National NC 300 information

Gary Schafer garyschafer at ATTBI.COM
Tue Nov 19 09:44:11 EST 2002

I am not particularly familiar with the NC300 or 303 but found out from a home
brew receiver I built several (many) years ago about product detector levels.

This is one of those items that gets drug out every few years to be improved upon.
For a long time I had too much IF drive to the product detector and did not
realize it. I did extensive work with amplified avc to try and clean up the audio.
I finally found that I had too large of a coupling capacitor from the IF to the
product detector. It would get over driven easily.

I would suspect that those National receivers could use some help in this area.
One of the problems on most of the old receivers was that the BFO would be picked
up by some of the IF amplifiers when the IF gain was up to maximum. If that
happens then the AVC will never function properly.

If you can do a little shielding or lead re routing to keep the BFO out of the IF
(easily checked by watching if the S meter goes up when the BFO is turned on) then
you can increase the time constant on the AVC line to make a satisfactory avc.

The next thing is to try and reduce the IF level into the product detector and or
increase the BFO level into the product detector. Decreasing the IF level into the
product detector can be as simple as decreasing the size of the coupling

There are probably many articles on doing those sort of things. Probably the late
50's early 60's CQ magazines will have those kinds of things.

Gary  K4FMX

Garey Barrell wrote:

> John -
> The '300 has a switch on the AF Gain control that is actuated when the
> control is full CW.  Normally, the "RF Gain" control is actually an "IF
> Gain" control.  For SSB and CW, the AF Gain is turned max CW to actuate the
> switch, and then returned to about half scale.  With this switch "on" the
> RF Gain control controls both the IF stage  _and_  the RF stage gain.  The
> volume is then controlled with the RF Gain control.  This was SOP with ALL
> receivers with diode detectors for CW and became imperative when SSB first
> reared it's ugly head!
> Don't know if the '303 does this or not.
> See, it  _does_  pay to read the directions!!  :-)
> Seems like I recall an FSN or perhaps a mod that changed some AGC resistor
> values and reduced the size of the coupling cap into the product detector
> to improve the signal handling capability of the 300, but I can't find it
> at the moment.
> 73, Garey - K4OAH
> Atlanta
> At 06:43 AM 11/19/2002, john wrote:
> >In both the 300 and the 303, it's imperative to turn the RF gain
> >way back and throttle the rx with the AF gain. The 303 manual says
> >that explicitly. I found it (naturally) after complaining about the 303's
> >lousy
> >SSB reception. Following the instructions, it now sounds good.
> >73
> >John wb5oau
> >
> >
> >At 09:19 PM 11/18/02 -0600, john w. king wrote:
> > >
> > >Looking for mods for my NC 300 to enhance SSB reception and strong signal
> > >handling ability. Any suggestions where info might be found.  How do I get
> > >to a reflector for National equipment? 73, and Thanks. John, K5PGW
> > >
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