Marshal TSL 60 channel switching from OD1 to OD2 problem

[jasonguitar]

I recently acquired a TSL 60 really cheap because it "needs a little work". It was humming really bad and wouldn't switch from OD1 to OD2. Switches from Clean to OD1 fine. I replaced the screen resistors(one had fried) and tubes and the amp biases properly and sounds great but I haven't been able to determine the problem with the switching from OD1 to OD2. The OD2 indicator on the front panel won't light up(the LED is good though) but when the footswitch is connected, the OD2 indicator on the footswitch does light up but not on the amp and no actual channel switching occurs. Someone has been in here doing a lot of hacking previously. You can tell they pulled out and reinstalled several transistors, mosfets and one IC. I started doing basic tests on a few of the devices I thought might be suspect and I did find one MOSFET F8 that was faulty(there was evidence that it had been previously removed and re-installed as well) but replacement did nothing. Any tips on specifically which devices could be the culprit before I go removing every one like the hack before me? I'm looking over the schematic and I'm not exactly hip to how these newfangled channel switching circuits work. Much obliged!TL60-60-02 (2003) iss7.pdf

[MarkusBass]

I don't know what you mean by OD1 and OD2. But if you are looking at channel switching, they are switched by IC3 - M5201 from Mitsubishi (double input, single output). It has also SW pin (#1) which is used for switching between inputs #1 and #2. Just check whether the voltage change on this pin.

Mark

[jasonguitar]

I don't know what you mean by OD1 and OD2. But if you are looking at channel switching, they are switched by IC3 - M5201 from Mitsubishi (double input, single output). It has also SW pin (#1) which is used for switching between inputs #1 and #2. Just check whether the voltage change on this pin.

Mark

Thanks for the reply. I was referring to the front panel labels for the channels which say OD1 CRUNCH and OD2 LEAD. So I am specifically trying to determine why there is no switching between CRUNCH and LEAD. Since there are two sets of switching functions happening, (1)CLEAN to CRUNCH which works properly and (2)CRUNCH to LEAD, I need to isolate the latter's probable suspects.

[MarkusBass]

OK, so this most probably OVERDRIVE1 and OVERDRIVE2. The schematic is not perfect and it seems to me that it does not tell which switch is responsible for this switching. I suggest that you check on the printed board what are the switch symbols for OD1 and OD2 switches. Then the switching is most probably done with JFETs. It looks like IC3 switches between clean and overdrive channels.

Mark