Peter Andersen wrote:
I have to NAD T762 on the table. Both with burned out (shorted) 5,1V
zenerdiodes (D718).



I have replaces them with a new 5W type instead of the original 2W.



But the diodes is getting extremely hot and I can't figure out if this is ok
(like the way it was designed). They are sitting directly in series with a
7812 from a 22V DC supply. The thought behind the design must bee to divide
the power loss between the zenerdiode and the 7812.

The amplifier is working, but I don't like those hot zeners!



Is there anybody out there that have worked on a NAD T762?



Peter




Peter (and others who have replied),

Unfortunately, some of these diodes do run hot. D716 is also doing the
same thing as D718 in the T762 for a different 12V regulator, so I'd see
how hot that diode (D716) is getting. Of course if the current load is
different on the regulator, the temperature may be different as well. I
don't have a T762 here to look at right now.

I would certainly agree with the other poster that the input capacitor
coming off the bridge rectifier (C704, 3300uf/35V) could be in trouble
and high ripple could cause problems. This can easily be scoped by
looking at the cathode end of either D718 or D716.

I have had problems on this series of NAD's with the bridge rectifier
diodes D701-704, especially if there is the brownish glue on them. They
often have horrible solder joints, and sometimes fail. I replace them
(originally they are 1N4004)with 1N5399 diodes which are rated for 1.5
amps. I also stand them off the board a bit for better cooling. If you
are running with only 3 of these diodes connected, you'll certainly have
higher ripple on the cap.

While you have the board out (lucky you) I'd also check D705-708 in the
same area and maybe replace them as well. Also check soldering in
general of the regulators. You don't want to have to keep removing the
board as it is rather time consuming and annoying.

Many owner's don't realize that surround sound receivers in general run
quite hot inside and need good air flow around them. I suggest at least
3 inches (8cm) of open air above the receiver if on an open shelf, and a
couple of inches (5cm) on either side and rear. The receiver should
also be on a hard surface, not carpet or such.

To other repliers: These supplies that we are discussing are low
voltage regulators for the input switching, digital board and front
panel, just about everything EXCEPT the power amps.

Regards,
Tim Schwartz
Bristol Electronics