NAD T742 in protect from a power supply issue alone

[srinath]

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.

 

[ticktock]

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

 

[srinath]

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.

 

[mjf]

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,......)

 

[srinath]

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.

 

[srinath]

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.

 

[srinath]

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.

 

[mjf]

did you measure the voltage drop across the emitter res (0,22 ohm)?
so you can calculate the bias current through the output transistors.......

 

[srinath]

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.

 

[srinath]

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.

 

[mjf]

......no, the volt drop across the em. res - from one end of the res to the other end (usually the bias current is around 50mA, so expect a volt drop of 50mA x 0,44 ohm = 22mV or sowhat....)
you can do this only in a working channel, not in a damaged one......

 

[srinath]

Q208 and 209 tested good.
I was going to test them in the Q218/219 location.
This amp is a nightmare to get to these parts.
I am swapping the screws to something I can get at from the side, like hex head, or may be try zip ties in the thing.
Cool.
Srinath.

 

[srinath]

......no, the volt drop across the em. res - from one end of the res to the other end (usually the bias current is around 50mA, so expect a volt drop of 50mA x 0,44 ohm = 22mV or sowhat....)
you can do this only in a working channel, not in a damaged one......

OK I will get to this in a day or 2. I am pretty certain now that the power transistors are OK. The problem is in the other predrivers or other parts.
If I cant do this in the dead channel, I am pretty much going to test the good channel.
I have to get the amp modded so I can remove the transistors without it taking all day.
Thanks.
Srinath.
Thanks.
Srinath.