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

[thoyer]

Hi Charlie, 

Well you helped me with my 51S-1's and now the R-274 - one stop shop for
advice! Thanks.

I'd be interested in the write up you are doing from your overhaul
experiences.

I have a couple other radios waiting in the wings for some attention that
may be easier then the R-274 (RCA CR-88A and a very early SP-600) but I'm
drawn to the R-274 right now. Both of these had been gon over before by
previous owners but I'm always leary of others work and will walk through
each to bring them up to snuff. This R-274 appears to be all original.

Looking through the parts list I see that a lot of the pF capacitors are
shown as being ceramic dielectric, I'm surprised they are not mica. Will
there be any issue replacing them with mylar or mica if needed?

I will look into the Tree Top Circuits board - but that is probably at least
a month away - lots to do first.

If anyone has interest, I'll be uploading some pictures to my website later
today  www.thdesignsinc.com/r-274.html


Tom
W3TA

 

-----Original Message-----
From: hallicrafters-bounces at mailman.qth.net
[mailto:hallicrafters-bounces at mailman.qth.net] On Behalf Of Charlie T
Sent: Monday, January 01, 2018 11:42 AM
To: hallicrafters at mailman.qth.net
Subject: Re: [Hallicrafters] SX-73 / R-274 Starting Resto

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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