[Boatanchors] National NC-300 HF oscillator problem

[James M. Walker]

Ok Phil,
Since all that seems to be working, This will sound weird, check the band
change
switch that section that switches the oscillator grid circuit. Many time
those switches
suffer fatigue at the contacts. Make sure the wiper is still there for the
grid of the
6AH6! Also check the line back to the tube socket from the switch. If the
wipers
lose contact, or a ground comes off, there will be no path for the tube to
start up
oscillations. Also check the continuity of T9 to ground!

Jim
WB2FCN

----- 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 5:05 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
> > >
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> > >
> > >
> >
>
>