[R-390] R-390A BFO Neutralization

[dog]

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
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