[Boatanchors] National NC-300 HF oscillator problem

[James M. Walker]

Plate voltes should be 69, screen volts 94,if these are low then I would
check the
0B2 regulator tube since you are in the correct area, and no other sources
for the
loading present themselves. What is the voltage at the 0B2 tube pins 1&5?
Regulators
go as well as resistors and caps, if the caps and resistors check reasonably
well, then
maybe the regulator tube is bad.

Jim
WB2FCN

----- Original Message -----
From: "Philip Atchley" <beaconeer at elite.net>
To: <boatanchors at mailman.qth.net>
Sent: Tuesday, May 18, 2004 4:09 PM
Subject: [Boatanchors] National NC-300 HF oscillator problem


> Hi, Me again,
> I'm working on a National NC-300 Hamband receiver for a friend.  It was
> given to him by a gentleman who couldn't get it to play.  He thought
"maybe"
> the HF oscillator for the first mixer wasn't working.  My friend Don,
> (W6LRG) tried also, but isn't a "strong" technical type, so he brought it
to
> me for repairs.  He also asked me to re-cap it for him along with anything
> else it might need.
>
> Anyway, when I first got it here I confirmed that apparently the first LO
is
> "dead".   I tried the usual, replacing the 6AH6 tube (twice), checking
tube
> Voltages etc.   I also used a digital portable radio to "look" for the
> oscillator 2215KC both above and below the operating frequency (1st IF is
> 2215KC) on a number of bands.  No signal detected.
>
> 1.  Plate Voltage is quite low, 30 Volts instead of 69.  Plate resistor
> (10K) is in tolerance, coupling capacitor to the mixer was disconnected in
> case it was leaky.  No effect on plate Voltage.  I suspect plate Voltage
is
> low because the oscillator isn't oscillating and hence has no grid bias
> developed, leading to high plate current.
>
> 2.  Screen resistor is in tolerance, Screen Voltage in tolerance.
>
> EVERYTHING in the (fairly) simple circuit has been either checked or
> replaced.  I even reached WAY under the bandswitch and replaced the 15pF
> grid capacitor leading to the tank circuit (NOT an easy task).
>
> Is anybody aware of a common failure mode in the oscillator of this
> receiver.  There REALLY isn't a lot of components in that circuit!
>
> 73 from the "Beaconeers Lair".
> Phil, KO6BB
>
> DX begins at the noise floor!
> Merced, Central California
> 37.18N  120.29W  CM97sh
>
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