[R-390] Tracking down low sensitivity 0 to 8 mhz problem

[Larry H]

Good questions, Jordan.  I use a vtvm that has 11 Meg Ohms input
resistance, but 10 M Ohms should be good enough.  For a probe that fits
snugly into E208 - E211 and E402, I use a piece of solid copper wire #16
gauge about 4.5 " long that has about 1/2 " of insulation removed on both
ends.

For the 1st and 2nd IF alignment, I assume you are referring to the IF
deck.  See my post that follows that is also at the end of the 'IF Deck'
section in the Pearls.  It is important to not straight tune an IF deck
that was designed to be stagger tuned, as the gain may be too high to
correctly adjust the IF gain R519 for correct operation of the AGC and
carrier level meter.

Regards, Larry

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Date: Sat, 11 May 2019 22:04:50 -0700
From: Larry H
Subject: [R-390] R-390A IF transformer stagger tuning

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 khz. 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 0.7 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).
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On Sat, Apr 13, 2024 at 6:58 AM Jordan Arndt <Outposter30 at shaw.ca> wrote:

>
> Hi again group...
>
> While replacing C327 with a mica cap did bring the 1st Xtal Osc output up
> to
> snuff, I still find problems below 8Mhz, so I have a few questions for
> those
> in the know.
>
> The service manual states that E402 should show from -4 to -11V when
> measured with a USM-116 VTVM which has a 100 Megohm input impedance on DC.
>
> What do you guys use to measure this voltage..?
>
> What type of tip/connector etc. do you guys use to insert into E402, E209
> etc..? Is there a standard "pin" that fits snugly in those connections...?
> I
> do have and use the specified CX-1363/U Test Lead, but I cut the alligator
> clip off the positive lead due to it's size and the tightness of the
> available space around the test points.
>
> Lastly, for now at least, Do you all follow the manual instructions for
> aligning the 1st and 2nd IF, or do you use your own methodology...?
>
> Thanks for your time and any possible responses...
>
> 73...Jordan VE6ZT
>
>