[R-390] Balanced input on higher bands correction

[Les Locklear]

BTW, spending a great deal of time to constantly zero each 1 MHz crystal
is a fool's errand. The on-off action of the crystal heater will cause the
beat frequency to swing up and down, sometimes as much as a hundred Hz or
so. I can see why RTTY operators would spend their entire military careers
sitting in some shelter, tweaking that adjustment for a couple of years.
(insanity may be the end result).


Thanks for the best holiday chuckle here in a while...I worked with some like that in the Air Force DOD.

Les Locklear

"You can have a college degree and legitimate parents and still be a dumb bastard".
 An Old Retired Mechanical Engineer





----- Original Message -----
From: Tisha Hayes <tisha.hayes at gmail.com>
To: R390A <r-390 at mailman.qth.net>
Sent: 12/30/2021 2:42:27 PM
Subject: Re: [R-390] Balanced input on higher bands correction
________________________________________________________________________________


I used the Twinax connection so I could have a (shielded) balanced pair
coming in to the lab; I even made a gas tube surge arrestor using GDT's
that had a triggering threshold of 65V (line/ground, line/ground,
line/line) with Twinax connections on each side and a brass bolt to pin it
to the copper bulkhead entrance panel.

If you shop wisely you can find the cable (and connectors) at junk-prices
(do not use R390 as part of the search term, that causes people to charge
an outrageous premium).

The baluns that come with the IBM cable IS HORRIBLE for anything to do with
radio. I tried it, just write off that idea, the impedances and frequency
range were just wrong-wrong-wrong. It has been so long ago that I do not
remember exactly what my testing had shown, because once I saw how bad it
was I just lopped off the baluns, tossed them out and wound my own on a
binocular core for a frequency range and impedance that was useful to me.

If you look closely at the schematics for the RF deck you can see that one
of the other differences between the unbalanced (50 ohm) and balanced (~100
- 150 ohm) connection are the antenna relay and an additional stage of
slugs in the RF deck. I had found one deck that had a zapped 1st RF
inductor (probably lightning) that affected only a range of 1 MHz frequency
selections (the RF transformers are selected in gangs through the rotary
switch). I was able to scavenge a new inductor off of a parts deck and
bring it back to life.

Actually, the biggest hassle I have ever had with the RF deck is on
intermittent connections on that long rotary switch that runs along the
bottom. It is sensitive to mechanical misalignment and can result in some
bands where you have to jiggle the 1 MHz bandswitch to get a good
connection. You can tell when that problem exists when you get a 1 MHz band
that is somewhat deaf. I would suggest that you use a crystal oscillator to
throw up a marker every 1 MHz at the midpoint of each band (1.5, 2.5, 3.5,
4.5...MHz) so you can just get the oscillator on frequency using a master
reference or by zero beating it against something that is known good (I
used to use WWV on 2.5 MHz) and then just turn the MHz switch and you could
tweak the trimmer caps on the crystal deck.

(BTW, spending a great deal of time to constantly zero each 1 MHz crystal
is a fool's errand. The on-off action of the crystal heater will cause the
beat frequency to swing up and down, sometimes as much as a hundred Hz or
so. I can see why RTTY operators would spend their entire military careers
sitting in some shelter, tweaking that adjustment for a couple of years.
(insanity may be the end result).

*Ms. Tisha Hayes*

*AA4HA*
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