Fender Champion 40 Power Amp Mute???

[Mr_bibbles]

Hey All, I'm pretty sure I have a very simple problem on a very complicated amplifier.

The Fender Champion 40 is a mostly SMD modeling amplifier, I have one here that is producing no signal (not even hiss) I've come to guess it's the power amp mute through these tests;

1. Tested alternate speaker
2. Cleaned Aux / Headphone jacks
3. Verified voltage change at headphone routing transistor Q3 when 1/8 jack plugged in / not plugged in
4. Audio probed the output transistor, finding good clean signal at the TDA7294. This pretty much means
5. Runnind an Aux in signal to no avail.

In my experience, fender amps use a power amp mute (which I can verify by the schematics attached) but I'm at a loss as to how to troubleshoot on something like this when it seems to be an Internal function of an IC.... or am I missing something more obvious??

The power board on this model (Champion 40) is "FAB P/N 2432ICT0007"

Thanks Y'all

[Jon Snell]

You have signal from pre amp out socket?

[g1]

You have signal from pre amp out socket?

He has signal into the power amp IC.

Schematic is not for the TDA7294 version.
Attached datasheet for TDA7294 should show you which pins for mute, standby, etc.

[g1]

Attached is Champion 20 - 40 schematic with TDA7294.

[Enzo]

3. Verified voltage change at headphone routing transistor Q3 when 1/8 jack plugged in / not plugged in

C'mon. Verified WHAT? If you tested voltages, state what they were and where, for example collector of Q3 got zerov and 3v toggling.

4. Audio probed the output transistor, finding good clean signal at the TDA7294. This pretty much means

Again, what where? YOu have signal up to the input pin? Or at the output pin? Note, the schematic you posted does not use a TDA7294.

[g1]

Note, the schematic you posted does not use a TDA7294.

See post #4 for 7294 version schematic.

[Jazz P Bass]

" Audio probed the output transistor, finding good clean signal at the TDA7294"
If you have signal going in to the 7294 and nothing coming out then.....
Replace the 7294.

[Mr_bibbles]

He has signal into the power amp IC.

Schematic is not for the TDA7294 version.
Attached datasheet for TDA7294 should show you which pins for mute, standby, etc.

Sweet! Thanks! I was confused because the revision sheets showed the correct number for the P board but something was rotten in Denmark.

If you have signal going in to the 7294 and nothing coming out then.....
Replace the 7294.

Thanks, it seems like my "test" of the output was not complete. Verifying signal and verifying signal OUT are different thing... I skipped a step based on the assumption that a shorted output would draw excessive current, which this amp does not. Must... not... assume....

C'mon. Verified WHAT? If you tested voltages, state what they were and where, for example collector of Q3 got zerov and 3v toggling.

Forgive my inexperience. The extent of testing routing transistors that I knew about was to verify a swing in voltage when a jack was triggered, it triggers the voltage up to about 6v from 0 if i remember correctly, which I again ASSumed would meana transistor like this was functioning correctly. I'll post full measurements and BEC changes when I can get in there next.

[Enzo]

Forgive my inexperience. The extent of testing routing transistors that I knew about was to verify a swing in voltage when a jack was triggered, it triggers the voltage up to about 6v from 0 if i remember correctly, which I again ASSumed would meana transistor like this was functioning correctly. I'll post full measurements and BEC changes when I can get in there next.

Hey, I still can't figure out women...

My point was that a transistor doesn't "have a voltage". It has three legs, and typically each leg sits at some different voltage. So it means nothing to say my transistor had 3v. We need to know if that is on the collector, the base, or the emitter.

[J M Fahey]

" Audio probed the output transistor, finding good clean signal at the TDA7294"
If you have signal going in to the 7294 and nothing coming out then.....
Replace the 7294.

Why?
It may very well be muted.
It is unmuted by applying positive voltage (or feeding positive current) to/into pins 9 and 10 at TDA7294

Said currents come through R406/407 connected to point called "mute 40" which through a pin called "mute40" and corresponding wire gets connected to Pin 5 at Jack J1 in the low voltage (+/-15V) power supply section where through R5 it gets connected to a +6V source.

So no positive voltage applied to mute/standby pins = deaf mute TDA7294.

Followbthese lines to check whether unmute voltage actually reaches the chipamp.

Now if they do and it still does not work, then yes, itīs highly suspect.

Personal rant: HATE these NEEDLESSLY complex amps, made ONLY "because they can" and that ONLY because of automated assembly.

I treat them as disposable, period.

Because I am not competent to repair them?

NO, simply because 1 hour (minimum bench time) is more expensive than a new board.
I am talking real production cost, of course.

If within warranty, let Fender (applies to all other brands of course) eat crow and supply a new board and pay for Tech time to replace it.

Outside of warranty? let USER eat crow for buying disposable stuff believing it is repairable for similar low cost. Buy another!!!!

[Mr_bibbles]

Hey, I still can't figure out women...

My point was that a transistor doesn't "have a voltage". It has three legs, and typically each leg sits at some different voltage. So it means nothing to say my transistor had 3v. We need to know if that is on the collector, the base, or the emitter.

Excellent information thank you, succinctly put too.

Said currents come through R406/407 connected to point called "mute 40" which through a pin called "mute40" and corresponding wire gets connected to Pin 5 at Jack J1 in the low voltage (+/-15V) power supply section where through R5 it gets connected to a +6V source.

So no positive voltage applied to mute/standby pins = deaf mute TDA7294.

Followbthese lines to check whether unmute voltage actually reaches the chipamp.

While I did not quote it, I truly enjoyed your personal rant. I am at the stage of my amp tech journey where I turn down NOTHING because the best way to learn is to take swings at bat (and honestly having an amazingily generous group of folks like yourselves to guide my inexperience). I have a full guarantee for amps I can't fix so usually everyone is happy!

I will take a look at the power amp mute voltages at standby pins and then work backwards to Enzo's suggestion if that yields no results.

THANKS AGAIN guys, for real. Hope anyone in the south is safe and warm right now... crazy in TX.

[Mr_bibbles]

Said currents come through R406/407 connected to point called "mute 40" which through a pin called "mute40" and corresponding wire gets connected to Pin 5 at Jack J1 in the low voltage (+/-15V) power supply section where through R5 it gets connected to a +6V source.

So no positive voltage applied to mute/standby pins = deaf mute TDA7294.

So, if I understand correctly.... voltage UNMUTES the P.A. through pins 9 and 10, so check for + voltage there?.... I found no positive voltage on either pin. That means it's muted!

So next step- where does the + voltage disappear along it's route? Fahey has explained it passes through 406/407, I found no voltages on either side which means it's not reaching the chipamp, so we can for now eliminate the possibility of internal failure? If that's the case we trace it back....

MUTE 40, as JM said is fed 6v+ from Pin 5 to pin 7 on molex connector J7 THROUGH r5.. so here we have a breadcrumb. I found 6V+ on pin, but not on pin 7... r5 reads good with an Ohmeter....

...but there is no continuity between pin 5 and R5!!! BINGO a teeny tiny SMD trace repair later we have voltage on the chip IC, and..... poor, blatty super quiet signal through the speaker. But the power amp is no longer muted, with definitive measurements to back up our (and by our I mean JM's) assertions.

I will dig into more of this later but seriously! Thank you. This was a wild ride but basically, we needed to supply + voltage to the chipset through r5, that was extremely illuminating. Now to trace back the audio signal and see where it's getting bad. I have a TDA chip on order just in case.

[Mr_bibbles]

SO audio probing it through the amp, and following earlier suggestions I've found good signal IN to the TDA at pin 3, but no signal out at 6. I have +27 and -27 at VCC and VEE so probably solution, replace the TDA?

I'll let ya'll know how it goes once the part arrives.

[Mr_bibbles]

New TAD went in, still no sound, checked again and the voltage had disappeared off the mute pin. Went ahead and bypassed the SMD r5 with a new discreet version, hardwired molex pin 7 to it and everything works! Thanks a lot guys this was a great learning experience.

-Matt