Fender Performer 650 Channel Switching Problem

[Stratfordade]

Spent most of today being driven slightly mad by problem with the channel switching of a Fender Performer 650. This is an amp I've had working perfectly and just set aside the other day to order a new reverb tank for it to replace the incorrect tank (input impedance only 8 ohm) in the amp when I acquired it, with resulting no reverb.

When I powered the amp up again with a 1kHz test signal the channel switching which was OK the other day no longer worked — the channel LED lit OK for Lead channel but was still on but slightly less bright on Clean. Going through test voltages as per schematic I found the DC voltage TP30 correct at the input to the switching op-amp U8B (DC measures 1.2V Clean, 0V drive) and attach scope trace of the signal at pin 6 of U8B. But at TP31 instead of +15V DC in Drive and -15V DC in Clean I found the drive voltage OK (15.3V) but the Clean voltage only about -1.6V not -15V. The scope output at TP31 (pin 7 output of the U8B op-amp) is attached too and is a square wave with a roughly 3:7 duty cycle +15V:-15V.

Because the Clean output is not a hard -15V the Q7 transistor is still being turned on during the +15V excursions and the LED is still lit. The test signal sounds very harsh and distorted but again was fine the other day so whatever is at the root of the switching problem seems to be affecting the signal too. It can't help having a 50Hz switching signal floating around!

It's very puzzling. Lost count of how many times I've installed/removed board from chassis to disconnect some components for testing. First thing I did was replace the U8 op-amp but it made absolutely no difference to the symptoms described. What makes it harder to diagnose is that the Clean input selected by the switch is not a steady DC but is derived from the +30V AC line.

Power amp section still working fine when I go in via the FX Return.

Been staring at the circuit for the channel switching and very grateful if fresh eyes on it could solve this mystery. I really thought things would go smoothly today but as it is I've not even tried the new reverb tank yet!

[g1]

It looks like the square wave is synced up and switching with the ripple at TP30. The threshold switching level is based on the voltage at pin5, check CR39 and CR40.
Also, have you tried with footswitch?

[Stratfordade]

Thanks g1 I was just about to post this:

Getting a little closer to figuring out what's happening. The signal at the non-inverting input pin 6 (derived via the 30VAC line and a couple of resistors and diode — trace on left above) is spending some time below the voltage held at the non-inverting pin 5 on U8B, so at those times the output of the op-amp shoots up to +15V instead of staying steady at -15V. Need to check all the components in the chain feeding pin 6 to see why this is happening.

The voltage at pin 5 held by CR39 and CR40 was about 1.5V but I didn't jot it down and takes me a few minutes to pop the board back in when I'll repeat the measurement. I did test the diodes in circuit with multimeter and they seemed to have correct voltage drop across them individually but I'll repeat the measurement and while board is out I'll desolder the diodes and check them out of circuit too.

I don't have the foot switch for this amp. But up till today it was always switching ok between channels just using the pushbutton switch.

[Stratfordade]

I've desoldered CR39 and CR40 and they're both good. After soldering them back in and popping board back and powering up the voltage at the top of CR39 is 1.27V so that's the threshold.

[g1]

Just to note that the switch contacts in the footswitch jack are critical. A bit of oxidization might drop the voltage at pin6.
Easy enough to clean the jack and/or jumper across the jack's switch contacts to test.

[Stratfordade]

Apologies for all the deletions above which relate to my confusing the Performer with a Marshall 8080 I've been working on at same time!

I've been incredibly dumb. I was just about to short out the foot switch contacts and was looking for R110 that connects the tip contact to the 30VAC line. I couldn't find it till I saw it right at the edge of the board. Only I'd replaced the 1K with a 10K resistor. I'd done that as the original power LED had failed and the replacement I'd put in was too bright. I did wonder why the brightness didn't change and was going to try an even higher value...R110 is right next to R109, which is the actual LED resistor.

Sound you can hear is me kicking myself very hard. All this time I'd been going over what work I'd done recently as usually problems can be traced to the last thing you've done. I'd dismissed the LED work as nothing to do with the channel switching part of the circuit.

Will change the resistor back to the original later and I'm sure it will solve the problem! Apologies g1 and any others following the thread for leading everyone astray!