Hallicrafters MK-3 update

Eric Lawson elawson at INFICAD.COM
Sun Jan 18 12:47:42 EST 2009

I've finally got some more time to get back to the Hallicrafters SX-101 that
I was working on last March.

The radio is more or less working.  After making a bunch of resistance
checks and fixing the stuff below, I brought it to life with some series
connected low wattage light bulbs.  Finally I took all the light bulbs out
of the circuit and "let it fly".

The 1/4A under chassis resistor that protects the oscillator filament
transformer was open.  Someone had wired the damp chaser (R-89) so it was
also protected by this fuse.  The damp chaser's resistance measured 80 ohms,
which was a bit less than the specified 8W dissipation would suggest.

I carefully inspected the resistor and found a tiny chunk of metal (solder
blob?) shorting part of the wire wound resistor to ground.  I dug that out
and made sure the insulation and such was OK and when I checked the
resistance, it was up to 3.3K.

Then I found the oscillator would only oscillate correctly on the "band 7"
(10MHZ WWV) setting.  On the other bands, the circuit was oscillating around
41MHz.  I tracked this down to another metal chunk.  The wire coming from
the tuning capacitor and the two caps that form the divider for the colpitts
oscillator use a spare terminal on one of the oscillator coils.  Amazingly
there was another chunk of metal this time looking more like a metal
shaving, shorting this terminal to ground.

The damp chaser is directly below where the oscillator coils are located, so
probably one event created all the "metal shouldn't be there problems".
Since I hadn't plugged the radio in before finding these things, and the
fuse was open, I don't think *I* caused the problem (but I'm probably biased
here!).  I guess I'll never know.

I can get the dial to read correctly on all bands, except 15M, but the dial
pointer set knob must be fully counter-clockwise.  The tuning mechanism is
correctly synched, and I tried deliberately unsynching it to get things to
read right, but I discovered that wasn't a good idea!  It's a shame I'll
have to touch the oscillator adjustments as they track within 1/2 a dial
division throughout the entire tuning range on each band. It looks like the
oscillator coils' adjustments are soldered in place....Oh well....

Next is a check of the pink capacitors.  Everytime I see them, they look
like a pink version of the BBOD caps..... :-)  And, finally an alignment.

At the rate I'm going, it will probably be another year or so....


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