Crate VC-3112 stuck on B channel. Help please.

[Dub8er_e]

I am repairing a VC-3112 for a friend of mine. I have the full schematic and have already tried replacing the dual switch and footswitch jack without any success. The amp will not switch back to channel A. If anyone has seen a problem like this before I would appreciate the help. My friend lost the footswitch but cant remember if he had the amp on the B channel the last time he was using the foot switch. I used to have a fender amp that would get stuck on channel B if the footswitch was unplugged while on channle B. Dont know why though. any help would be greatly appreciated.

[Prairie Dawg]

Originally Posted by Dub8er_e

I am repairing a VC-3112 for a friend of mine. I have the full schematic and have already tried replacing the dual switch and footswitch jack without any success. The amp will not switch back to channel A. If anyone has seen a problem like this before I would appreciate the help. My friend lost the footswitch but cant remember if he had the amp on the B channel the last time he was using the foot switch. I used to have a fender amp that would get stuck on channel B if the footswitch was unplugged while on channle B. Dont know why though. any help would be greatly appreciated.

they use optoisolators for channel switching, don't they? Maybe one of them is on the fritz. You could probably try bridging them one by one to see what happens.

[Dub8er_e]

Thanks for the quick reply.Yes they do use optoisolators for the switching. Now I just have to find the one thats on the fritz. Visually they all look good.

[Enzo]

Let's not guess. Your footswitch turns something off and on. Plug it in and see if there is a control voltage coming and going with the switch.

SInce you can;t switch to A with or without the footswitch, I suspect the amp circuits.

Is the solder OK to the FS jack?

Look at teh schematic. The FS jack has a resistor to +v - ab0ut +15v. and a 3.3v zener to clamp the voltage. With a blank plug shoved in the jack, is there about +3.3VDC there?

Now note the switching circuits, they run on -10v, NOT positive volts. The +3.3v through the two diodes is enough to keep Q1 turned off. Per the test points, that means the collector of Q1 will have -10v. And since Q1 control,s Q2, whatever is on Q2 will be the opposite of Q1. SO when collector of Q1 is -10, then the collector of Q2 will be zero.

And when we ground the tip of the FS jack - as a stomp switch should do - that kills the +3.3v No +3.3 coming through D3,D4 means the base of Q1 can be pulled up to -10v, and that in turn will bring the collector to zero. And at that point then Q2 will turn off bringing its collector to -10v.

So, the question is, when you click the channel footswitch - or flip the front panel switch without a FS - does the voltage at the base of Q1 change from about +2 to about -.5? Does the collector of Q1 toggle between -10 and zero, and then the collector fo Q2 toggle from zero to -10? Assuming the 3.3v zener is not shorted.

[Dub8er_e]

Thanks Enzo. Unfortunatley my friend lost his footswitch but with the info you gave me I should be able to do a complete diagnosis without wasting time. This was exactly the kind of info I was looking for. Thanks again. I will post a status report when I have completed the diagnosis.

[Pitsi]

Hello,

Sorry to change slightly the title.

I was given an old Crate VC-3112W as a gift to my son who plays the guitar.
Browsing in the internet I saw this forum and I saw that Dub8er_e who is also reparing the same amplifier has the schematic diagram.

I was wondering if he has an electronic copy of it to send it over to my email address: dimispitsi@yahoo.gr.

My Amp has one of the EL84 tubes broken and also the only 10,000uf/16V capacitor is swollen. Those are the "visible" problems.

I would also like to have the user manual of this little beast as the link in Crate site for this model manual is not working

Finally I would like to ask if anyone knows whether the two pairs of the EL84 tubes are matching pairs and where I can find a full set of new tubes the SOVTEK EL84 & the SOVTEK 12AX7W or 12AX7LPS for better sound .

If I manage to have this Amp up & running by Christmas I would make a big surprise for my 15year old son !

Many Many thanks in advance to anyone who can help.