National NC-109 Need some help
 

I have a NC-109 that I am trying to get working. The radio does receive and
all the functions work somewhat.
The problems are the S-meter will not zero and the tuning does not track
across each band. On each band the radio will align on the lower end and as
you tune up in freq across the band it goes off freq up to 2 mhz with
decreased sensitivity. The broadcast band (A) does track ok but has
decreased sensitivity on the upper end.
The schematic set I have was down loaded from BAMA. This schematic shows
that both the H.F osc and the meter amp should have 105 volts on the plate
which is fed from a 0B2 vr tube. There is 58 volts on these plates. The
vr tube circuit does not match the schematics, nor does the ssb/cw det tube
circuit. My question is should I attempt to try to return the unit to match
the schematic or should I assume that this was a factory change to the unit.
The wiring does look different than the original in these areas.
Thanks for your help.
Bob WC0D

"Bob Fay" wrote in message
...
I have a NC-109 that I am trying to get working. The radio does receive
and
all the functions work somewhat.
The problems are the S-meter will not zero and the tuning does not track
across each band. On each band the radio will align on the lower end and
as
you tune up in freq across the band it goes off freq up to 2 mhz with
decreased sensitivity. The broadcast band (A) does track ok but has
decreased sensitivity on the upper end.
The schematic set I have was down loaded from BAMA. This schematic shows
that both the H.F osc and the meter amp should have 105 volts on the plate
which is fed from a 0B2 vr tube. There is 58 volts on these plates. The
vr tube circuit does not match the schematics, nor does the ssb/cw det
tube
circuit. My question is should I attempt to try to return the unit to
match
the schematic or should I assume that this was a factory change to the
unit.
The wiring does look different than the original in these areas.
Thanks for your help.
Bob WC0D



it's likely several resistors have gone waay up in value. It's not unusual
to find some that have gone up 50% that may explain the low plate
voltages. I'd take an ohmmeter to most every resistor in the radio. If it
measures higher than spec by more than 10%, I'd replace it. If it measures
lower, it could still be bad and have a parallel shunt path around it.
These you have to lift one end out of the circuit to check. Tedious, but
necessary.


Also, did you replace any capacitors? Most of the coupling capacitors are
likely to be leaky, especially if they're the early plastic-cased kind.
Leaky coupling caps are going to kill the bias vltages, leading to an
off-zero S-meter and lowered B+.

I'd replace all the caps of 0.005 uf and more. The ones smaller than this
are probably ceramic or mica and those hardly ever leak.

It's possible somebody misaligned the radio, or dust and grease has
increased the stray capacitance across the tuning capacitor, or maybe it
just needs realignment. There should be a bunch of little trimmer caps that
set the oscillator and tuned front end circuits at the top end of each band.
These may need alignment and/or cleaning.

Good luck,

George