NAD T742 in protect from a power supply issue alone - diyAudio
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Old 28th November 2014, 04:46 PM   #1
srinath is offline srinath  United States
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Default NAD T742 in protect from a power supply issue alone

I recently bought a dead Nad T742.
It would abruptly go into protect. It would work anywhere from 2 sec to 15+ min and drop into protect.
Then I found if I tapped 1 spot with a plastic ruler on the power supply/surround amp board, it will instantly drop into protect.
Then one of those taps caused it to spark from the power supply 1/2 of the board and it now will drop into protect @ power up.
Can it be just a power supply issue.
Has anyone opened up a Nad T742. The problem seems to be common enough though.
Thanks.
Srinath.
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Old 29th November 2014, 02:26 PM   #2
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Quote:
Originally Posted by srinath View Post
I recently bought a dead Nad T742.
It would abruptly go into protect. It would work anywhere from 2 sec to 15+ min and drop into protect.
Then I found if I tapped 1 spot with a plastic ruler on the power supply/surround amp board, it will instantly drop into protect.
Then one of those taps caused it to spark from the power supply 1/2 of the board and it now will drop into protect @ power up.
Can it be just a power supply issue.
Has anyone opened up a Nad T742. The problem seems to be common enough though.
Thanks.
Srinath.
schematic and pictures
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Old 29th November 2014, 06:13 PM   #3
srinath is offline srinath  United States
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Default Opening it tomorow

I'll get pics tomorow, schematic I'll post after that, but my question was just hypothetical. Can it go into protect if ripple is too much, or if a regulator is gone. I guess there isn't a definitive answer, atleast not one that people can say, never can happen, or yes happens a lot.
I'll get into it and post it.
Thanks.
Srinath.
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Old 29th November 2014, 06:53 PM   #4
mjf is offline mjf  Austria
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if you tap at the pcb and then it fails...........that could be a bad soldering point, a rent in a copper trace, a bad (cable)connection or a little short somewhere........;

annother possibility is: broken parts (regulators, electrolytics,......)
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Old 30th November 2014, 02:18 AM   #5
srinath is offline srinath  United States
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I have done one better.
The first few taps yielded the thing going into protect.
Then the next tap broke the camels back, it made a lovely blue spark and then its now in protect always.
Tomorow I'll get it apart.
Thanks.
Srinath.
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Old 30th November 2014, 10:47 PM   #6
srinath is offline srinath  United States
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OK 1 pair of transistors in the surround amp being removed from the circuit makes it not go to protect and best of all work in the same gutless fashion a 50 w a ch NAD receiver will. As in, it makes good nice musical sound and ... dont expect it to drive any difficult loads or try to host a birthday party.

I'll now probably test those transistors, and if they test fine, swap em into the other channel ...
It was sitting with the solder side up, and it was basically begging to be desoldered before I messed with it any further.
Cool.
Srinath.
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Old 30th November 2014, 10:59 PM   #7
srinath is offline srinath  United States
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Anyway this is the schematic. Q208 and q209 are the suspect transistors.
I was gonna test em with my transistor tester, and is they test fine I was gonna swap em with the Q218-9 pair and see if it follows them or stays with the present set.

Thanks.
Srinath.
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File Type: png Nad_surround_amp.png (243.3 KB, 16 views)
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Old 1st December 2014, 02:53 PM   #8
mjf is offline mjf  Austria
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did you measure the voltage drop across the emitter res (0,22 ohm)?
so you can calculate the bias current through the output transistors.......
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Old 1st December 2014, 02:55 PM   #9
srinath is offline srinath  United States
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Its a nightmare getting that board to where I can see it. Its easier to replace these 2 transistors than it is to test anything else. I have not even seen the board. It was just easier to desolder first and ask questions later.
Oh, I can find the location and test it on the solder side. Let me check it today.
Thanks.
Srinath.
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Old 1st December 2014, 08:45 PM   #10
srinath is offline srinath  United States
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I believe I have located the emitter resistor R213. .22 X 2 ohm, 5 watt.
I should measure voltage to ground from the end that connects to the Q208 emitter ?
Should I connect 208/209 ?
If I do that, I dont thinnk that section even energizes, the amp shuts down in fractions of a second.
Thanks.
Srinath.
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