[Boatanchors] National NC-300 HF oscillator problem

[Philip Atchley]

Hi Jim,
Thanks for the info.  Guess I should have been more explicit.  The voltage
on the 0B2 regulator (and supplied to the Plate/screen resistors of the
oscillator) is 105.4 Volts, right where it should be (107 is spec).  The
drop across the plate resistor is excessive, and it's 9.78K Ohms, within 10%
of the specified 10K. The RF choke L15 is good too.

Like I said, it appears to have high plate current (no oscillation) but I
can't find anything "wrong".

73 from the "Beaconeers Lair".
Phil, KO6BB

> 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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> >
> >
>