SX-28 Power Transformer

Tom

unread,

Sep 20, 2009, 6:08:08 PM9/20/09

to

I bought it last year and restored it because my Elmer had one during
the early '50's and it always seemed the thing to have. I enjoyed the
restoration process (skiped the caps in the one RF section) and
alignment and had it working fine. Then the 5Z3 rectifier lit up
brightly a couple of times upon turn-on and then later, it blew a
fuse.
>I recapped an SX-28 and did the usual cleanup etc and then realigned it with care. All the voltages were within 10 to 15% and it worked well for a few months. Then the 5Z3 rectifier lit up with a flash twice after it had been off for a couple of weeks. Then while operating it blew a fuse. At that point I replaced the fuse and it blew immediatly. I then used a Variac and ampmeter and noted it would draw an amp with only 5 to10 volts on the transformer primary.

> I unhooked the caps on the line side, pulled the rectifier and jumper plug in the rear to isolate the transformer and it will still draw an ampere with only 5 volts on the primary. The line cord is not shorted and the primary is not shorted to ground and it does measure low resistance ( a couple of ohms) end to end which seems normal.
>
> It appears the HV secondary is shorted - it measures 19 ohms to CT on one side and 16 ohms on the other to CT and 3 ohms end to end. No shorts to ground.
>
> I have a parts SX-42. Do you know if there are any problems in using the power transformer from the SX-42 in the SX-28? They measure physically about the same and the schematics are the same but I don't know about the power capabilities. Do you have the transformer numbering for T7 or the SX-28? Any advice on the substitution?
>
> Thanks for the help and guidance.

Tom Ruhlmann, W9IPR
262-377-6945

Roger Basford

unread,

Sep 21, 2009, 4:07:29 PM9/21/09

to

Not a lot of help maybe, but when my SX-28A mains transformer burned out I
had Danbury Electronics here in the UK make a replica, and they did a nice
job, incorporating the 115/230 volt switch and top cover onto their own
laminations and core. Cost was 100 GB pounds, not cheap but the RX is worth
it. I tried two transformers from two junker RXs before going to Danbury,
wish I'd saved the hassle as I found the two transformers in the junkers had
faults too. Danbury reported that the wax in the original had basically
turned to a sort of dust, they believed that the wax was organic, not
mineral.

73

Roger/G3VKM

"Tom" <teruh...@sbcglobal.net> wrote in message
news:132e9ea7-aa0e-49a4...@l31g2000vbp.googlegroups.com...

Tom

unread,

Oct 31, 2009, 9:22:59 PM10/31/09

to

I completed the installation of the SX-42 power transformer into the
SX-28. Mechanically it fits just fine - no problems.

The SX-42 transformer is 250 VAC either side of CT while the SX-28
transformer was 290 VAC either side of CT. This results in a 15%
reduction in plate voltage but the transformer runs at a normal 110
degrees F and the receiver seems to operate fine with no notable loss
in sensitivity etc.

Be sure to note the transformer terminal numbers per voltage etc.
befor attempting the replacement and write them on a schematic or
drawing.

Tom : W9IPR

Bill Baka

unread,

Oct 31, 2009, 10:16:09 PM10/31/09

to

Guys,
I designed test equipment and then more stuff to test the stuff we were
building, so here is what I did for the test floor stations. The power
had 2 settings, one for first power had 25 Watt Christmas tree bulbs in
series and a bright light meant trouble, but at least saved the smoke
routine. The second setting was straight power for the final test.
Simple?

Bill Baka

paul @removeppinyot.removecom Paul_P

unread,

Nov 4, 2009, 12:18:17 AM11/4/09

to

> Guys,
> I designed test equipment and then more stuff to test the stuff we were
> building, so here is what I did for the test floor stations. The power had
> 2 settings, one for first power had 25 Watt Christmas tree bulbs in series
> and a bright light meant trouble, but at least saved the smoke routine.
> The second setting was straight power for the final test.
> Simple?
>
> Bill Baka

That be called a Dim Bulb tester. Not referring to the user but the device.
See hw to make one here at:

http://www.antiqueradio.org/dimbulb.htm

Paul P.

Bill Baka

unread,

Nov 4, 2009, 12:23:27 PM11/4/09

to

Like I said, it isn't rocket science. At home I use a Variac and a Fluke
77 on current range (20 Amps) to monitor it. I got lucky and have one
with a housing and voltmeter (5 Amp). On a new find I also use a light
bulb to protect my Variac. All you need to make is the light bulb
fixture and the above URL from Paul_P points to a picture of a good
example. With a Variac the switch can be eliminated.

Bill Baka