Sx-25/power supply

[N9axl]

My new project is restoring a hallicrafters sX-25 super defiant.  The name alone makes the project worthwhile. I have little idea if this is considered a quality réceiver but the price of $28 made this a no brainer.

Now, I intend to test this with a variac obviously after I recap, etc. Should this have an isolation transformer permanently installed? I've been looking for examples of how this has been retrofitted to have a modern safe grounded plug but not seen one.

[KX5JT]

Sure it's a worthwhile project!  The SX-25 Super Defiant is a classic example of a mid-level radio from 1940.

Concerning the 3 wire grounded plug, have you seen this page?  I think it's mostly about the AA5 type radios however.  The SX-25 has a power transformer so isolation is not an issue but you probably want to put a 3 wire plug on it, or at the very least a polarized plug.   The other issue is maybe to use a bucking transformer to bring the voltage down to 110 vac ala 1940.

http://www.geojohn.org/Radios/MyRadios/Safety.html

[WQ9E]

As John noted there is no need for an isolation transformer since the SX-25 is a transformer operated set so its chassis is already isolated.

I have but very rarely use a Variac.  Having the proper fuse installed will offer protection against most major failures/errors.  With its high vacuum tube rectifier little B+ will be produced until the rectifier filamentary cathode is up to temperature so the variac doesn't provide a smooth start up in this case anyway.

The SX-25 is a decent radio, it was a step down from the SX28 but a big step up from the S-20R, S-19R, etc.  It is an upgrade from its little brother SX-24 which uses a single RF stage and single ended audio while the SX-25 has two stages (mostly providing better image rejection on the higher ranges) and push/pull audio.

You may well find a few resistors out of tolerance so check them while recapping.  You are looking for increased resistance and in general there is no need to lift one lead, most resistors can be checked in circuit.

Be sure that a proper fuse is installed, Hallicrafters was sort of hit or miss using fuses during this era so if there isn't one add it.  You can use an inline holder to avoid drilling holes.  Be sure to install modern line rated bypass caps in place of the original line bypass caps.  I use 3 wire cords and install the "hot/load" side bypass cap on the protected side of the fuse.  The fuse and power switch should both be in the "hot/load" side of the line.

It is common to find that the crystal element used in the IF filter is contaminated by this time and not working properly.  Carefully disassemble the holder and clean the element and its contact plates with isopropyl ("rubbing") alcohol, dry with clean paper towels and reassemble without touching the element or holders directly with your fingers.  If you have a signal generator and scope or sensitive RF voltmeter you can do a quick before/after cleaning test.  Connect the hot lead from the generator to one crystal terminal and the hot lead to the scope (or voltmeter) to the other.  Attach the ground leads from the generator and scope to each other.  Slowly sweep the signal generator around 455 Khz. (the crystal could easily be 1 or 2 Khz. either side and final IF alignment is done to the exact crystal frequency).  You should see a very sharp and distinct peak from a good crystal element.

I cannot remember whether Hallicrafters uses high or low side tracking on the highest range so if calibration and tracking appear poor try tracking on the opposite side.  All other ranges use high side tracking but I vaguely recall some of the Halli receivers of this era using low side tracking on the highest frequency range.

[n1ps]

All great advice.  Further thoughts....

On the fuse, I tend to use a snap in fuse holder in the underside.  There often isnt room on the rear apron.

I like a variac to test.  I use a clamp meter in association with the variac and monitor the AC current while powering up.  To get the meter in a readable range, I use 10 turns of wire on the hot side and clamp that. The meter reads 10X what the actual value is in AC current, but it is very accurate.  The clamp meter is one of the cheapies from ebay (years ago). 

Peter

[KA0HCP]

www.radiodaze.com  has snap in fuse holders that can be epoxied or bolted on.  Radio Shack used to carry Littelfuse fuse holders.