Fender Super Champ X2 Channel 2 and FX not working after op amp switch

[pipokoeie]

Hi everyone, I hope your weekend starts better than mine!

I recently acquired a Fender SCX2, and after seeing the board when re-biasing I felt confident enough to swap out the first op amp for a socket to try out some different ones. I could not get the PCB out so I it was a bit tight to work on, and at first my efforts resulted in a fail. Fizzy and faint sound from both channels. I tested for continuity on all traces going to and from the socket and all tested ok but it didnt work. After closer inspection one solder joint seemed a bit iffy and lo and behold, after fixing it I had sound! I put the amp back together and channel 1 worked like a charm. Now after a while I tried switching on some effects, but no cigar. Switching to channel 2 also did not work. The light does not come on and its not switching to channel two and giving no sound. Its just as if the whole front is not working, except the treble and bass and volume knobs. Also when I turn the FX switch knob, there is a small (non permanent)drop in volume. ( I did double/triple check all the ribbon connectors)

So I found the schematics online (attached) and being a newbie with this stuff (yes i know maybe i should have refrained from switching op amps ) i 'm not sure what to measure and what conclusions to draw. For instance, on the pcb itself, there are a lot of testpoints but almost only on the breakout board with the dsp on it. So when the schematic references TP16, there is only one to be found and thats on the breakout board. And when I measure the points there, I do not get any readings that make any sense. Take TP16 for instance. I set the multimeter on V~ and it gives me 0.38 with the scale set to 20. But wouldn't that be 380mVaC? Also if I try to measure in the circuit (where the line is pointing) I get nothing.

Also if I measure R52 it gives me 13.21K resistance with the meter set to scale 20K, but the schematic says it should be 1M. The color code on this one is brown-black-green-gold, which should be 1M if im not mistaken. R91 next to it on the otherhand is giving me 1.94K@20K scale so that's good i guess.

I hope some of you can point me in the right direction!

[Jazz P Bass]

For reference, the TP voltages marked in a 'square' are Volts dc readings.
Those in a 'circle' are the expected Volts ac readings if the test parameters are adhered to.

Also. 0.38 Volts ac is 380 millivolts.

[pipokoeie]

tnx, I thought so. The schematic says it should be 0.05 there so i'm wondering if the problem lies there (maybe because the 1M resistor is too low?)
Ill try to read up some of this stuff. Thanks for the reply!

[nosaj]

tnx, I thought so. The schematic says it should be 0.05 there so i'm wondering if the problem lies there (maybe because the 1M resistor is too low?)
Ill try to read up some of this stuff. Thanks for the reply!

Is the resistor measured with one leg lifted? in circuit resistances do not always measure correctly.

[pipokoeie]

I will try this, thanks. I was a bit hesitant because i never worked on an amp before, but I got a broken champion 600 off the internet and fixed it. So when I have tiime I will take the x2 apart and measure all parts in the preamp section. Will post an update!

[patlaw]

I have to wonder if you boogered the PCB when you changed the op amp. The holes are plated-through. If you're not careful, you'll pull the plating out of the holes, and the top traces of the board do not connect reliably to the bottom traces. It's just a thought.

[pipokoeie]

I have to wonder if you boogered the PCB when you changed the op amp. The holes are plated-through. If you're not careful, you'll pull the plating out of the holes, and the top traces of the board do not connect reliably to the bottom traces. It's just a thought.

That is a good suggestion actually, thanks for pointing that out.
The strange thing is that apparently the usb connection still works: The pc sees the amp, and the controls change on the screen along with the real ones as I turn them. Just none of the FX work and the channel switch.

I have some time this week and will post an update is I find anything!

[patlaw]

Did you remove the effects board from the main board? If so, make sure there is not a bent pin on the daughter board connector (if there is one. I'm not familiar with this amp.)

[pipokoeie]

Yes, the dsp/fx board is there and I did remove it (pic is attached on first post). I thought I checked the pins but will check again when reassembling.

[pipokoeie]

Well I found the culprit it seems! Transistor Q1 was the problem, I think. I am not sure how that part of the circuit works, but I started from the switch and before Q1 there was nothing but after (seen from the switch side of the circuit) there was voltage. I jumped the outer legs of the transistor and everything was working.
Removing the pcb from the chassis is a bit of a hassle so for now I clipped the transistor and soldered a jumper between the outer legs. I will replace it later to see if it was indeed the transistor or maybe the footswitch jack that was bugging. I do remember it did not come out of the chassis easily.