[Hammarlund] T35 on the SP-600JX-17

[James A. (Andy) Moorer]

I answered a question on another list that I thought others might 
appreciate.

The question was about T35 in the SP-600JX-17. This is the torroid - the 
only one in the SP line - that both drives and receives the VFO signal 
to/from the SO-239 connector on the back of the frequency control unit. It 
is always broken. Always. I've never seen an intact original one. Here is 
how I replace them, followed by some editorial comments. Your mileage may 
vary.

========================
I rebuild the torroids as follows:

I start with a core from Micrometals. A T37-6 or T44-6 will do. If your eyes
are good, you could use a smaller one.

I wind it as follows:

Tap 1 followed by 4 turns
Tap 2 followed by 10 turns
Tap 3 followed by 4 turns
Tap 4

(Tap 1 and Tap 4 aren't really taps - they are the ends of the wire)

The original was 2-10-2, but that doesn't put much voltage on the output. 4
turns puts about 1.5 VRMS. Note that whatever you feed it with has to put
out that voltage.

I then cover the whole thing with epoxy. I hold it to the PC board with
nylon washers with a #6 screw and nut.

Remember not to twist the wire together when you bring out the taps. That
just adds capacitance (bad!).

I have restored a number of JX-17's (I'm not sure how many) and I have never
seen one without a broken T-35. You can't just glue it together - when it is
broken, it also breaks the magnetic circuit. Note that this means that you
can't put two standard JX-17's together and expect it to work, since both
their T-35's will be broken. With this replacement in both receivers, you
can. I have tried it and it works pretty well.

I believe the problem is that the torroid was made early in the history of
ferrite materials and they didn't take stresses from thermal cycling into
account. A little humidity in the air plus a red-hot tube receiver and *pop*
goes the transformer.

This coil does not resonate - it is just an autotransformer.

Note there is another cool usage for that VFO input - you can feed it with a 
classy crystal-controlled oscillator, like an HP 8640 or the like. You have 
to offset the frequency by the IF frequency, but it otherwise works pretty 
well. You still have to tune it in on the dial to align the RF coils.

I think I have noted before that you also get a big surprise when you find 
out how far off the resonance in the RF amplifiers is when you get away from 
the two frequencies that you align to. It is supposed to track across the 
band, but it doesn't - not by a long shot.

Hope this helps.

James A. (Andy) Moorer
www.jamminpower.com