Fender Super Champ X2 Sound increases when turned on even if volumes are minimum.

[Simon baldwin]

Hello everyone.

New member here from Northants in the UK. Very pleased to meet you all.

I'm a recent convert to repairing and modifying music electronics. Started dabbling during lockdowns for Covid. First project was modding some old Boss pedals, which has led onto some pedal repairs, and recently been doing my own pedal builds on Veroboard or various PCB's that are available for clones. After the pedal mods came the amps, done a few small repairs and restorations on some old ones i've purchased. Much to the Mrs's dismay.... i've now accumulated a bit of a collection of kit at home!!

Anyway, i've acquired a Fender Super Champ X2 Head that is behaving somewhat strangely. When you turn it on without a lead in the input jack, the lights come on, tubes glow etc as it should... but no sound at all from the speaker, not even a pop or hiss of any kind when you adjust volume pots, if you then turn it on with the guitar lead plugged in, it starts to make a load of hum and feedback as well as the actual guitar sound that just gets louder and louder, i assume until it hits it's maximum level, even though i have the volume pots turned right down. None of the controls seem to make any difference at all, i've tried different guitar leads, i've tried different guitars and i've even tried different speaker cabs and it's not made any difference.

Any advice on where i should start to fault find it would be much appreciated.

I've downloaded the schematic.... fender_super-champ-x2-combo_sm.pdf

I'm going to pop the chassis out and start checking voltages on all the test points first, but wondered if any of you may have come across this issue before, or have any ideas about where i should look first. I've looked up threads on here about the amp, but couldn't find any with the same symptoms.

Many thanks,

Simon Baldwin

[loudthud]

Is there something missing from the schematic, like a digital board with a DSP processor ? There are pots that just control a Voltage and send it off to who knows where on P5. Doubleyouteeeffcolonleftparenthesis!

Edit: According to the service manual, the DSP board is not serviceable, you have to call Fender for a replacement. There is a Mute signal that is deactivated when a cord is inserted in the input, that seems to be working. Any way to reset the DSP board ? I didn't see any instructions to do so in the manual. Maybe Fender can tell you.

[g1]

What keeps getting louder, everything? Or just hum, or just guitar?

[g1]

Edit: According to the service manual, the DSP board is not serviceable, you have to call Fender for a replacement. There is a Mute signal that is deactivated when a cord is inserted in the input, that seems to be working.

As far as I know, the board is no longer available. If it turns out to be bad, there is a workaround/bypass here: https://music-electronics-forum.com/...3&d=1481678850

[Helmholtz]

If it turns out to be bad, there is a workaround/bypass here: https://music-electronics-forum.com/...3&d=1481678850

I don't understand why the volume pot is only 50k (well, probably to re-use an existing pot) and is wired as variable shunt resistor
This results in very low input impedance (vaying between 60k and 10k).
If wired as regular pot, input impedance would at least be constant at 60k.

I would want a 1M vol. pot using standard pot wiring.

[The Dude]

My recollection is that the controls are all DSP controls and not in the signal path. Hard to tell from schematic, which doesn't show DSP board.

[Helmholtz]

My recollection is that the controls are all DSP controls and not in the signal path. Hard to tell from schematic, which doesn't show DSP board.

I was speaking about the workaround which eliminates the DSP board from the circuit.

[Simon baldwin]

Many thanks for all the responses.

I'll try and answer as best i can....

Is there something missing from the schematic, like a digital board with a DSP processor ? There are pots that just control a Voltage and send it off to who knows where on P5. Doubleyouteeeffcolonleftparenthesis!

Edit: According to the service manual, the DSP board is not serviceable, you have to call Fender for a replacement. There is a Mute signal that is deactivated when a cord is inserted in the input, that seems to be working. Any way to reset the DSP board ? I didn't see any instructions to do so in the manual. Maybe Fender can tell you.

I Couldn't find a schematic for the DSP. My searches also seemed to suggest it wasn't available, as is the board. I've done the reset which is just 'turn on, press and hold tap tempo button for 4 seconds, led comes on, when it goes off again the reset is complete. Process seems to work as it should, but makes no difference to the behaviour.

What keeps getting louder, everything? Or just hum, or just guitar?

As far as I know, the board is no longer available. If it turns out to be bad, there is a workaround/bypass here: https://music-electronics-forum.com/...3&d=1481678850

Everything gets louder. Guitar sound, hum, feedback.... it's a whole cacophony of noise
I did see that workaround, and assumed that would be where i'd end up if it turned out to be the DSP board that was at fault.

My recollection is that the controls are all DSP controls and not in the signal path. Hard to tell from schematic, which doesn't show DSP board.

I gathered from my searches previous to this thread that the DSP board controlled all of the pots on the panel, which you have all just confirmed.
Should i be preparing for the blown DSP workaround? Or is it worth me popping the chassis out and getting started on the TP testing?
I've also seen a thread where someone has removed the DSP and baked it in the oven to reflow the solder. Is that worth a try? or am i likely to do more damage that way?

Many thanks,

Simon

[loudthud]

I see three possibilities.

1) A component on the DSP board has failed. It could be one of the chips that were custom made or programmed for Fender, it may be a smaller component, R/C/small semiconductor that would fix the board if you could figure out which one it is.
2) A solder connection on the board DSP has failed. This could be found by close visual inspection under high magnification. Big Stereo Microscopes are popular for this. Baking the board is a last resort, it has to be done very carefully to avoid frying some components.
3) A solder connection on the Main board or a connection within one of those IDC ribbon cables has failed causing the DSP board to malfunction. Could be the connector where the DSP plugs in.

It would be a shame to put this amp in a landfill. It has a nice little tube power amp.

[Simon baldwin]

I see three possibilities.

1) A component on the DSP board has failed. It could be one of the chips that were custom made or programmed for Fender, it may be a smaller component, R/C/small semiconductor that would fix the board if you could figure out which one it is.
2) A solder connection on the board DSP has failed. This could be found by close visual inspection under high magnification. Big Stereo Microscopes are popular for this. Baking the board is a last resort, it has to be done very carefully to avoid frying some components.
3) A solder connection on the Main board or a connection within one of those IDC ribbon cables has failed causing the DSP board to malfunction. Could be the connector where the DSP plugs in.

It would be a shame to put this amp in a landfill. It has a nice little tube power amp.

Ok, i think i'll pull the chassis out in a bit and then power it up and get the poking stick out and see if i can find any loose parts. Then i'll remove the DSP board and see if i can find something to have a look at it with. Not sure what i'll do with it if i do find something... the components are tiny!! although i did fix an Electro harmonix metal muff pedal which had a faulty transistor a while back... my first and only SMD project so far!
I'll check for continuity on the ribbon cables too while it's out.
If that doesn't show anything up then i'll get the DVM and start to test the TP's

I've also tried new tubes in it this morning... still doing the same.
I did however notice this morning that the channel led doesn't turn on and off like it's supposed to when you switch between the channels. Not sure if it's done that since i swapped the tubes or if it was like it before.

[g1]

I didn't mean to jump straight to the DSP board, there is other stuff that can be checked first. Power supply voltages, particularly the 3.3V to the DSP, also the controls themselves.
I'm also thinking you can probably bypass the DSP with jumpers to test before doing the whole mod procedure yet. You would have to use the guitar volume though, as amp would be set 'full volume' without the DSP.

[Simon baldwin]

Ok, chassis pulled out.

Firstly, I couldn't see anything obviously loose or damaged.
I've gently poked with a big stick and not found any points that when touched change the behaviour at all.
I've not tested ribbon cables yet, but did start to test the TP's...

Only 1-15 so far as according to schematic they are tested with no input

First figure is stated power, second is what i actually measured

TP1 17.12 Vac - 17.4 Vac
TP2 17.12 Vac - 17.4 Vac
TP3 +20.5 Vdc - +22 Vdc
TP4 -20.5 Vdc - -22 Vdc
TP5 +12.6 Vdc - +12.2 Vdc
TP6 -12.6 Vdc - -12.2 Vdc
TP7 +9.8 Vdc - +9.9 Vdc
TP8 +3.3 Vdc - +3.25 Vdc
TP9 +1.7 Vdc - +2 Vdc
TP10 +70 Vdc - +90 Vdc
TP11 +1.3 Vdc - +1.26 Vdc
TP12 -1.3 Vdc - -1.26 Vdc
TP13 70 Mvac - ?
TP14 -8.3 Vdc - -6.2 Vdc
TP15 +7.7 Vdc - +4.6 Vdc

Looks like it has some power issues. 1-9 are all ok within the stated +-20%, as well as 11 & 12.

TP10, 13 & 14 are all outside the +-20%
TP13 i don't seem to have any reading on

[Simon baldwin]

Tested all the ribbon cables, which all seem good.

If TP7 measures correct at 9.9v, and TP8 is good at 3.25v, then why would TP15 be low at 4.6v when it should be 7.7v? The LM1117 fixed regulator is seemingly good as it's putting out 3.3 to spec as the input to it is still higher than the output. R64 seems good at 7.5 ohms, although i haven't popped an end out to test off board, so is either C56 or C48 affecting the voltage? or is it more likely R64 and my DVM test is just bad?

As far as I know, the board is no longer available. If it turns out to be bad, there is a workaround/bypass here: https://music-electronics-forum.com/...3&d=1481678850

Is this workaround for the Super Champ X2 also? PDF only says Super Champ XD and it looks to be a little different.

I'm also thinking you can probably bypass the DSP with jumpers to test before doing the whole mod procedure yet. You would have to use the guitar volume though, as amp would be set 'full volume' without the DSP.

If i was going to bypass the DSP to test the amp section, would i jump LIN to LOUT & RIN to ROUT?

[g1]

If i was going to bypass the DSP to test the amp section, would i jump LIN to LOUT & RIN to ROUT?

Yes. Remember amp will be at full volume so start with guitar volume control (or source signal) at zero.

As far as the test points go, some of the TP numbers are duplicated, so be sure you are at the right ones by referencing component designations.

[Simon baldwin]

Yes. Remember amp will be at full volume so start with guitar volume control (or source signal) at zero.

As far as the test points go, some of the TP numbers are duplicated, so be sure you are at the right ones by referencing component designations.

Many thanks for the help on this. Much appreciated.
I've bridged the DSP board connectors, and the amp has no noise now, and seems to behave normally. Volume goes up and down with the guitar input as would be expected. Assuming that definitively points to the DSP board as the cause of the fault?

I have run the tests on TP1-15 again while it's bridged, and it gives me different results.... should they be different to my first test, or should they be the same??

This was my first set of figures...

First figure is stated power, second is what i actually measured

TP1 17.12 Vac - 17.4 Vac
TP2 17.12 Vac - 17.4 Vac
TP3 +20.5 Vdc - +22 Vdc
TP4 -20.5 Vdc - -22 Vdc
TP5 +12.6 Vdc - +12.2 Vdc
TP6 -12.6 Vdc - -12.2 Vdc
TP7 +9.8 Vdc - +9.9 Vdc
TP8 +3.3 Vdc - +3.25 Vdc
TP9 +1.7 Vdc - +2 Vdc
TP10 +70 Vdc - +90 Vdc
TP11 +1.3 Vdc - +1.26 Vdc
TP12 -1.3 Vdc - -1.26 Vdc
TP13 70 Mvac - ?
TP14 -8.3 Vdc - -6.2 Vdc
TP15 +7.7 Vdc - +4.6 Vdc

This is what i have with the DSP board bypassed....

TP1 17.12 Vac - 17.14 Vac
TP2 17.12 Vac - 17.07 Vac
TP3 +20.5 Vdc - +21.63 Vdc
TP4 -20.5 Vdc - -21.76 Vdc
TP5 +12.6 Vdc - +12.13 Vdc
TP6 -12.6 Vdc - -12.09 Vdc
TP7 +9.8 Vdc - +11.45 Vdc
TP8 +3.3 Vdc - +3.27 Vdc
TP9 +1.7 Vdc - +2 Vdc
TP10 +70 Vdc - +89.5 Vdc
TP11 +1.3 Vdc - +1.27 Vdc
TP12 -1.3 Vdc - -1.28 Vdc
TP13 70 Mvac - 55Mvac
TP14 -8.3 Vdc - -11 Vdc
TP15 +7.7 Vdc - +11.38 Vdc

The biggest change in the second readings are around resistor R64, which now seems to have a high reading into it, as well as out of it. The regulator is still reducing it to 3.3v however. Would the changes be due to the bypassed board for some reason? or because the amp is effectively on full power when bypassed and so the load is considerably higher?
I assume C53 could be the cause of the high voltage into R64?

I'm thinking as i'm a novice at this and trying to get as much learning as possible from this exercise, that i'll swap out C53 and see if it makes any difference, and then i'll swap R64 out if not.