GK Backline 600 channel problem

[catstrat]

I guess my last post didn't get posted. I'll try again. I did some voltage test and here is what i have. Voltage at R 15 on the rail side is at a steady 14 volts +/- 200 mv. The voltage on the LED side of R 15 is swinging wildly between 13.5 volts and 3 volts. This happens after the amp is on for about 10 minutes. I changed R15 to see if that affected it and ,no change. This morning i removed U6 and installed a socket. I have some analog switches on order. These voltage tests are done with U6 out of the circuit. I removed the panel switch a few weeks ago and the channels still randomly switched. Enzo suggested maybe the LED itself but i subbed a new LED and still no change. The supply voltage seems ok and the problem starts on the LED side of R15. Baffling. Soooooo, i'm running out of things to check.
I told the customer, if push comes to shove ,maybe i can rig the amp so it stays on the clean channel only. He plays jazz and blues and he never uses the distortion channel anyway. But it would sure be nice if i can fix it right. It's almost like something in a parralel circuit is shorting creating an alternate ground for the switching circuit. But what? Are there any zeners that aren't shown on the schematic? Thanks for all the help.

[Jazz P Bass]

In a properly working circuit, the only way to illuminate the LED & activate Ch B is to ground the LED side of the 22K resistor.
That can be accomplished by the pushbutton switch (which in my case, was the culprit) or the footswitch jack.
Measure the top of the 221K to ground when Ch A is selected.
Insert an open plug in the footswitch jack to rule out the contacts.
I'll bet you will get a reading.
In that case it is the pushswitch.
Pull the 221K & Ch B will not activate.

[catstrat]

switching issue

Hi Jazz p. Thanks for hanging with me on this problem. I removed the panel switch and tested the amp and the LED still flickers. I put an open plug into the footswitch jack and it's still flickers. I'm thinking it's time to choose option 2. Rig it so the clean channel is on all the time. I look at the schematic and everything has already been replaced. So let me go and revisit the schematic again and spend some more time with it. Thanks.

[Jazz P Bass]

I don't know.
There is a 0.1 uf cap in there.
It looks like a debounce cap.
Again. The LED needs a ground reference to illuminate.
Where is it coming from?
From the bottom of the LED to ground. (amp off)
What do you read?
It should read infinity.
Remove the 221K resistor & read it again.

[g1]

There is a 0.1 uf cap in there.
It looks like a debounce cap.

Thinking JazzP has it there. The cap is probably breaking down after your 10 minutes or so. It probably measures fine when powered down, so remove it and see what happens.

Edit: I see Enzo had already mentioned this in post #11, "C45 could be trying to short, we could remove it and see if it makesd a difference".
Not sure which drawing is correct, the cap could be called C21 or C45, or C22. In any case it is at the footswitch jack.

[Klep]

catstrat, I picked up a used BL600 a week ago, and it seems to have nearly the same problem. In my case, the overdrive LED is always on, but gets brighter when pushing button or foot switch. Whichever channel you're playing on cuts out after a while. Fan never comes on.

I've cleaned all the contacts and checked what I can, including setting bias to 5 mV, as mentioned in another post, but I'm not nearly as handy as all you guys. Eager to hear what the fix is, whether it disables channel B or not.

Does your fan ever spin? 9V battery makes the fan spin, but it never gets going while hooked up to the amp...

Thanks!

[catstrat]

HI Everyone and thanks for the support on this. I finally got in touch with GK and they will sell you a new pre amp board for about $100.00 bucks. That cured it. Sometimes it's easier to do it this way. Thanks.