Crate Volume Control Problem

[RWood]

The one remaining problem with this Crate TV-120 is with the volume control. It functions as it should from 10 down to 4, then the rest of the way down to 1 it does not lower the volume, but changes the tone, kind of like a wah.

I thought "bad pot" and removed it from the pcb, only to learn that it tests fine. A gradual 1 Megohm to 0 ohms from the wiper to each leg.

In looking at the schem (portion attached) I see that the signal leaves the volume control and goes through an optocoupler en route to pin 7 on V1. I assume this is to power the channel-indicator LED.

Would a bad optocoupler (this one is a VTL5C6) cause the problem I'm hearing? Or does something else come to mind?

Thanks,

Richard

[Robert M. Martinelli]

Hi,
if the pot is good, with the wiper set to "0" ( GND ) it is impossible for the signal to get to the valve grid ( pin 7 ) through OC3, so I'd say that some signal is leaking through the other LDR ( OC2 ).

Bear in mind that VTL5Cx are not simple opto-couplers ( an assembly of an LED and a photo-transistor ), but LDRs ( an assembly of a light emitter, in this case an LED - a lamp back in the old days - and a photo-resistor ). This means they're not "on-off" devices, but "modulating" ones. Check voltages on the LED side of the OCs, with voltages below the LED threshold the photo-resistor should read some MOhms, increasing the voltage over the LED threshold and the FWD current the photo-resistor should read less and less resistance. If some signal leaks through an LDR this means that either the LED side of the LDR is still energized ( check the channel switching ) or that the LDR is defective.

To test an LDR, pull it aside from the circuit, connect an Ohmmeter to the LDR side by means of alligator clips, then supply the LED side paying attention not to exceed the LED ratings.

Hope this helps

Best regards

Bob

[Enzo]

I assume this is to power the channel-indicator LED.

???????


The LEDs in the optos control the resistance of the opto. They have nothing to do with panel indicator LEDs. Other than that the circutis might turn them on and off at the same time.

[Robert M. Martinelli]

+1.
( I ignored that paragraph because I was not sure about what he was meaning ).

Richard, your problem is signal-related, and LDRs in your circuit are along the audio signal path, they are usually controlled by the channel/sound switching circuitry.

With the volume pot to zero ( wiper to GND ) no signal can pass through OC3, and, since you stated that below 4 the volume does not decrease and the pot behaves like a tone control, my guess would be that OC2 lets some signal through.

HTH

Best regards

Bob

[RWood]

???????

Heh heh..... I had a feeling if I said something wrong I would get Enzo's attention! I should have said "channel-switcher", right?

Thanks for the responses, though. I have a lot to learn about these new-fangled amplicators.

From what you are saying, Bob, it is not the LDR associated with the Normal channel (which by way of the volume control has the signal grounded and silent when the pot is on 0) but the OTHER channel's LDR - OC2 - which is leaking some signal through, even when that channel is not on?

So the short answer sounds like a "yes" and it looks like I need to track down a VTL5C6. For Crate parts like this, is it effective to go through Loud Technologies or is it better to go through a electronics supply dealer? I see that Allied has them for six bucks.

If you think I should remove it and test it before ordering a replacement, let me know. The one thing I didn't follow about your description was how to energize the diode in there, so that the resistance side can be measured.

Thanks,

R

[Enzo]

Generally one goes through an OEM like Loud for specific parts that are not otherwise available, like pots, power transformers,and such. Electronic components are usually best ordered from a general parts supplier.

[RWood]

Thanks, Enzo, I did get them from Allied.

I replaced both OC2 and OC3 and am still experiencing the same phenomenon.

Something I noticed this time is that the 'noise floor' tapers all the way down when lowering the Channel One volume control. It's just that the guitar volume dips at around 4 and then levels out, as the pointer goes down to 0. Channel Two, btw, functions normally with its two volume controls.


At first I installed the LDRs upside down, so that the polarity on the LED side was reversed. Then corrected that and am getting results similar to my original problem. Did I do any damage when the LEDs were wired backwards? When on and in-circuit, each LDR's diode gets around 2vdc, switching to 0vdc when off.

I now have a standalone low-voltage supply source, so I'llable to test them all while they are out of the circuit next time I flip the board.

Any further thoughts on why Channel one is behaving this way?

Thanks

[RWood]

Well, I tested the old LDRs, out of the circuit, and they both seem to work. When voltage (2vdc) is applied to the diode side, the resistor closes up and signal flows through, and when no voltage is applied it opens up and has resistance of over 2M.

I am assuming that the new LDRs are functioning that way, and I did some tests on them IN circuit - they look as though they are working. I am pretty much stumped.

Is it possible that the volume pot, which looked fine under test while out of the amp, is behaving differently in the circuit with signal going through it?

Richard