[Hallicrafters] SX-17 Recapping - NOT

[Barry H]

Hi Bill & list:

Yup -- excellent example of a radio that triggers the controversy between wholesale recapping vs. don't fix what ain't broken camps - and then there is the authenticity ideology.  Personally, I'm torn between all of 'em, and the ultimate answer is "it depends".

Ironically, some of the old wax caps still have more life in them than the whiz-bang molded paper ones (black beauties) that followed, but may well be failure prone.

Given that it's so difficult to get to them -- and it's true -- there's a possibility of unintentionally violating the physician's credo of "do no harm", I suggest a conservative approach with this one.

Since you just recently repaired it and things are relatively fresh in mind, you can do some additional detailed PM.  For example:

1.  Visually inspect each wax cap for wax migration for lack of a better term -- where the seal is about to go, wax cracked or seeming to ooze out of the ends.  Either replace or reseal those in place with heated tip of a screwdriver, etc.  I suspect one failure mode is that the wax seal fails from ambient heat, then the insides are exposed, then ....  So, resealing might buy some time -- maybe a lot of time.

2.  Do a full set of tube pin voltage and resistence measurements -- if that's available in the service literature.  If not, do what you can with an ohmmeter across the caps in place.  As they're mixed in the wiring that will take some interpretation and a grain of salt, but you might find some low resistance readings where they shouldn't be.  If so, and sure, just replace those suspect ones.

3.  Study the schematic for those caps most likely to do serious collateral damage, like bypass caps going to ground, whereby a short would take out a coil or transformer.  If accessible, it might pay to pre-emptively replace those. 

4.  Like the rest of us, I'm sure you have other radios and won't be running this one 24/7.  So, "baby it" -- run it periodically to keep it "energized" but not stressed.

5.  Check your line voltage.  The older the rig, the lower the AC voltage assumed in the design.  I suspect the SX-17 design assumed 110-115 VAC, tops.  Line voltage has crept up over the years.  I have a bit over 120 vac at home, and as high as 126 vac at the office.  This also varies seasonally, with partial brownouts during the summer.  If your line voltage is on the high side -- 120 or more, use a variac/autotransformer to reduce it or a bucking transformer.  If using a variac, monitor its output continously with a meter or, if it has a built in voltmeter, verify its accuracy.

You could further reduce stress on it by bringing it up slowly on a variac each time you power up, however some of this is false security.  Tube rectifiers don't turn on until about 90 v or so, thus as you gradually turn up the juice, nothing is happening on the B+ until a certain point and then it's turning on all at once.  Some temporarily or permanently replace tube rectifiers with solid state to get around this, but they you'd need a dropping resistor for the B+.

The main thing, though is to ensure that the old caps and other components aren't being overstressed due to high line voltage.  There are no guarantees, but it couldn't hurt.

Hope this helps in your deliberations -- though sounds like you made your mind up already.

Barry








Bill wrote:

Since I have finished repairing my wonderful SX-17 (addition of replacement
"S" meter), I decided, while it is on the bench, do a recap.  When I
received the radio, the electrolytics had been replaced under the chassis
with the original on the chassis.  Nice and the radio works great.

Upon flipping it over and removing the cover plate, I was shocked at the
density of the components and dismayed at the inaccessibility of lots of wax
paper caps.  Recapping this wonderful, fully functional, completely aligned
receiver would be extremely difficult if not impossible.  Some of the caps
would require the removal of the entire bands switching section to get at
them.  Not a very appetizing prospect.  Others are packed so tightly with
other parts, I would probably do more damage than good.  Resistors look like
the very first types ever made with strange color coding.  And to make
matters worse, the schematic does not have resistor or cap values printed on
the schematic just component labels.  You must refer to the parts list to
get the value of C11 then back to the schematic to confirm C11 is correct,
then back to the receiver chassis and locate C11 which miraculously is
soldered 180 degrees out so you can't read the label.  I wonder if this was
done deliberately.  I'm too old to do this.

So, it stays as is with fingers crossed these caps last at least another few
years.

K2WH





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