[R-390] R-390A IF transformer stagger tuning

[Larry H]

I recall quite a few discussions on the R390 list about whether the R-390A
IF transformers T501 - T503 need to be stagger tuned or not.  So, I spent
some time going through the tech manuals and the 'IF Deck' section of the
'Pearls' yesterday (thank you Wei for doing a nice job on this) and found a
lot of good information about this.

First, let's get to the ones that should NOT be stagger tuned - the initial
release of the R-390A.  This was confirmed by the early tech manual TM
11-856A, January 1956.  It clearly says to 'straight' tune them.  The
biggest problem noted in the Pearls was how to identify them today, as they
get moved around (swapped) so much.  In order to do so, consider the
following.  As many of you may know, the original run of IF decks did NOT
have trimmer caps on the top and bottom of the mechanical filters.  These
same decks had straight tuned IF transformers.  The later IF decks (made
after 1954) are identified in 4 ways: 1. The side of the deck has 4 holes
below the filters with trimmers in them.  2. The 2.25" x 2.5" square cover
that goes over the top of the filters is about 1/4 " taller and requires an
indentation in the top center for the nut to hold it in place.  3. They may
have  'mod 2' stamped on the IF chassis.  4. The year code in the contract
order number.  So, if the IF deck does NOT have any of these indicators,
the transformers are different and they should be straight tuned.  These
were mostly Collins units and made BEFORE 1954 and they do NOT have  'mod
2' stamped on the IF chassis.

Now for the one's that should be 'stagger' tuned.  R-390A's made after 1954
and those with mod 2 stamped on the IF chassis or otherwise identified
above.   Most of the tech manuals from December 1961 and after say that
'MOST' should be stagger tuned.  The 'most' refers to the exception of the
first run of them.  So, yes, the decks with the trimmer caps on the filters
that were manufactured that way should be stagger tuned.  The consensus in
the Pearls is that the reason for the stagger tuning was to ensure that in
the 16 kc bandwidth position, the bandwidth was indeed 16 kh.  Another
reason sited was to ensure that there was not too much gain to cause
unwanted oscillations or feedback.  However, it was also stated that
straight tuning these decks probably won't be a problem although the 'Q
spoiling' components in the transformers are different than the 1st run.
The main concern was that it might have too much gain.  So, if it can still
be adjusted to the '-7 vdc on diode load', it's probably ok.

However, two very strong R-390A technical experts state that they should be
stagger tuned.  Chuck Rippel made a very nice write up about IF alignment
and it is at the very front of the 'IF Deck' section of the 'Pearls'.  It
is for stagger tuning.  Dallas Lankford is also a strong proponent of
stagger tuning.  Who am I to argue with them.  Chuck's stagger tuning
procedure is a little different than the tech manuals.  His last step is:
• Select the 4kc filter then peak top and bottom of T-503 only.  I agree
with this minor change because using the narrower bandwidth will make it
easier to center the T503 adjustments on 455 kh (T501 and T502 only are
stagger tuned in the manuals and his procedure).

Regards, Larry