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

[thoyer]

Update 1/8/18

Been working through the chassis area replacing the out of tolerance
resistors - which is basically all of them! I had on 3.3 ohm resistor
measure over 200 ohms!

So far so good. I should finish up the replacements tonight then I'll start
in on the bathtub caps. I'm going to use some of the esisting screws to
mount a terminal strip near the electrolytic cap and wire new caps in its
place leaving the original in place, just not wired.

The AC power input connector housing is pretty rusted so I'm going to have a
blank off plate machined at work to enable me to mount a new three prong ac
connector to the existing chassis holes. I'll use a Corcom power entry
module. While I strive to keep things original (don't ask me what my other
hobby is....) I feel this is more of a safety improvement and can easily be
reversed if needed.

Once I get the caps sorted and the power connector done I'll reassemble
everything and break out the variac. Yippee!

Latest pics of progress can be seen here:
http://www.thdesignsinc.com/R-274.html

One note: I do need a 455khz crystal for the crystal filter any suggestions?

Tom
W3TA


-----Original Message-----
From: hallicrafters-bounces at mailman.qth.net
[mailto:hallicrafters-bounces at mailman.qth.net] On Behalf Of Richard Knoppow
Sent: Saturday, January 06, 2018 1:28 PM
To: hallicrafters at mailman.qth.net
Subject: Re: [Hallicrafters] SX-73 / R-274 Starting Resto

    I would just leave them alone. They may even be temperature
compensating. Unless they are leaky (very unusual for ceramics) or have some
other problem there is no point changing them. Mica caps are fine where one
needs great stability. In fact polypropylene caps are even more stable.
Drifted carbon comp resistors seem to be endemic to Hallicrafters rigs. All
carbon comp resistors drift up and the higher the original value the more
they drift. No matter what brand any resistor over about 100K should be
checked and its likely those over about 250K will have gone quite high. My
experience is that Ohmite were the most stable, Allen-Bradley the next best.
I seldom see IRC resistors so I don't know about them. IRC must have either
been expensive or there was some other problem. They advertised fairly
heavily so I've always wondered why there are not more in commercial
equipment. Anyway, Hallicrafters seems to have bought bargain basement
resistors. So did National. Modern carbon film or metal film resistors are
extremely stable, new ones will last forever.

On 1/6/2018 9:24 AM, thoyer wrote:
> 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

--
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL
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