SWR Workingman's 10 and TDA7293

[acorkos]

This amp specifies a TDA7294 output chip, but all i have on hand is a TDA7293. This substitution will work, won't it?

Assuming it will, I'm having a problem. The PCB where the chip connects is pretty badly mangled from a previous repair...most of the solder pads are gone. I believe I have the TDA7293 connected correctly to the remaining pads and 3 jumper wires i had to add....but i still get no sound (actually only very very faint sound). I can trace signal all the way to the chip input (pin 3), but no signal out from pin 14.

I do get signal from the tuner out and effects send.

So there's something I've done wrong or something else I've overlooked. Can someone help point me in the right direction? Thanks!

[Twist]

I haven't had much luck using the TDA7294 in place of the TDA7293. You might check D1 in the mute circuit.

[Enzo]

But he wants to go the other way. The 7294 is a 100v part and the 7293 is a 120v part - or something like that - otherwise they are about the same, so yes the 7293 should drop in for the 7294. I would not go the other way, I agree with Twist.

And I also agree to check the mute. Rather than D1, though, I'd suspect a shorted C6 or C5, or less likely an open R9,5,15. Is the +42v present on the top end of R9?

And of course... check the speaker fuse.

[acorkos]

speaker fuse - ok
D1 - ok
c6 - ok
c5 - ok
r9 - ok
r5 - ok
r15 - ok

-42v present on the top end of R9



there is a low hum coming from the speaker. very faint sound when i play ipod music through the input. same with the headphone.

should there be signal present at pin 14 (out) of the tda? pin 3 (in) has signal, but nothing at pin 14.

[jrfrond]

MCM has the TDA7294 for $7.39. Why even monkey around with a sub?

[Enzo]

I hope that's a typo, there should be +42 at the top of R9. SO is there about 5v at the bottom of R9? Is that getting to the IC pins 9,10?

Since the schematic is not complete, is the ground return for the speaker intact? is there a cutout contact on the headphones jack that might be messing with you?

Well hell, check C1,2 as well.

And power supply - both pos and neg 42v each go to TWO pins on the IC. Are they present at both sets of pins? One pair is for the power stage, and the other pair for the signal stages.

[acorkos]

sorry...that was a typo. +42 at the top of R9. +4.8V at the bottom of R9.
Also +4.8V at pin 9, after the 22k R5.

Pin 10... -0.02V. R15 has +3.5V where it connects to D1 and R6....-0.02V on the other side of R6 at pin 10. is this OK?





C1 and C2 check out ok.

+42 on pins 7 & 13
-42 on pins 8 and 15

[teemuk]

No. Mute and standby pins should receive a voltage potential that is higher than approximately 3.5V in order to turn those functions off. The components create an initial startup delay but even after that the amp seems to be permanently stuck on mute. Are you sure that D1, R6 and C6 test OK?

I wonder why pin 11 is connected to pin 10. It does not have any use in TDA7294 (datasheet suggests not connecting) - BUT - in TDA7293 the pin is the “buffer driver” node, used for “slaving” other chips. It should not be tied to a DC voltage potential.

[acorkos]

i bet that's it...I'll disconnect pin 10 from 11 tonight and let you know what happens. Thanks!

[Enzo]

I'd be surprised. It might be there in case they want to use the 7293 for some other purpose here. But I will be surprised if that is the trouble.

If it isn't that, then check R6 for open, and if OK, then lift C6. I don't know how you tested C6 but it could be too leaky for the circuit to work.

And measure resistance to ground at pin 10 in case you have a solder blob or something grounding it off.

[acorkos]

well...as it turns out, it was the pin10-pin11 connection that was the problem.

but because the connection to R6/C6/D1 was through pin 11, i had to clip pin 11 off the chip.

once i did that and removed the pin 11 lead from the board, it worked fine.

i guess i could have solved this problem a lot quicker had i used a tda7294 in the first place. oh, well. thanks for your help, everyone.

[Enzo]

I was wrong on this. I apologize to anyone I misled.

Live and learn.

[ragenreality]

hi all.i am having this problem with my marshall mg100hdfx.can anyone help me out.i need to buy the TDA7293 chip and board together.anybody where i can find this or does anyone have one for sale?thanks for any and all help.

[Enzo]

Any Marshall dealer can orer one, or try

www.marshallparts.com