[Boatanchors] [National] NC 100 Receiver B+ question

[Robert Nickels]

Greg Mijal wrote:
> Hi:
>
> Does anyone know what the B plus voltage should be in a normal running machine.  Mine is running at 150 vdc or so and the transformer is buzzing a bit.
Timely question, Greg, as I've just been re-capping mine the last couple 
of days.   My original power transformer developed shorted turns, so I'd 
advise caution and as was suggested, making sure your filter caps are in 
good shape.

I just measured 255 volts AC on either side of the transformer secondary 
and 223 volts DC at the input to the first filter section.   I'm seeing 
about 170 volts DC on the plates of the 6K6 output tubes.   This sounded 
a bit low to me, but none of the hodge-podge of NC-100 family manuals 
I've found actually specifies what the B+ voltage should be except for 
the NC-100asd version that's on bama, and you can't really go by that 
because it's only got a single 6V6 audio output stage. 
However, National sold the NC-100 in both AC and DC-powered versions and 
the sales literature specified that the DC version requires filaments 
and "180 volt B+", so I think mine is close enough.  The speaker I have 
is marked "Zenith" and its field coil could have higher resistance than 
the original Jensen, which could account for slightly lower B+.

I'd suggest re-reading Chuck McGregor's note on the National reflector a 
few days ago about the NC-100 bias circuit.  After rewiring mine I'm now 
seeing the proper -40 volts from the B- point to ground but someone had 
previously tied the two together, totally ruining the AVC and limiter 
operation.  (Also when recapping, note that 8 uf between the B- point 
and chassis ground should have its positive lead connected to the chassis).

Good luck, after recapping I'm pretty pleased with mine.  Can't go wrong 
with a great "band-scanner" dial and black wrinkle!

73, Bob W9RAN