the flicker sensor but people tend to listen to your ideas when you have).

[J M Fahey]

What does "no resistance" mean?

[drewl]

well the first thing I did was replace the .47uf caps, the 1Meg resistor is good, the driver transistor is not shorted and as I mentioned if I drop the 47K between the caps to like 1K, the oscillator runs but is fast with no speed control.