Repairing an MTX TA7801, seeking some assistance - diyAudio
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Old 8th May 2015, 05:02 PM   #1
kibble is offline kibble  United States
diyAudio Member
Join Date: Sep 2009
Default Repairing an MTX TA7801, seeking some assistance

Hello all,

I was given an MTX TA7801 to repair. Upon opening the unit, I immediately noticed that someone had attempted repair on it as some of the components appeared to have been swapped out at some point. The positive 15V regulator section was fried and both N5532s were dead. A couple of SMD electrolytics on that rail were ballooned. Nothing else was noticeable. I replaced all bad components I could find and tested the optos out of circuit to make sure they were ok. They were fine. I have the 15V rails at +15.542 and -15.177. Fast Forward some and I've got the amp functioning and it will produce audio, but something still doesn't seem right. I seem to be getting a pretty high idle current draw, just above 4A, and the two output inductors get pretty toasty after a couple of minutes sitting there, I measured them around 160F after about sitting for around 5 minutes. All the electrolytics at the output terminals also seem to be warming up some. Does any of this sound typical for this model amp? I've never repaired a class D amp that produced so much heat at idle.

When first powered on, the current draw is around 2A and then jumps up another 2 when the output section is un-muted. I removed all output MOSFETs and tested without them. No excessive current draw when they're not in circuit. All the output MOSFETs in the circuit right now test ok. I put only one MOSFET on each rail back into the circuit and I was getting the same results afterwards, so I decided to just install them all back in. Here's some pics of the waveforms present at all the input/output pins on the servo 5532. The other 5532 all the pins sit pretty much at 0V. I have the scope set to 5V/Div, 10uS/Div and DC input so I could see where the voltages were sitting at.

Pin 1:

Pin 2:

Pin 3:

Pin 5:

Pin 6:

Pin 7:

Here's the waveform present at the HCPL-3120 optos pins 2 and 3:

And here's a pic of the scope directly across the output of the amp:

I stumbled on the thread about a 7801 repair from a few years ago, but didn't really find anything that lead me in the right direction with my issue. I'm hoping maybe someone is more familiar with this amp than I am and can maybe give me some tips on what else to look for.

Thanks in advance!
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