Randall LB103 Channel Switching Issue

[tdlunsfo]

This amp showed up on the bench today with weird channel switching problems. Upon opening the amp I saw a dreaded microcontroller that appears to control this function. I found a close schematic below. Does anyone have any experience with this amp?

The amp will not switch channels from the front switch or the foot pedal. Depressing the front switch, I lose all channel lights. Plugging in the footswitch yields similar results. No output on amp even though tubes are lit and fuses are good.

https://el34world.com/charts/Schemat...H_20091211.pdf

[The Dude]

I'd first check that all power supplies are working.

[tdlunsfo]

I actually just found the schematic and was thinking the same thing. Weird that I have no sound, and that kind of leads me right to the supplies. You'd think even if the microcontroller was bad it would be stuck on one channel and play.

[tdlunsfo]

Something was dragging down the 5v rail and after pulling the microcontroller the load dissapeared. I checked all the optocouplers and they seem fine. All the switching transistors check fine. It took me a bit but figured out that if I ground the collector of Q4, the clean channel pops in. If I remove the ground from Q4 and apply 5v to R200, I get OD1. Similarly, if I put 5v on R201, I get OD2. I guess if Randall can't come through with a new programmed microcontroller I can simply put the 3 modes on a rotary switch. It would function, but would loose the footswitch capability. Any other thoughts? I don't see the clock running on the microcontroller and placing my sig gen in its place doesn't yield any results.

[tdlunsfo]

For anyone that stumbles on this post and runs into this issue in the future I have bad news, Randall no longer supports the microcontroller for this amp. Although, you can still make channel switching work by installing a switch that will send 5v to r234 for clean, remove 5v from r234 then send to r200 for OD1, remove 5v from R200 then supply 5v to r201 for OD2.Your original channel switch button and footswitch will no longer function but the amp will be usable. A three pole, three position switch will get it done.

[Jon Snell]

The chips was made by Holtek in Tiawan, still available I believe.
They can be pre programmed and the programmer is available to those who want to make an EPROM reader/writer from the schematic.
I would use a simple A-D circuit, based on the DMX system, to carry out that function. A 339 comparator will do the job nicely, allowing for three foot switches and a single guitar jack lead to drive and supply the 0/2.5/5volts or whatever voltage you deem are required.

[Phil_E]

Mine is collecting dust for over 3 years now. Jon please elaborate more if you can so I can get it working! Cheers