[Boatanchors] National NC-300 HF oscillator problem

[Jack Taylor]

At this point I'd say either the grid circuit or the socket itself is
suspect.  Pull the tube with the
power off  and measure the resistance on the plate and grid pins to ground
and adjacent pins.

73 de Jack

----- Original Message ----- 
From: "Philip Atchley" <beaconeer at elite.net>
To: "James M. Walker" <chejmw at acsu.buffalo.edu>
Cc: <boatanchors at mailman.qth.net>
Sent: Tuesday, May 18, 2004 2:05 PM
Subject: Re: [Boatanchors] National NC-300 HF oscillator problem


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
> >
> > _______________________________________________
> > Boatanchors mailing list
> > Boatanchors at mailman.qth.net
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> > ** List Administrator - Duane Fischer, W8DBF/W9WZE **
> > ** For Assistance: dfischer at usol.com **
> > $$ For vintage radio info, see the HCI web site $$
> > http://www.w9wze.org
> >
> >
>

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