Marantz SR7500 burning output transistors - so far 2 channels are gone. Help!

[Daniel Larsson]

Hi all,

Will start with the question and the full story follows below.

Can someone please let me know what voltage I should see per pin on the protection circut IC81, TA7317P?
My voltages per pin with the IC removed, only measuring on the soldering pads now, are:

1 -0.39
2 0.0
3 0.0
4 0.0
5 -12.5
6 11.94
7 0.0
8 11.74
9 11.74

The full story is that my Marantz SR7500 shut down a couple of days ago stating "CHECK POW5".
I checked the service manual to find out that the protection circut IC81, TA7317P, senses a power failiure on the amp board and is triggering shutdown to IC36 on the power board.

I found that the SAP17 on the SR channel was shorted and probably burned so I removed that PNP/NPN pair and also the resistors and diodes that clearly burned.

After stripping all the key components from SR channel I could start it again without any check pow5, and I was happy to enjoy music for roughly 20 seconds. Then again, smoke, shutdown and check pow5.
This time it was the center channel.

I have now removed the damaged SAP17 pair from the center channel as well but the check pow5 remains and I don't even dare to connect the BH+ and BH- as probably more will burn.

While I only dare to hook the amp board up to the bench power supply I managed to find a faulty NJM2068, IC71, that was shorted to ground which I have now replaced and the short is gone.

Any help or guidance is greatly appreciated to hopefully bring this one back to life.

[petehall347]

hope you didn't power it up as it is in the picture .

[Daniel Larsson]

hope you didn't power it up as it is in the picture .

It was powered up by the bench supply like that to measure the voltages. The current limiter was set to 0.5A and the card was not pulling nearly that, and showing stable voltages.

Could you let me know how come you hope I did not?

[petehall347]

It was powered up by the bench supply like that to measure the voltages. The current limiter was set to 0.5A and the card was not pulling nearly that, and showing stable voltages.

Could you let me know how come you hope I did not?

because it's very likely to burn up the output devices in little time ..they did extremely well to last 20 seconds without cooling .

[Daniel Larsson]

because it's very likely to burn up the output devices in little time ..they did extremely well to last 20 seconds without cooling .

Oh you are refering to when it played for 20 seconds?
I had the receiver fully assembled with proper cooling back then.

At this point on the test bench (that the photo shows) there is no heat at all. No AC connected, I currently have it like this to troubleshoot the bias and the protection circut to try to figure out what is blowing the outputs when the AC is connected.

Any ideas?

[petehall347]

firstly verify the power supply