bad TDA7293?

[Marcos - Munky]

A friend have a 127V Marshall MG100 which was connected to the 220V wall plug and got the TDA7293 burned. Easy fix, just replace the TDA, correct?

Wrong answer! I replaced the TDA and got no output. Then I did the usual testings: audio probe, voltage checking, effects loop connection checking. Everything is fine, but the output. So I guessed I got a bad TDA and just got another one. Same results. I got the signal tracked to the TDA input and the voltages are fine, but I don't get any output. Both chips were bought from the same store.

So my question is how common is to buy a bad/counterfeit TDA7293? I don't remember reading about problems with TDA7293, but I checked for the prices and they are so different comparing different stores that it's somehow suspicious if they're original or counterfeit.

[R.G.]

It is quite common to get them counterfeited - as you know. The stores often get taken by counterfeits, just like you do.

On the other hand, the TDA7293 has the reputation of being fragile. When these first came out, I bought a half dozen, expecting to use them. Then I dug in in depth on the data sheets, and there are tricky specs on power supply and start up. The LM3886 is so much more reliable and easy to use that I still have that plastic tube with a half dozen TDA7293s in my desk drawer.

Hmmm. Here's a thought. I seem to remember that the TDA7293 has a mute pin or function somehow. The chips may be good but the mute parts are holding it muted. Worth checking into.

[anotherjim]

I would suspect a standby/mute function too. Those functions are usually operated by a time delay from the power supply input to keep the power amp silent until the supplies and circuits are up and running normally. Since it's operated from the power supply input, it often gets damaged by an overvoltage. I found an MG100 scheme that shows the function as standby, but also written as "STBY" on the scheme, and if you're lucky, it may be silk-screened on the boards.

Look for TR2, D14 (6v8 zener) - possibly near a 5v regulator and fan connector. It looks like "STBY" should start up at 0v and by the time it reaches the 6v8 zener voltage the chip amp should be alive.

[Marcos - Munky]

Yeah, after a few readings I found out this is indeed a fragile chip and it's easier to get an counterfeit than an original one. Maybe it's a nice idea to just remove the power section of this amp and build one with a LM3886 - actually, I suggested my friend to think on building just the preamp section he uses and use a class D amp with a 1x12 cab, so he'll be have a very compact rig for the concerts he play.

Anyway, I checked the standby/mute function. The mute pin is tied to V+, and the standby pin should receive something above 6V to work. But this one is at 0 V. So probably TR2 or/and D14 is damaged. I'll replace both and check how it goes.

[Marcos - Munky]

I replaced both TR2 and D14 and got the amp working! So I decided to try the other TDA7293 I got from the same store and... no luck. Probably I was "lucky" enough to get an original one and a counterfeit one from the same store . Well, I'll just put back the working one and throw away the non working one .