[R-390] R390A alignment question

[Tisha Hayes]

On most radios the IF's are all set by peaking while injecting a signal into
the receiver. If done right this usually results in all of the IF's being at
the same frequency. Hopefully this is the same frequency as whatever filters
the receiver is using (crystal, mechanical, LC, etc..

I have seen that not happen. ie, something has happened to the IF in a big
way, either through a component failure or some well meaning person just
"tightening down" those loose slugs (don't laugh, I did that on my first
ever radio back in the 70's). You can end up with the IF strip set to one
frequency (like all stages set to 460 khz) but the filters maybe at 455 khz
. In the R-390A I have had a similar experience when I tried to use the BFO
oscillator as my reference, only to find out that my adjustments made it all
worse because the BFO in center position was 'not' at 455 khz (the frequency
counter was on vacation that day). In general, you end up with horrible
performance across the board.

In a perfect world every filter would be dead on at 455.000000 KHz but we
are talking about old radios. Things can happen inside of the mechanical
filter that would either make it non-linear across it's passband or shift
the frequency. There have been some dissections done on the mechanical
filters used in the R-390A's and it appears that the foam falls apart and
makes a mess of everything. Even on the manufacturing line there had to be
some sort of +/- tolerance for what would be considered a good filter.

On the R-390A, since the radio was intended to provide a flat response at
it's widest bandwidth setting (16 KHz according to the switch) they had to
stagger tune the IF to give it a broader passband.

Just brainstorming here but you may be able to do the tuning without a sweep
generator... Use a signal generator with an unmodulated carrier. Set the
frequency of the generator spot-on the desired frequency and peak the first
IF. Then the generator frequency to be slightly higher (without retuning the
radio) and set the second IF, now set the generator to be slightly lower
than the center frequency and peak the third IF. You will probably need to
bounce back and forth a couple of times through the IF deck but as long as
one IF is center, one IF is high and the other IF is low you should be able
to accomplish the same thing. The +/- spread is shown in the manual.

On Tue, Jan 12, 2010 at 1:10 PM, Ben Loper <brloper at gmail.com> wrote:

> To help me understand, if each of the filters in a R-390A has a slightly
> different bandpass + or - 455 and the IF is tuned precisely to 455 won't you
> get some loss of gain anytime you select a filter slightly off of 455.  I
> now understand why aligning a radio with one filter you tune for max using
> the center of the single filter.  With the R-390A isn't the stagger tuning
> to account for the multiple filters giving ample gain for each filter?  I
> ask this question because I don't have the equipment to stagger tune the IF
> and wondered exactly why it was stagger tuned in the first place.
>
>
>