Marshall AVT-150: Blown resistor & tda7293 chip

[xCxHxRxIxSx]

Recently I went to turn on my AVT-150 and the light on the main switch came on but nothing else; no channel light, no fans, nothing. So I did some research and discovered that usually the internal fuse will blow or one or both of the TDA7293 chips will burn out.

I then opened up the amp and tested the fuse and it's fine, and there were no visible burn marks on either of the chips. Upon further research I read about the r202 and r223 resistors burning out and sure enough they both blown. I also momentarily jumped each resistor and the amp would come to life.

So I replaced both resistors but, I failed to read the instruction on the PCB to space them 10mm from the board. When I plugged the amp in and turned it on it worked fine, then when I switched to OD1 the TDA7293 chip furthest away from the tube lit up like a Christmas tree.

My question now is why did the chip blow? Was it because of improper spacing on the resistors or is there another issue with my amp that caused r202 and r223 to burn out in the first place?

Any help would be greatly appreciated,

Chris

[Enzo]

Why did the chip blow? Hard to say. But I don't think you realize it was ALREADY blown. MAybe not completely, maybe intermittantly, but failed nonetheless. Those 2.2 ohm resistors are there to limit current into the rectifier. The whole power supply current - less the tube part - runs through them, so they generate some heat. You are supposed to mount them a cm above the board to give them some air space to cool themselves better. If you snugged them to the board, they will have shorter lives. But it had ABSOLUTELY nothing to do with damaging the IC. In fact it is way more likely the failed/failing IC is what toasted THEM.

Those two TDA chips work together, and when one blows it is a REAL GOOD idea to replace both, even if one of them seems to still be working.

[xCxHxRxIxSx]

Thanks so much for the quick response.

Tomorrow I will order two chips and replace them both for good measure. Now to my understanding these chips just give out over time right? So I can assume that by replacing the chips I will most likely solve my problem with the AVT as the chips seem to be the number one cause of this issue? Also the resistors are mounted over a cm off the board.

Thanks again,

Chris

[Enzo]

It isn't time so much, they could last forever. But they are connected to the real world, so bad speaker cords, problems with the speakers themselves, cords that pull partway out of jacks and short the amp output, static discharge into the jack. Loads connected or disconnected while the amp is running. SO many ways to stress them.

[Amp Kat]

Thanks so much for the quick response.

Tomorrow I will order two chips and replace them both for good measure. Now to my understanding these chips just give out over time right?
Thanks again,

Chris

Not really. They can last forever if the design is right but there are other things that can happen like faulty filter caps or blown diodes and drifting resistors that can put more stress on them and obvious end-user abuse like running speakers under the maximum required load and pushing it hard. The transistors aren't like tubes to where they ware out from heater or plate dissipation. It is usually a heat related issue or something else that kills them.

[R.G.]

AK is correct. If the conditions are right, they will last longer than you will.

That being said, the TDA7293 and RDA7294 chips have a reputation for being somewhat touchy about conditions being right.

One thing that is absolutely critical in all chip amp installations is to make sure they are attached to their heat sink properly; that means held to the heat sink with the right amount - not too little, not too much - force by their mounting arrangement, with a thin layer of heat sink grease between them, any insulators, and the sink, and insulated from the sink if the power supply arrangements make there be a voltage between the heat sink tab on the chip and the heat sink itself. Many chip amps will become Light Emitting ICs (however briefly) if this gets done wrong.

[xCxHxRxIxSx]

It's been a while but between school and work I finally installed the TDA chips. However the amp still does not work. This time however, both fans come on and so does the channel indicator light, and I can switch between all channels without anything exploding but, there is no sound (yes the speaker was plugged in and both the guitar and amp volumes were turned up).

So now I have replaced the r203 and r223 resistors and both TDA7293 chips. Could something else have burnt out from the initial failure or should I suspect my soldering job?

If you guys think its my soldering job can I buy the little boards with the caps and TDA chips already installed, because changing those chips is a pain in the ass?

Thanks again for all the help.

[Jazz P Bass]

Monitor TDA7293 pin #10.
That is the 'mute' pin.
It should be low at power up & slowly rise (5 volts, I think).
The complete datsheet is here: http://www.st.com/internet/com/TECHN...CD00001887.pdf

[xCxHxRxIxSx]

So I went back and touched up my connections and I blew up one of the new chips. Not sure if it was because of my soldering or if another problem still exists. Can I get the AVT50-62-00 boards complete from somewhere?

[mike9369]

So I went back and touched up my connections and I blew up one of the new chips. Not sure if it was because of my soldering or if another problem still exists. Can I get the AVT50-62-00 boards complete from somewhere?

I'm new to this forum also, and have this same problem. I've read enough on it to suggest that the TDA7293 is the most likely suspect (power light, but no other indicators and no sound). Likewise, I am considering the complete board, because it seems to be a simple plug and play fix. I'd be interested in people's comments on that approach as well. A quick google search shows some boards that look similar to what comes out of the AVT150, but a little different. Not sure if these would work (is this a standard design), or not. Would take any input on where to get the board for the AVT150 if the generic options are not recommended.

Thx,

-Mike-

[Jazz P Bass]

The most important , often overlooked, fact is that the power supply MUST be drained before you reconnect the board.
If there is continuity between the power pins & the output pin, the IC is bad.
Removing the bad IC is relatively straight forward.
Cut the old pins off.
Wet each hole & lightly tap out the pin piece (or use a solder sucker).
I hold the board up to a light to make sure there are not any bridged connections before & after soldering.
Another point is to not push the IC all the way down on the board.
That in itself can cause a short.

[mike9369]

The most important , often overlooked, fact is that the power supply MUST be drained before you reconnect the board.

I have read this before. What is the procedure for draining the power supply?

Thx,

-Mike-

[52 Bill]

You drain off the voltage charge on the main filter caps with a resistor. Use a medium value resistor and remember to do both neg and pos caps. For the first few times, you should measure the voltage with your voltmeter to see how it works.

I have a few power resistors around with alligator clips attached to the leads for this purpose. My old cap meter doesn't pre-discharge a test cap, so I need to make sure that the cap is discharged before metering.

[mike9369]

I'm new to this forum also, and have this same problem. I've read enough on it to suggest that the TDA7293 is the most likely suspect (power light, but no other indicators and no sound).

Just to follow up on my post above, and to provide an update. One of the two current limiting resistors (R202, R223) in my amp was toast. The other ohmed fined, but I replaced both anyway. Did not replace the 7293's though. Primarily, I wanted to see what the fix was rather than replace several parts and not really know which one really fixed it. I didn't have 15v anywhere that I could find on the main board, so I was particularly suspicious of the current limiters. Desoldered them with an acetylene torch, drilled out the holes with a reamer, and installed the new ones with my MIG welder. Works like a top now.

Actually - it almost felt like I was using tools like that. The eyes are not as good as they used to be, the hands and fingers don't work as well as they used to, and patience is something other people have these days. But... oh well...

So to recap - my problem presented with the following symptoms:
- Upon power up - Crackling noise for several seconds, fading out to nothingness
- No lights on the front panel or on the foot pedal, though the power switch did light up
- No sound out of any channel
- Sad look on my face
- My fuse was not blown
- My 7293's showed no visible sign of damage
- No magic smoke escaped
- No visible damage to R202 or R223 but an ohm meter showed one of them was bad (inboardmost one - can't remember which one that is)
- No 15v visible on the main board

Hope this recap contributes to the greater knowledge of this amp, and its peculiarities. Thanks to the folks that offered up advice and ideas. I actually learned a lot about this amp from this forum which enabled me to apply some almost forgotten knowledge, and to make some logical deductions. Good stuff..

-Mike-