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

Mon Jan 1 16:11:31 EST 2018

     I am not sure what capacitors Hallicrafters used in the R-274. It 
was intended to be their version of the SP-600-JX. I think by the time 
it was built the problems with the infamous Sprague Black Beauty caps 
were known. That resulted in a military work order requiring all paper 
caps in the SP-600 to be replaced with disc ceramics. Hammarlund also 
began using disc ceramics in its other receivers. Have a look and see 
whats in the R-274, if it has either disc or dog-bone ceramics leave 
them alone unless there are symptoms of one being bad.
     There may also be temperature compensating ceramics in the rig, 
they are usually tubular ceramics. There is only one in the SP-600. The 
AR-88 has many dog-bone TC ceramics in the RF section, also almost 
always good. RCA compensated nearly all of the RF tuned circuits.
     The Western Historic Radio site has material on the R-274 and I 
think an article comparing it to the SP-600.  There is also a good 
article on the AR-88 family. I had to replace all the bathtub paper caps 
in my AR-88 because the seals around the terminals had disintegrated and 
the oil leaked out. A mess. The paper filter cap is still good!
     The AR-88 has no electrolytic caps in it. Some of the war-time 
receivers have Micamold or Solar flat paper caps in the RF section. They 
will all be bad and should be replaced with polyprolylene plastic caps. 
The older versions had RCA-made mica caps, which are odd lozenge shaped 
things, they also will usually be OK.
    The R-274 had a current regulator on the oscillator and mixer 
filaments. The regulators are getting hard to come by. National did the 
same thing on the HRO Sixty. They do help stabilize the receiver when 
there are line voltage variations.
    Hallicrafters is notorious for drifting resistors (also National). 
As a rule of thumb any carbon composition resistor, in any receiver, 
over about 100K will have gone up in value and should be checked but 
some lower value ones will also have drifted.
     Good luck, these are supposed to be somewhat difficult receivers to 
work on but are good enough to justify the work.

On 1/1/2018 11:46 AM, thoyer wrote:
> 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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-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL