[Hallicrafters] SX-25 restoration info

[Mike Everette]

Neither the 6AC7 nor 6SG7 are tubes original to the SX-25.  The 6AC7, I assume, is in the RF amp socket.  Not a good choice, because the 6AC7 is a sharp-cutoff type which doesn't respond well to AVC.  The 6AB7 is the remote-cutoff equivalent; but it's not nearly so commonly available as the 6AC7 which was used in a lot of early Teevy-sets.  

The 6AB7/6AC7 are supposedly higher-gain and lower-noise than the standard 6SK7; but in this receiver it probably didn't make a lot of difference.  The 6SG7 is a suitable replacement for the 6SK7 as an RF and IF amp; but again, in this receiver it probably didn't make a lot of difference.

When working on my SX-25, I did note one difference between the pre-war and post-war schematics.  The cathode resistor on the mixer tube in the early sets is listed as 400 ohms.  Later-production receivers used 250 ohms.  

Changing this resistor to 250 ohms would probably make the most noticeable improvement in overall gain; more even, than swapping tube types (these radios have, and probably always did have poor performance on Band 4; just accept that fact and live with it).  Getting to it is difficult at best; in fact it's almost impossible without a pair of angled-jawed needle nose pliers and a VERY thin soldering iron with a curved tip.  BE CAREFUL of hair-thin wires in this area which connect the coils and band switch.

I replaced 95% of the resistors in my SX-25.  Many of them were over 100% out of tolerance; the rest were significantly out.  The aforementioned mixer cathode resistor was marked 400 ohms, but actually measured 750 ohms.  Obviously, this was one of 'em.  Even one of the big 15K voltage-divider resistors was way off, measuring something like 23K.  I replaced both of them; the originals were rated at 3 watts, carbon type and I subbed 5-watt wire wound types.

PATIENCE.  It CAN be done!  Study the socket layout carefully and plan the surgery before ever heating up the iron or reaching for the wire cutters.  PATIENCE is your most important tool.  If you ain't got it, either get it, or give up.  It's that simple.

And, DO NOT attempt to disassemble the band switch or remove the shield baffles.  The hex head screws which secure them are likely very brittle.  They WILL break, as I learned early on, and which fortunately deterred me from attempting a potentially disastrous disassembly.  I was able to remove the broken screws (one under the tuning condenser) using a pair of heavy duty, VERY long-snouted slim longnose pliers, S-L-O-W-L-Y backing the broken threaded portions out of the chassis.

DO NOT attempt to remove any screws or nuts in an SX-24 or SX-25, and likely other early Hallicrafters sets, without soaking them overnight in P-B Blaster or Liquid Wrench to break any corrosion around the threads; even if you don't "see" it, it is there, I assure you.

The SX-25 restoration was a great learning experience, and when it was finished I had a receiver that looked really nice and "played acceptably" except on Band 4, where it would only hear the loud ones "no matter what."  Tracking the two RF stages was challenging, during alignment; but it finally worked.  Be prepared for a lot of fiddling, in this area.  The SX-24, with one RF stage, is somewhat simpler to align.

Good luck....

73

Mike
W4DSE



--- On Sat, 11/24/12, Glen Zook <gzook at yahoo.com> wrote:

> From: Glen Zook <gzook at yahoo.com>
> Subject: Re: [Hallicrafters] SX-25 sn/H-104930
> To: "DJ" <fourzulufive at yahoo.com>, "Hallicrafters at mailman.qth.net" <Hallicrafters at mailman.qth.net>
> Date: Saturday, November 24, 2012, 2:34 PM
> As for "C":  The two wires "wrapped"
> form what is called a "gimmick" capacitor which has a very
> low actual capacitance.  This type of capacitor was very
> common for coupling the BFO in receivers.
>  
> Glen, K9STH
> 
> 
> Website:  http://k9sth.com
> 
> 
> ________________________________
>  From: DJ <fourzulufive at yahoo.com>
> To: "Hallicrafters at mailman.qth.net"
> <Hallicrafters at mailman.qth.net>
> 
> Sent: Saturday, November 24, 2012 12:43 PM
> Subject: [Hallicrafters] SX-25 sn/H-104930
>  
> Hi all,
> 
> 
> So after almost 5 years after acquiring an SX-25 and
> promptly putting it on a shelf to wait for that rainy
> day yesterday became that rainy day. I decided to start
> the overhaul with a full recapping of which I finished most
> of today.
> 
> Having "restored" an SX-16, S-22, SX-42, SX-62 and a few
> others I feel pretty confident I can handle this one too,
> but having said that all it really means is that I can
> follow direction and know a little bit about radios...
> 
> So my questions to any of you that have gone down the SX-25
> road are now that it's on the "bench" are:
> 
> A. What are the standard improvements if any?
> B. Is there any good reason why there is a 6AC7 AND several
> 6SG7 tubes being used instead of the 6SK7s?
> C. There are two wrapped wires twisted together, but not
> joined between the set of pads for the OSCILLATOR and for
> the 2nd IF.  Is this normal?
> 
> There are a few other things, but I'll start with these.
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