Oscillating noise problem with Acoustic amp

[chikov]

Hello fellow amp gurus. Someone brought me an Acoustic G60T amp head And it was making a loud oscillating noise. At some point it got whacked on the front right two knobs. I replaced them and the noise has gotten less but it’s still there. When I pull the V2 tube, the noise stops but when I pull V1 it gets louder ( see the video, but be careful, it gets loud in the very end). My main question, besides what could be causing it, is What does this misterious Vactrol part do and If it failed can it be causing all this? In the meantime the volume1 and volume2 toggle switch is not working, LEDs are not working and the sound is very quiet.

https://youtu.be/gtV9CgPRg0Y

[professormudd]

I had an issue with the flux becoming conductive. The pics are pretty small, so I can't see for sure, but it looked like there might be some goop left. I used a paint brush dipped in nail polish remover (acetone) to scrub it off. It dries very fast.

[chikov]

Thank you for your help. Indeed the board was very dirty and I had to clean up front and the back from flux residue and the corrosion. And surprise surprise, the amp started working! Isolation went away but the second channel is very hot and when I turn gain up past 20% it starts squealing. I mean, the channel 1 works great but the double pot with split shaft Was damaged and even after I tried to fix it I may have to replace the whole thing. I will keep you posted but I see a huge progress

[Phil_S]

What does this misterious Vactrol part do and If it failed can it be causing all this?

Assuming the T in the model number is for tremolo, the Vactrol probably controls the oscillation when the tremolo is engaged. Vactrol is a brand name of optoisolator. A Vactrol is usually a lamp with a light sensitive diode. I'm doubtful it is part of the problem. Be careful with it. These things are impossible to find, or nearly so.

[JD0x0]

What does this misterious Vactrol part do and If it failed can it be causing all this?

Assuming the T in the model number is for tremolo, the Vactrol probably controls the oscillation when the tremolo is engaged. Vactrol is a brand name of optoisolator. A Vactrol is usually a lamp with a light sensitive diode. I'm doubtful it is part of the problem. Be careful with it. These things are impossible to find, or nearly so.

I believe the 'T' might stand for Tube as they had a SS or hybrid version of this amp. The Vactrols are for the channel switching.


The oscillation is probably from the poor lead dress and layout. I took a few feet of wire out of my amp just shortening runs that were unnecessarily long. I believe these amps have a reputation for oscillating.

[pdf64]

My guess is that both vactrols leds are ‘on’, which would create a low resistance path around the overdrive stages, turning it into an oscillator.

[chikov]

My guess is that both vactrols leds are ‘on’, which would create a low resistance path around the overdrive stages, turning it into an oscillator.

That sounds like the answer to my problems... is there any way I can fix it?

Another video:
https://youtu.be/WxzLEUy0gFI

[pdf64]

You need to verify whether my guess is correct, eg have both vactrol leds got Vdc across them?
Or maybe a vactrol LDR element has failed such that it’s a constant lowish resistance? Dunno how likely that is, LDRs tend to fail high, but it's worth checking for.
Or maybe there’s a low resistance path where there shouldn’t be, due to gunk and dirt mentioned previously?

There’s some things for you to be getting on with

[chikov]

You need to verify whether my guess is correct, eg have both vactrol leds got Vdc across them?
Or maybe a vactrol LDR element has failed such that it’s a constant lowish resistance? Dunno how likely that is, LDRs tend to fail high, but it's worth checking for.
Or maybe there’s a low resistance path where there shouldn’t be, due to gunk and dirt mentioned previously?

There’s some things for you to be getting on with

What makes it difficult is that each VActrol has seven legs and I have no idea what they go to. Do you have a link to their schematic - I looked but haven’t found yet. I did clean The circuitboard one more time but the oscillation did not go away

[chikov]

I am still not sure , what do vactrols do, but Would a failure of these components cause the channels to be shorted together? And that would explain the oscillation and hi squeel?

[Phil_S]

A Vactrol has two elements. One is a lamp. The other is a LDR (light dependent resistor) or maybe some other light activated semiconductor. If it fails, I can't imagine any scenario where it fails with the lamp on. If it failed with the lamp on, the lamp would burn out. This particular Vactrol has an incandescent, suggesting if it could fail in the on position, it wouldn't stay on for very long until the lamp burned out. I found and attached the data sheet. I didn't look closely, but I expect it explains the 7 legs.

My conclusion is a failed Vactrol would fail as an open circuit. The result is that the tremolo can't be activated. In a Vactrol controlled tremolo, the wiggle is produced by the circuit cycling the lamp on and off.

Does anyone see this differently?

[pdf64]

The vactrols used in the Acoustic I’ve seen were an early type in a metal can
Schematic https://el34world.com/charts/Schematics ... g60t_2.pdf
When the footswitch or front panel Vol1-2 switch opens the circuit, the incandescent lamps of Vactrol1 and Vactrol2 go dark, and their LDRs go high resistance, which attenuates signal going into the Vol 1 clean channel, and disables the muting of the Vol 2 drive channel.
My guess is that Vactrol2 lamp is dead, so the drive channel is always operational, doesn't get muted; and the switching is stuck (or always left) in the clean channel mode. Hence with sufficient drive channel gain and vol, the drive stages turn into an oscillator.
So, try and identify which vactrol pins are which and so verify if the V2 lamp is open circuit.
In which case V2 can be replaced by a 5C3/2 https://www.ebay.co.uk/itm/Xvive-Audio- ... SwrVRcWJW1

[chikov]

Thank you for this much needed info. I identified both Vactrols. When the amp has no power, both LEDs between legs two and six show 51.7 ohms resistance. However the V1 that is located on long narrow circuit board with potentiometers has no resistance between legs one and seven and three and five ( light sensitive resistors). On the V2 that is located on the main printed circuit board I have 27 kOhms between the leg one and seven and 2.1 megohms between the legs three and five. At this point I’m not sure which one is showing abnormal resistance. Would you be able to tell me?
Update. When my amp is in channel 1 ( the clean), Both LED lights get 8.7 V DC. When you switch to channel two, both loose that power. When the LEDs are on I get about 1000 ohm reading on V1 ( both LSR resistors) and about 500 ohms on Both of V2 LSRs

[pdf64]

So when switched to Vo 1, do the Vol 2 gain & vol controls have any effect?
And when switched to Vol 2, does the Vol 1 vol control have any effect?

...the V1 that is located on long narrow circuit board with potentiometers has no resistance between legs one and seven and three and five ( light sensitive resistors). On the V2 that is located on the main printed circuit board I have 27 kOhms between the leg one and seven and 2.1 megohms between the legs three and five...

What do you mean by 'no resistance'?

...When the LEDs are on I get about 1000 ohm reading on V1 ( both LSR resistors)...

So do those change when the V1 bulb is off?
I think these will have a filament bulb, rather than an LED, between pins 2 and 6.

[chikov]

So when switched to Vo 1, do the Vol 2 gain & vol controls have any effect?
And when switched to Vol 2, does the Vol 1 vol control have any effect?

...the V1 that is located on long narrow circuit board with potentiometers has no resistance between legs one and seven and three and five ( light sensitive resistors). On the V2 that is located on the main printed circuit board I have 27 kOhms between the leg one and seven and 2.1 megohms between the legs three and five...

What do you mean by 'no resistance'?

...When the LEDs are on I get about 1000 ohm reading on V1 ( both LSR resistors)...

So do those change when the V1 bulb is off?
I think these will have a filament bulb, rather than an LED, between pins 2 and 6.

When I am on volume one channel everything works perfectly well - loud and clear, And when I move potentiometer on volume two - nothing changes. When I switch to volume two, The part on the volume one has no effect on it. The outer ring, the Vol2, works adding the volume , but as soon as I go past 2 on the inner (Drive) shaft, The amp starts squealing.

When I said that the legs one and seven and three and five have no resistance that means that my multimeter shows Open Line.

Yes , when I switch from volume one to volume two voltage to the filament bulbs disappears on both Vactrols

Should I remove both Vactrols And test them then?