NAD T762 Plays analog CD with the Tuner

pdlemen

Member
2011-01-10 6:44 pm
I have an NAD T762 which is combining the Tuner and CD (and a few others) at the Audio input selector IC U301. When the volume is changed, the left channel is killed by the same input selector IC U301 TC9273F-004. Vcc + and - 12 is nice and clean along with the clock and data lines coming into it. I thought it unlikely that both (U304) selectors would be bad in the exact same way so I exchanged them with the same results. I believe there is somthing wrong with the 9273_DATA line but it looks good on the scope when functions are changed. That leaves me with the data itself being corrupt or invalid from U209/U201 on the front panel. I believe U209 lets 6 different data sources talk to 2 ports on the main micro, P3.4/DO and P3.5/T1 pins 16 and 17. Can anyone help isolate this problem down a little further? With 6 different sources as oportunities for a problem, my head starts to spin a little.
 

pdlemen

Member
2011-01-10 6:44 pm
Also found that I can hear the tuner's audio when using the coaxial inputs as well. Weird part is that the tuner portion is only there when an input is given to one of the coaxial inputs. If there is nothing plugged in,it is silent. One other thing; the combining of inputs does no happen until a function is utilized. If the unit is initially turned on in the CD input for example, only the CD audio is heard. If the volume is adjusted, it then combines the FM with every other input. It stays this way unil power down. If a function is never used, it will continue to play just the CD audio.
 
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pdlemen

Member
2011-01-10 6:44 pm
Thanks for the reply Mooley. Both input selectors have been changed with the same results. Further, none of the data lines are shorted to ground or to each other. I don't see how it is possible to combine audio from a coax and an analog input. I have not found a connection via a single component or data line besides the main micro and bidirectional tranceiver.
 

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
I guess its one of those things you need in front of you...

Only other things I can suggest are,

1) Does it have a backup PSU for the micro and/or RAM anywhere such as battery or cap. If so then discharge all rails and slap a piece of tinfoil over the micro for a few minutes to remove all stray charge. Be absolutely 100% sure all rails are nominally discharged first.

2) Has it worked in the past, and this is a genuine fault or is it an acquisition with unknown history ?
 

pdlemen

Member
2011-01-10 6:44 pm
It doesn't have a back up cap but the micro in question is located flat inside of a socket. Would this be to try to reset it? It has RAM on the DSP board. I am well familiar with what bad connections on RAM can do to data busses. It ain't pretty. Fought that battle with a few Denon AVR's. More specifically, AVR2805 and models from that era. I put all of the sm IC's under the microscope and saw no bad connections. Reflowed them anyway just for good measure. I found this thread (http://www.diyaudio.com/forums/solid-state/212901-nad-t762.html) and it is doing pretty much the same thing. Except his did not cancel the left channel when volume was changed. As far as it working in the past, the person who owned it said that it would go into protect after 10 minutes and that was cured by replacing the power supply caps on the main board and getting rid of the conductive glue everywhere. It may have been doing this for him as well but he never mentioned it. I am looking a little closer at the electronic volume control IC's...two of them being M62446AFP. I can hear the steps of the rotary encoder (v.c)as it is turned and I know that isn't right.
 

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
Yes it was to reset it. I've had many instances where leaving tin foil over a micro seemingly "cures" the problem. Some Sony VCR handsets have this issue (all soldered... no sockets) and the awkward ones need at least a week or more left like this to be cured. Sounds like your issue might be more deep seated and the unit obviously "has history".

I don't really know what to suggest to you other than (generally), that its never the big chip :D Micros and LSI chips are supremely reliable (as a rule) but there is always the exception.
 

pdlemen

Member
2011-01-10 6:44 pm
I'll admit I once replaced a micro for a no turn on problem just to have the same problem after replacement. Replacing a stuck tact switch cured it. I whole heartedly agree. It's not the micro. It is one of 30 busses and 20 devices trying to talk to it that is the problem. It would be easy enough if any of them were low resistance to ground or didn't have data present when required but sadly that is not the case.