Vox V9310 Cambridge 30 Reverb - low output power and lots of distortion

[DFuller]

Schematic attached.

Backstory: The stock LT9914 optoisolator was bad - common failure. Replaced with VTL5C3 (supposedly an appropriate, more reliable substitute). VR1 may be faulty as well - new part on order.


So the issue at hand now is that seemingly no matter how I adjust VR11 and VR12, I either get very little output, hard clipping, or both.

[Jon Snell]

Have you got a clean audio signal on IC7 pin 1?

[DFuller]

Have you got a clean audio signal on IC7 pin 1?

Just pulled the scope out to check, 1KHz sine checks out clean at pin 1. Clipping seems to be isolated to the TDA2050.

[Jon Snell]

As the TDA2050 is only a car radio amplifier, check the output is clean, if it isn't replace the TDA. Anything will do. TDA2030, TDA2050, LM1875 .. whatever you have to hand, that will check the problem.

[DFuller]

I'll need to order a suitable replacment, but that's on the list of things to try.

[DFuller]

LM1875T ordered. Will report back Friday when I've got it and the new clean volume pot installed.

[DFuller]

Alright, LM1875T did not solve the problem.

Seems I get clean signal up to the output of IC6A/input of the power amp but the amp distorts like crazy at the power amp with any moderate level of signal swing.

[g1]

D3 & D4 are good? Speaker is verified good?

[Enzo]

Does it distort through headphones? PLug a good speaker into JK4, the extension speaker jack, is it clear there?

[DFuller]

D3 & D4 are good? Speaker is verified good?

Will check on the diodes. Speakers are all shop speakers or my dummy load.

Does it distort through headphones? PLug a good speaker into JK4, the extension speaker jack, is it clear there?

Headphones no clue, that's my next check. Doesn't matter if it's attached via the ext jack or the speaker wires (to a shop speaker).

[DFuller]

Checked the diodes - D4 reads 0.524V and D3 0.667V. Neither seem to be at the point where I'd consider them bad.