Another AVT-150/TDA 7293 Thread

[Allen.Gordon]

I apologize to all in this thred for the emotional reaction to my disappointment in thread replies.

I articulated my thoughts clearly and EXPECTED well articulated replies. As Enzo pointed out, questions WERE answered, just not as I expected.

I'm sorry for any disrespect anyone may have felt from my unrealistic expectations.

I hope you will all forgive me and we can move forward.

[Allen.Gordon]

Before powering the modules, I checked the headphone outputs. The output seemed normal there. I'm not familiar with the emulations of this amp enough to tell if the audio sounded "good," but it was clear and free from DC.

When I plugged the "fresh" $130/pair modules in, the noise (LOTS of DC on the speakers) was just as bad, AND the fuse blew.

I will now have to trace the circuits. This is what I get for not adhering to my boundaries of not accepting Solid State amps.

The time I'm spending on this amp is pushing me further behind on amps of other customers that need service as well.

[Enzo]

If it is of any comfort, I dislike those amps too. And I refused to take in MF350s at all.

Unless the little board itself is damaged (burnt) you can generally put a new TDA7293 on them, instead of buying whole new ones.

Always work with NO speakers or loads until we know the amp is stable and not producing DC.

[J M Fahey]

FWIW I used TDA7294 in my small amps with not a single problem, BUT:

1) never ever bridge/parellel/bridge-parallel them.
If I need more than what a single chip can give, then straight go discrete.
The purpose of a chipamp is to make design, construction and in general my Life simpler; as soon as it approaches discrete amp complexity, it loses all attractive, period.

2 or 3 in parallel to get 160W is absolute nonsense, and there are no words to describe the MF450 thing.

2) I pull only about 65/70W RMS into 8 ohms, period.
Pulling 100W out of two TIP31 side by side, which is what that case amounts to, is nonsense
About what LM3886 puts out all day long ... using the exact same case size ... maybe there is a lesson to be learned from that.

3) my (single) chip amp is less than 2" away from power supply caps, joined by fat tracks (use thickest possible and then add fills to increase copper area)

4) Marshall chips joined to pcb through 4'-6" ribbon cables?
Through 0.1" computer grade connectors?
Hilariously unreliable.

[Mick Bailey]

Those connectors are ridiculously inadequate. Even if you run up the amp with new modules and there's no DC on the output, those ICs can still fail as soon as the amp draws power with a load and input signal if the connectors are in the least bit slack or high-ish resistance.

They will also die if there's anything that interrupts the output load when in use - bad speaker cable, flaky speaker terminal connection, slightly oxidized contacts.

A customer killed his MF-350 by plugging the speaker in after he'd turned the amp on.

The only basis on which I'll take these amps in is that I'll repair it, demonstrate it working with the (checked over) customer's speaker and lead, but no warranty.

[Allen.Gordon]

If you tried fresh complete modules (not just power IC's), and had the same bad sound, then I think it's safe to assume the fault is not on the output modules, and may even be a second, separate issue.

Thanks for the input. I concur, something else is now the problem in addition to the modules or IC's.

Keep in mind when the amp arrived, it was working with about a 45 minute drive/ 15 minute cooldown for 60 minutes of use. No other issues...

Accepting my fault in not draining the caps anymore accurately than I do before handling an amp, I must have killed something else in the amp.

My forte is NOT solid-state. I understand electron theory applies to all electronics, but I'm very familiar with tube topology. I have repaired MANY amps since taking this amp in.

What I can't manage to imagine is what would allow DC on any component other than a failed/shorted cap? How far back should I trace the audio path to locate DC.

Treating this as I would a tube amp, I check the PSU from the wall to the various rails in search of DC passing caps. No progress so far!

I have printed all 9 pages of the official Schematic from Marshall, and the Data Sheets from SMT Micro for the TDA 7293. So far, with this wealth of information, I've only managed to validate the few calculations I can make. They are covered in higlighted portions and notes... Even the PDF is so covered in comments/markers that I had to download it again from Marshall.

Since the last time I presented this issue, I have taken it to yet another local tech in the area. That was a waste of $20 bench fee and another set of TDA7293's.

My client has already purchased a set of Modules ($130) so I have to cover the cost of anything up to the cost of the new modules as a respectable businessman.

Would any of you care to provide a little direct interaction to help me troubleshoot this amp more specifically?

Thank you, in advance, for your time and assistance!

Sincerely,
Allen Gordon

PS - I have since enrolled in a 2 year program as an Electronics Technician to broaden my skills from just tube amps, but I need to get this client's backup amp working properly ASAP!

[allante666]

For what its worth, I also initially had a problem with these blowing straight away, I tried replacing the caps around them as suggested, I spoke with the technician at Marshall itself and he didnt see I was doing anything wrong except maybe not discharging properly. he said they were very static sensitive.
So, since that day I have NEVER had a problem with them and I have replaced countless ones. My method is that when I put the new chip into the little board to solder it, I discharge the board completely by shorting everything on the board with tinfoil and hold it there while i put the chip into the holes. Then, when its soldered and Im going to plug it back into the mainboard, I short all the pins out of the ribbon cable where they leave the mainboard and keep the foil pressed on while I plug the other end in. This has worked for me every single time. These chips are very very susceptible to voltage spikes and you would be surprised how often when shorting the pins out on the ribbon there is a little click of a discharge where some previously discharged cap has built a bit of voltage back up by itself.
Whether this is good practice or not it has worked for me every time. I had exactly the same problems as you did before I started doing this. I couldn't believe that it would make that much difference but it does.

[Allen.Gordon]

Whether this is good practice or not it has worked for me every time. I had exactly the same problems as you did before I started doing this. I couldn't believe that it would make that much difference but it does.

Thank you for your reply and for the technique. That is something I can wrap my head around.

I will definitely go pristine with this attempt and follow your advice!

Thanks again, for your time and assistance!

[Hanger-18]

Wish I could get some cheaper. My avt project came without this board and the cheap($10) little fan

[Hanger-18]

Your advice is really good. I've saved a lot of trouble by following your comments. Please keep it up anyway you can, thanks.

[Hanger-18]

Question about this...I have the output board missing tda7293 I got from you. I got a valid ic from digikey. I have mica spacers and plastic transistor washers. Do I use silicon grease between the tda7293 and the mica and the heatsink for better heat transfer? I'm using the bolt insulating washer to separate the chip from ground I've also got a larger 60mm instead of a 40mm 24v fan and am putting in a better heatsink. Because of all the chip problems I'm trying to make this a solid repair. I've never dealt with ic's that have to be any more than silicon greased to the heatsink.

[The Dude]

Yes, use the grease and the mica washers for better heat transfer and electrical isolation.

[Hanger-18]

Yes, use the grease and the mica washers for better heat transfer and electrical isolation.

Thanks! I thought so. Never dealt with the isolation style. Ready to finish this!

[Hanger-18]

Thanks!!