NAD T762


2012-05-19 1:17 am
When using optical input #1 sound works fine but if you use OSD to switch to optical input #2 you get the sound from input #1. Also it seems to bleed over to coaxial #1 and #2 but not #3 or #4. Also none of the coaxial inputs will work or the optical #2. Confusing as hell. The digital input board is verified good. Since the digital inputs go to the DSP board next could that possibly be the problem?? Also all other features of receiver work properly.
Here's the service manual from the T762, looks a like a complex device.

After rotating the document clockwise, in the upper left corner of page 9 is the digital input board. The selection for which channel to relay to the DSP board is made here (by U1203), commanded by the CPU (U202). The schematic shows the control lines (251_A..251_C) are coming from U202 (on keyboard-board) and travel over the DSP-board to the digital input board). First thing would be to check that the connectors still make cood contact with the counterparts on the pcbs (oxidation).

Since the digital input board only outputs one channel to the DSP, it's unlikely the DSP is the cause of the fault here. You write that the digital input board is confirmed OK, so that most likely leaves only the CPU or wiring as the cause. A complication is that line 251_B seems to double for IR duties too (IR_IN), so it's woth checking the circuit around Q206.

Guessing by your desprition and by looking at the truth table of the 74HC251 (U1203), I'm inclined to think that control lines 251_A and 251_B are stuck to L. To confirm this, you would have to connect a source to coax #3 and select that source on the amp and listen. If it plays coax #3, then switch the amp to coax #4 but leave the source connected to coax #3. If it keeps playing coax #3, then that confirms 251_A and 251_B don't change state which would explain why optical #1 plays in any of the following settings: optical #1, optical #2, coax #1 and coax #2.
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When using optical input #1 sound works fine but if you use OSD to switch to optical input #2 you get the sound from input #1. Also it seems to bleed over to coaxial #1 and #2 but not #3 or #4. Also none of the coaxial inputs will work or the optical #2.

I missed the bit that only optical input #1 will work.

If the digital input selection doesn't work anymore and all control lines 251_A..251_C are stuck at L, then all inputs should play optical #1, including coax #3 and coax #4. But that doesn't seem to be the case.
If 251_C still works with 251_A and 251_B stuck at L, then coax #3 should still work (but also with coax #4 selected), but now that also doesn't seem to be the case.

Could you measure the states of all three control lines in all the digital source settings on the amp and see if they look like the truth table below?
251_A = A = pin 11 of U1203
251_B = B = pin 10
251_C = C = pin 9.
L = 0 V, H = 5 V.

Truth table for first 6 states of 74HC251:
L L L -for optical #1
L L H -for optical #2
L H L -for coax #1 (coax #3 in schematic)
L H H -for coax #2 (coax #4 in schematic)
H L L -for coax #3 (coax #5 in schematic)
H L H -for coax #4 (coax #6 in schematic)

If the results are identical, then the control lines (and CPU) are fine.
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Thank you for trying that. That confirms my initial suspicion that the control lines 251_A and 251_B remain low all the time and that 251_C still works.

As a result only L L L and H L L are available from the truth table in my second post. That explains why you can only hear optical #1 in the first four settings and coax #5 for the last two.
I wouldn't be surprised now if your measuremtents reveal the above.

Please also measure these three control lines at the other side at pins 15..17 of connector J203A (or if you feel up to it, right at pins 39, 40 and 41 of U202, note that U202 in the T762 schematic is mirrored on the vertical axis, see attached image for normal pin-numbering). Please also check if the two control lines are somehow shorted to ground.

If these read the same as at the other end and there's no short to ground, then wiring and connections are OK and comes the hard part of finding out why the CPU doesn't change the state of 251_A and 251_B. Do not automatically assume the CPU itself is at fault. From what I understand from the datasheet, these pins are bidirectional, either functioning as input or output (and in case of line 251_B, it does serve both as input and as output). The IR_IN circuit is connected to 251_B and a fault there might also affect proper functioning of the digital input selection, but I don't see how that would affect 251_A unless the CPU gets "stuck" because of it. Is or was there anything connected to the IR_IN jack at the back of the amp?

I have a hard time understanding what the function of transistor Q206 is, so if someone else could have a look and explain?


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Nad T762

Turns out I can get away with using optical #1 and coaxial #5 for now and will get other digital inputs checked locally later if needed. I want to thank you very much for taking the time to help me out. You gave me a lot of things to check with hopes that one of things I did would of worked. Thanks again...Will check with you here in future if I decide to get it fixed to let you know what the problem was.....
Hello, Seems like I'm in the wrong place to post a problem, but you guys seem to know this amp pretty well, so hopefully you can help.
I have a NAD T762 which would turn ON and then shut OFF (or go into protection) after several minutes. I'm looking for the schematic as well as any assistance from any of you that may have experience with this amp.
It certainly would be greatly appreciated.
A quick visual check shows signs of electrolytic leaking on part of the main board, as well as this gummy yellowish glue that I understand turns conductive after several years. I took a quick check on the supply board and measured ±67VDC or thereabouts. Any ideas where I should begin?
Thanks for any ideas and direction. :)