[R-390] Synchronous AM Reception

[David Wise]

Fraser Bonnett <fraserbonnett@adelphia.net> wrote:
> > on modifying the R-390A to achieve
> > carrier synchronous demodulation, and was 
> > wondering if anyone has any experience with it,
> > and if it is worth attempting:
> > 
> > An NRC reprint from 1992 uses:
> >     a coil of wire around the anode of V508
> >     100K (or 47K) resistor
> >     a 5-50 pf trimmer grounded to the carrier meter adjust nut
> >     a 47K (or 22K) resistor in series and connected to pin 
> >     socket 1 of V505

Tom M. [mailto:courir26@yahoo.com] replied:
> 
> I've never used that method, but it sounds too easy to be 
> true.  Seems like a true sync detector needs to measure
> and actively alter the BFO frequency needed
> to keep things straight.

A sync detector needs to control the BFO slowly
and continuously, so in between dropouts it
remains at least approximately on frequency.

I don't think the NRC mod's worth trying.
Judging from your description, it's more like a
synchrodyne* detector than a true synchronous.
The latter contains a PLL, locked onto the IF.
The NRC mod locks the BFO to the IF (via "pulling",
usually a problem, here used intentionally), but
only when there's signal.  On every little dropout it
will instantly revert to its free-running frequency.
The synchronous detector's PLL drifts too,
but so slowly it doesn't make a beat note
during most dropouts.

* Uses amplified, amplitude-clipped IF as BFO
into a product detector.

If you can't find an SE-3 for a price you're willing
to pay, Tom's recommendation holds.  Use any
sync-detector-equipped radio that can tune your IF.
Once in a while you can find one really cheap because
it has a blown front-end.  It might still
pick up the R-390A's _loud_ IF output.

Regards,
Dave Wise