[Boatanchors] SX-100 Power Transformer (Long)

[David Franklin]

Three years ago, I picked up a Hallicrafters SX-100 Mark 1A on eBay.  The 
radio was one of those that looks worse in real life than in the pictures. 
The case had been painted over with black spray paint and the calibrator was 
missing.  I found a calibrator, cleaned the radio and put it on the bench. 
Next I replaced all of the leaky capacitors and out of tolerance resistors. 
The dial was off so I did a realignment.  After alignment the sensitivity 
seemed to be OK though the radio was still ugly. ;-)

I put it in my listening position and all went well for a few days.  Then I 
noticed that the oscillator began drifting more than normal.  During 
warm-up, the dial frequency would drift up and then down never stabilizing. 
I put it back on the shelf for future analysis.

Last month, I pulled the radio out and placed it on the bench to 
troubleshoot.  I fired it up, tuned a broadcast station then went outside 
for about 15 minutes while she warmed up.  When I returned smoke was coming 
from the radio along with a frying sound and wax running from beneath the 
chassis.  The transformer appeared to be at the point of igniting when I 
pulled the plug.  The ohmmeter showed one side of the center-tapped HV 
secondary winding to be shorted.  I began a search for a replacement 
transformer.

My search led to a Hammond FX-270.  The specifications are good for the 5V & 
6.3V filament current and HV current is rated at 150 mA.  The HV AC output 
is 275V from each phase to the center tap. The main problem was the 270 uses 
a vertical chassis mount instead of horizontal like the original.

Measurements showed the end bell bolt dimensions to be the same on the 
vertical as the horizontal.  So why not use the original end bells and mount 
the transformer horizontally?

I removed the end bells from the 270 and rerouted the leads to one side to 
accommodate the end bells from the original transformer.  The original end 
bells were cleaned and painted and mounted with the original bolts.  The 
Hammond tag was left on the transformer for future identification.  See 
pictures for details.

http://www.mindspring.com/~dfranklyn/xfmr/270FX.jpg
http://www.mindspring.com/~dfranklyn/xfmr/CoverRemoved.JPG
http://www.mindspring.com/~dfranklyn/xfmr/Top.JPG
http://www.mindspring.com/~dfranklyn/xfmr/Bottom.JPG
http://www.mindspring.com/~dfranklyn/xfmr/Covers.JPG
http://www.mindspring.com/~dfranklyn/xfmr/Reassembled.JPG
http://www.mindspring.com/~dfranklyn/xfmr/UnderChassis.JPG
http://www.mindspring.com/~dfranklyn/xfmr/ChassisTop.JPG

The transformer cured the drifting oscillator problem.  Final measurements 
after installation: Line Volts (120VAC)   Line Current (0.78A)    B+ Volts @ 
pin2 of 5Y3 (300VDC)    B+ Current (95mA)

The next step will be repainting to cure the ugly part.

Thanks for the bandwidth,
Dave Franklin
Cornelius, NC