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Post subject: Super Champ X2 channel 2/fx not working
Posted: Sat Feb 21, 2015 10:01 am
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So I recently got a SCX2 (combo), great amp! I decided to check the bias and it was a bit under. Set it to specified 40mvDC and reassembled. I noticed the opamps were pretty accessible and a friend of mine had some lying around soo..... i decided to desolder two of the TL072s and replace them with OPA2132s. I actually put in sockets so I could swap them out and see whats what. Everything went super smooth, but... After playing the clean channel I tried to switch to channel 2, but the button did nothing. Pressed or depressed, channel1 stays on. The only controls that are working now are volume treble and bass. I found a schematic and tried to measure various test points but my troubleshooting skills aren't that high that I can look at the schematic and readings and deduce what is wrong. I thought maybe the dsp died but if so there should be no sound at all right?
Anybody got a clue where I might look?
Cheers!
(yes i know i totally voided my warranty, but i just had to try! :P )


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Post subject: Re: Super Champ X2 channel 2/fx not working
Posted: Thu Mar 26, 2015 7:28 pm
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U1, a TLO72 is used in the channel switching circuitry. If I read it right, TP 10 &11 are the input and output of U1A.
If that's one of the ones you changed, it probably wont work right with a different chip.
This chip feeds data voltage to the CPU. The CPU is looking for 4 specific voltages to decide what channel to turn on.
If the internal gain or input resistance of the new op amp is much different than the original, it sends the wrong voltages to the CPU and confuses it.
If you've got them in sockets, you've probably put an original back in by now and it works.
Hope this helps. Better late than never.


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Post subject: Re: Super Champ X2 channel 2/fx not working
Posted: Fri Mar 27, 2015 1:58 am
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Hi and thanks for the reply!
I did manage to "solve" the problem in the end. Found out that Q1 was not working properly(it might be because of different voltages like you mentioned). I tested all components in the switching circuit, and when i arrived at Q1 i found a trick to test the transistor in circuit (http://www.geofex.com/fxdebug/trantest.htm). It gave some erratic readings and at moments the fx and channel 2 would spring to life. I removed it and jumpered the collector and emmitter points. All working now but I guess it will not sense a footswitch or disable the front panel with a switch inserted. Seeing i do not have one for now im a happy camper.
I did a lot of reading on electronics and circuits in the mean time, but not really understand what is going on around Q1 (other than some switching instead of amplification i guess). If you do, would you mind explaining a bit? Thanks!


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Post subject: Re: Super Champ X2 channel 2/fx not working
Posted: Fri Mar 27, 2015 12:40 pm
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You're right when you say there is no signal on Q1. It is set up as a voltage level shifter. The voltage divider of R9 & R10 turns Q1 on part way. I'm guessing @ 9 volts on the collector
The line going from the collector to the footswitch is partly grounded by the footswitch and the channel switches. This lowers the apparent voltage at the collector.
That voltage is divided again by the resistors R7 & R8 and fed to U1. Those are the voltages listed on TP10. The output voltage, as listed from TP11 of U1, is fed into the pin 2 of a multiplexer IC where it is matrixed and fed to the CPU to determine what switching should be done.
Shorting the transistor is not the best idea, but it it got you through .... It could overload R11 the 220 ohm resistor on the emitter of Q1
Q1, the 2N4403 is a common ordinary number with lots of equivalents. I would replace it if possible and that should restore the channel functions
Also, it looks like there is a chassis ground on the footswitch connector. It's just a little tab of metal on the front of the jack. It will only make a good connection when the jack nut is tightened properly. Check the nut and see if that stabilizes the operation of the channel switches


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Post subject: Re: Super Champ X2 channel 2/fx not working
Posted: Sat Mar 28, 2015 1:35 am
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That is really helpful, thanks!
I remember the jacks gave me a hard time when removing the pcb from the chassis so it might very well be that the grounding tab is no longer connecting. When I have time I will give this and a new transistor a shot.
Thanks again for the info and i'll post an update when it's done (when I fail miserably i'll post too :) )
Cheers!


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