[R-390] R-390A BFO Neutralization

[Larry H]

I was finishing up the IF deck I was working on and got to the BFO
Neutralization part.  Many times I have thought that there must be an
easier way to make this adjustment.  Well, I think there is.  I started by
looking through the large amount of data we have online, but could not find
anything easier.  So, I thought, what could I do?

I started by thinking, what was the purpose for this function?  Well, it is
to minimize the BFO oscillator signal from infiltrating into the IF output
and the AGC system (ie: minimizing the BFO signal at the V504 signal
grid).  This is done by feeding a small amount of 180 degree out of phase
BFO signal to this grid.  This is accomplished by the special tap on the
primary of T503 and the 'neutralization' cap C525.  The reason this is
needed is that some of the BFO signal is coupled to the grid through the
plate capacitance within the V504 tube.  Having this correctly adjusted
allows the AGC and IF output to operate correctly.

The documented procedure does work, but there is an easier way to do it.
This procedure depends on the IF being properly aligned and the BFO center
frequency set to about 455 kh.

1. Disconnect the 2 signal input cables (J513 and J518) from the IF deck.
Turn RF gain down about 1/2 way.  'OR', remove the first IF, V501 (I think
this is easier).  The idea here is to stop the IF signal from getting to
V504.
2. Connect your RF millivolt meter (ME-30 or alternative) to the IF output.
It needs to be capable of measuring 1 millivolt full scale.  If you don't
have such an animal, perhaps a scope will go that low (mine does even
without the amplified probe).  If you have neither, all is not lost.  You
can connect a scope (best method) or high impedance -1.5 VDC voltmeter
(poorest method) to pin 1 of V509, the AGC detector.  You can use a tube
test extender to do this.
3. Turn on the BFO and set it to + or - 1 kh.
4. Adjust C525 for a dip with an INSULATED tuning tool.  Even if you have
additional capacitance across C535 from the BFO to the audio detector (to
increase the BFO injection for SSB), it should still dip.
5. Reinstall tubes and/or cables as appropriate.

I've tried the procedure in the books and the above in various
combinations, and the resultant adjustment turns out the same.  The
procedure I like the best is to pull V501 and connect my millivolt meter to
the IF out.  This leaves all cables in tact and works great.

There you have it.  What do you think?

Regards, Larry