Marshall TSL100 Channel/Switching Problems

[The Dude]

Brain fry. Been battling a TSL100 for a while now. Here's the deal. The crunch channel volume control does nothing. You can get sound out of the channel, but it's only level control is the gain control. Even if the volume control is all the way down, you can still get output using the gain control. Basically there's no "master volume" control on the crunch channel.

Here's what I've done so far.
1) Checked the volume pot. It's fine.
2) Scoped the volume send to the lead channel board. It's there and the level changes with the volume knob.
3) Checked all of the opto's. All of the LED's in the optos check good with a DVM.

Other curiosities:
OPT6 on the lead channel board (which is fed by the crunch channel volume out) with about a volt of input has about .1V on the other side. I'm not sure how much loss if any is to be expected.
If OPT6 is indeed defective, why would you still get audio controlled by the gain control.

I'm happy to provide any other info needed to get this thing off my bench.

I tried to upload the schematic, and the site wouldn't let me. I'll try again later.

[The Dude]

Still can't upload, but here's a link:

http://www.webphix.com/schematic%20h...sl100_100w.pdf

[g1]

Is there something that shows where CON2 and CON3 on page 6 go? Or is it just supposed to be intuitive .
It's not like they're the only CON2 &3, seems to be one on most every page.

[Enzo]

There actually is a page that covers it, a chart of connectors for each board, and what connector on what board it goes to. Not sure where to find it now. yes, every board has its own CON 1,2,3... The schematics are usually of boards, not the system.

When a volume pot stops having effect, it usually means the bottom end is not finding its way to ground. Get out your ohm meter and check.

Checking the LEDs in the otpos is fine, but the opto is a variable resistor the LED controls. What you need to check is if the opto resistor changes when the LED goes off or on.

[The Dude]

Had already checked the ground side of the pot and it is grounded. After further investigation, OPT6 is defective as I suspected. I checked it against another known working one in the amp.

Good part= Off state is infinite, on state is around 550 ohms across resistor.
Bad part= Infinite on and off across resistor.

I'm going to track one down and go from there.

Thanks for your help!

[g1]

Here's a link to that missing connector diagram, thanks to DrGonz78 for posting it elsewhere.
http://music-electronics-forum.com/a...nnections2.jpg

[The Dude]

For those playing along, a replacement of OPT6 has solved the problem. Thank you all.