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
Well,
I have a theory..
If the Zener is in fact in series with the feed to the 7812
from the 22 volt source? May I suggest that maybe 22 volts isn't
the real voltage it should be? If this is the case, it would place
the zener in a high resistive state and it will get hot!..
The reason say this is because the 7812 (last time I checked) can
handle around 36 volts.
Since your Zener is 5.1 volts, that leaves 2 things in conclusion.

#1.
It really isn't being used in Series but in parallel with a feeder
resistor to be used as a reference! In which case, the feeder as
reduced in size of the load from the circuit has been removed!.

#2
The 5.1 volt zener was picked to bring the Vcc source voltage down
a bit with in the safe operating range.

I would be willing to bet that it's suppose to be more than 22 volts.




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