[R-390] R-390A BFO Neutralization

[Larry H]

Yes, you should be able to measure the RF level at the IF output jack or
the pin 1 of V509 (the AGC detector).  The RF level at the IF output is
about .1 millivolt, so I don't know if a loop coupling will work.  The
other important factor is to stop all RF from getting into V503 (by
removing V501 or other means).

Regards, Larry

On Tue, Apr 2, 2019 at 7:04 PM dog <agfa at hughes.net> wrote:

> I use a spectrum analyzer (Moto service monitor) to neutralize my
> transmitters. I wonder if it would work with the BFO, don't see why not?
> Just need to make a loop probe and put it in the proper place while tuning.
>
> On 4/2/2019 19:53, Larry H wrote:
> > 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
> > ______________________________________________________________
> > R-390 mailing list
> > Home: http://mailman.qth.net/mailman/listinfo/r-390
> > Help: http://mailman.qth.net/mmfaq.htm
> > Post: mailto:R-390 at mailman.qth.net
> >
> > This list hosted by: http://www.qsl.net
> > Please help support this email list: http://www.qsl.net/donate.html
> >
> ______________________________________________________________
> R-390 mailing list
> Home: http://mailman.qth.net/mailman/listinfo/r-390
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:R-390 at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>