[Hallicrafters] SX-73 / R-274 Starting Resto
thoyer
thoyer1 at verizon.net
Sat Jan 6 12:24:27 EST 2018
Hi Richard, (and list)
Thank you for the info. I'm working the tuning section today and have found
most of the resistors way out of tolerance.
I also did a sampling of the capacitors. They are spec'd in the BOM as being
ceramic and look like the dog bone type. I checked a couple of the 33pf
coupling caps and they measure over 50pf.
Now my question becomes - since they are spec'd as a ceramic dielectric
should I replace them with ceramic or go to a mica style which is what I'm
more accustomed to seeing in RF circuits?
Tom
W3TA
-----Original Message-----
From: hallicrafters-bounces at mailman.qth.net
[mailto:hallicrafters-bounces at mailman.qth.net] On Behalf Of Richard Knoppow
Sent: Monday, January 01, 2018 4:12 PM
To: hallicrafters at mailman.qth.net
Subject: Re: [Hallicrafters] SX-73 / R-274 Starting Resto
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 O 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 8O 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
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