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Peter Andersen

wrote:
On Aug 29, 8:41 pm, "Arfa Daily" wrote:
Well I must say, Jamie, I am having some difficulty following those
lines of reasoning. If the nominal supply is 22v, and a 5v1 zener is
glued in series with that to the input of the 7812, that's going to
leave around 17v on the input to the reg. So why is that a problem
in as much as you think that
the starting voltage should be higher than 22v?

I thought he was suggesting that the starting voltage should be ~40V,
and that's why the zener is needed (to get below 36V). Since the amp
is working with 22V as the starting voltage, this doesn't seem likely.

However, something else just occured to me. The OP was taking about
zeners in the plural - two I think. So is that two zeners, in series
with each other, and then in series with the input to the reg?

As I read it, there are two amplifiers, and the diode at location D718
is the zener in question. Or is D718 a part number?

That is the partnumber on the schematic (if there should be a NAD technichan
out there he would know..)

No, there is only ONE zener in series with the 7812. The supply is 22V. The
7812 can be driven with 22V directly, so I located the _shorted_ zener by
the smell :-) In the other amp the zener is leaking with 300ohm in both
directions.

I got a mail from someone that confirms the theory about the divided heat
dissipation. This 7812 is supplying the tuner and preoutboard (with digital
volumecontrol, a lot of opamps etc.)

The question is now: is there drawn to much current from this zener/7812
circuit or is it normal(bad bad design!). Who knows?

Peter