Fender Vibro Champ XD No Sound Help

[misterc57]

Amp has no sound. I am tracking a signal to the effects board through R47, but no signal coming out of the effects board. I am also seeing -10 VDC on both sides of R47 which I do not think should be there?

I also see a Mute connection to the effects board but not sure what that is all about.

Greatly appreciate advice. Thank you, MarkO

Schematic attached but not the best legible as you zoom in.

Fender-Vibro-Champ-XD-Schematic.pdf

[g1]

No signal at line out jack?

[g1]

Sorry, I missed the -10VDC at R47.
Can't read which IC R47 is across, is it U6B ?
Which ever it is, post DC voltages at pins 4,5,6,7, & 8.

[Enzo]

No, -10v do not belong there.

Unplug the DSP from P1 there. Is the -10v still present on R47? That voltage is either from a defective U8, or coming from the DSP board. This is a quick way to find out.

Your ICs run on +/-12v. Are both 12v rails present and up to voltage? I mean at the chips?

Your signal normally returns from the DSP, so with the DSP removed, probe those returns with a signal. Noise, test tone, whatever. Does that come out the speaker?

[Enzo]

Here, I like the pdf better.

[misterc57]

Thank you everyone!

Following done with the P1 connector removed.

rails; R50 +12 VDC, R51 -12 VDC

U8 pin 4; -11.7 VDC
U8 pin 5; -1 mV
U8 pin 6; -1 VDC, keeps rising when measuring
U8 pin 7; -2 VDC, keeps rising when measuring
U8 pin 8; 11.7 VDC

R47 -6 VDC, keeps rising when measuring. Audio signal at R47

I have audio signal at P1 LIN connector pin. No signal at any other pin. No signal at R14 or R15. No audio signal at line out.

[Enzo]

With the DSP rmoved you of course have no signal at the LOUT pins, that is why I suggested to INJECT some sort of signal there to see if the rest of the amp after the DSP is working.

[misterc57]

With the DSP rmoved you of course have no signal at the LOUT pins, that is why I suggested to INJECT some sort of signal there to see if the rest of the amp after the DSP is working.

I see now. Will do. I will put a signal on R14.

[misterc57]

I connected a signal at LOUT + and LOUT - . The signal can be heard at the speaker of the amp. The volume and gain knob have no effect on the signal.

[g1]

Thank you everyone!

Following done with the P1 connector removed.

rails; R50 +12 VDC, R51 -12 VDC

U8 pin 4; -11.7 VDC
U8 pin 5; -1 mV
U8 pin 6; -1 VDC, keeps rising when measuring
U8 pin 7; -2 VDC, keeps rising when measuring
U8 pin 8; 11.7 VDC

Schematic shows 2.5VDC at pin5 of U8. I guess it is supposed to come from the DSP board. There are 5V and 3.3V supplies shown at the DSP connector, are they getting to the DSP board?

[Enzo]

Oh foo, my bad. When we pull the DSP that reference voltage is lost to U8.

[misterc57]

With DSP connected. Readings at U8

U8 pin 4; -11.7 VDC
U8 pin 5; 1.3 VDC
U8 pin 6; 1.3 VDC
U8 pin 7; 1.3 VDC
U8 pin 8; 11.7 VDC

Not seeing proper V at P1. See photo.

Problems at TP7, reading 1.37 VDC should be 11.2 VDC. TP8 reading 3 VDC, should be 5 VDC. F2 is good.

4 VAC at P6 / P9. This should probably be above 12 VAC? Possible bad PT?

[misterc57]

Took another measurement with P6 and P9, this time removed from the board. Reading 9 VAC coming from the PT. Spec shows that is correct.

p-t290abx_specification_sheet.pdf

[misterc57]

I snugged up the connectors at P6 and P9. Amp is working properly now! Although they felt like a good connection I guess they were loose enough to prevent the full 9 VAC from getting to the board.

Thank you everyone!