AVT50 no output

[DigiDan]

Hi, I am new here. My AVT50 does not produce any sound. My guitar and cable are fine. I have the schematic, but lack the test equipments (scope, audio gen.), but I do have the ohm meter. Anyway, I do have a little background in electronics. Does anyone here have any kind of similar problem with their AVT50? Please drop me a line. Thanks

[Enzo]

That amp uses the TDA7293 IC as a power amp. It is on a little board mounted to the heat sink. They sometimes - ahem - fail.

Look for open fuses on the main board, make sure operating voltages are present.

Does ANY sound come out the speaker? Low background hum or noise?

[DigiDan]

Thank you, Enzo. I measured the speaker itself and the reading was around 6 ohms, so, I guess the speaker was fine. I looked at the schematic and also suspected the same IC that you suggested, but really had no way to check it yesterday. I will try to gather some test equipments and feed in some low level audio signal and start tracing. I have never troubleshot tube amp. before and that was the reason for my question, to see if those "tubes" do fail often.

[Enzo]

This is not a tube amp. I don't care if they stick a tube in it, it is still solid state. Little on board fuses OK? Any burnt aroma to the little power IC? ANy signs of soot on it?

[DigiDan]

All the minus and positive bias voltages on all the ICs are all there (I measured 32 volts, + and - on the onboard amp. TDA7293), so the fuses and power are ok. The IC1 (power amp.) felt fine and not hot to the touch. Nothing burnt or blown up as far as I can tell. No funny smell, either. I need a scope probe to do signal tracing. Will post what I find here. And yes, like you said, it is a solid state amp., with one tube in it. Sorry I called it a tube amp., and it is not even a hybrid.

[Enzo]

Well, befoer you go buying a scope for it, check the cutout contacts on the jacks - external speaker jack, headpnones jack, most any jack. CHeck the solder to them.

If the output chip is ppowered, touch something to its input pin and some hum or small noise should come out the speaker

[DigiDan]

I already own a scope and audio generator from a while back. I just need to possibly borrow a scope probe. I also tried to touch on the pins of some pre-amp and the power amp chips, but no hum coming out from the speaker. But I will check the jacks like you suggested. I have to wait for a few days on the scope probe. Thanks Anzo

[DigiDan]

Switching Amp. M5201

Anzo, I finally got everything together after a stop at Fry's Electronics. Anyway, here is what I've found: The output at IC2 pin 7 is fine. It has a gain of x10 or more, I did not pay much attention because I was merely looking for low or no output. But the output at IC3 pin 5, a switching amp., is greatly attenuated to the original input level, at best. All the supply voltages are there. I adjusted the pots (VR1 and VR5), and they seemed to be fine. I have been out of electronics for so long, so I don't know what exactly this switching IC supposed to do relates to VR1 (Clean Gain) and VR5 (O/D Gain). If you know how it's supposed to work, please drop me a line, Enzo. Thank you.

[DigiDan]

Defective headphone jack

Anzo, never mind my previous post because I did not know how to use all the front panel controls (I am fixing this amp. for a friend and never used a guitar amp. in my life and that was why I didn't understand how all the controls supposed to work). Anyway, all the audio outputs were healthy and I traced it all the way to the amp. out. For a while, I couldn't understand why the outputs (yes, two output of equal amplitude) were present at the speaker out and yet, no sound came out. Now I understand that only a single output (a sinewave in this case, from my signal generator) needs to be there and not both, for the speaker to generates any kind of sound. The culprit was the headphone jack. The center tap wasn't making contact and always open, and thus, did not ground one of the speaker out. I am just going to solder something to help close up the gap when not in used. Whew ! I've learn a lot from troubleshooting this amp. I thought it was a simple process of looking for a missing audio signal. Thank you so much, Anzo for helping. Have a great day.

Daniel