[Hallicrafters] SX-73 / R-274 Starting Resto

[Charlie T]

Along with the usual capacitor suspects, carefully check all those low value
resistors in the IF strip.
Basically, the tuned circuit bandwidth of the IF is around 2 - 3 kHz which
is fine for SSB.

Various broadening low Ω shunt resistors and/or coupling coils are used to
widen the bandwidth in steps.
I just did two of these radios and ALL of those resistors were WAY off in
value, some measuring 3 X their nominal value.
With those higher than spec resistors in place, the bandwidth is never
widened as far as it was designed to be.

Also, the S-meter & AGC circuit must have been designed by a team of Boris
Karloff & Bela Lugosi.
I never could get it all to work right after screwing with it for many
hours.

I used a beautiful circuit * board from TREE-TOP circuits in Canada (Where
they have STRONGER Beer) to drive the AGC & S-Meter as well as provide great
sounding SSB AND AM detection.

Another minor suggestion is to change out the audio output transformer for
one that allows direct connection of a 4 or 8Ω speaker, but this isn't all
that necessary, just a slight convenience.

I am in the process of writing a restoration article describing all that I
did.  I will be selling one of these radios and will keep the other. They
both are in about the same condition.

73, Charlie k3ICH

*A TREE-TOP CIRCUITS "SB-75" was used with a few slight mods to the board
for the SX-73.
This circuit board is designed to almost be a plug & play mod for the
Collins 75A-2 & 3.
I have used it for the Collins and it works great, so after many
conversations with Bob at Tree-Top, I decided to give it a try for the
Hallicrafters.
I am very happy with how it turned out AND Bob's advice was priceless too. A
very nicely done board AND a clever design.


-----Original Message-----
From: hallicrafters-bounces at mailman.qth.net
[mailto:hallicrafters-bounces at mailman.qth.net] On Behalf Of thoyer
Sent: Monday, January 01, 2018 10:59 AM
To: hallicrafters at mailman.qth.net
Subject: [Hallicrafters] SX-73 / R-274 Starting Resto

I'm getting ready to start working on an R-274 that I picked up this past
fall. A quick survey of the internals shows that many of the capacitors show
evidence of decay (cracking) and the resistors I did a quick measurements on
are out of tolerance.
 
First order of business will be a complete disassembly and cleaning (it is
pretty dirty and smelly.....). Then replace any out of spec parts,
reassemble and see how it plays. It is mostly original from what I can tell
with the exception of a replacement 455kc crystal.
 
Looking for advice from those who have "been there, done that" with one of
these radios before. What should I look out for - what parts are typical
troublemakers, etc.......
 
Just finished an overhaul on an R-390A that came out well and have been
tinkering with these boat anchors for many years so I have some experience
and a bunch of test equipment. Never been inside one if these so looking for
advice.
 
Regards, 
 
Tom
W3TA
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