[Hallicrafters] I "Stabilized" my SX-71 some

[Philip Atchley]

Hi Y'all,

As some (most?) on this list are probably aware of, I've been complaining 
about the stability of my SX-71 receiver for some time (actually ever since 
I overhauled it a couple years ago).

After warm-up, it was "fine" for AM reception right on up through the 15 
Meter SWL bands (may need a slight occasional tweak), but was NOT a very 
enjoyable set to use in CW or especially the SSB modes on any band as it 
needed constant "tweaking" of the tuning dial to keep the voice "decent", 
even after several hours of operation.

Anyway, I'd tried various things like propping the lid open for ventilation, 
setting a 12 Volt fan on the top of the set to "suck" the hot air out etc. 
That last item dropped the internal temperature a LOT and helped stability 
SOMEWHAT, but it still needed tweaking.

Last week I decided to try an internal fan to blow air out the top and just 
generally keep the air "stirred up".  I took a little 4 post terminal strip, 
mounted it using one of the screws that holds the right hand dial lamp to 
secure it.  I then took one 1N4007 diode and one 2200uF 16 Volt capacitor 
and made a half wave rectifier circuit, robbing 6.3 Volts from the same dial 
lamp lead.  This provided 8.5 VDC for a fan (about an even 8 Volts under 
load).

I then connected a 12 Volt, 3 inch fan to this terminal strip and mounted it 
to the underside of the lid, OVER the power transformer and 5Y3 rectifier, 
blowing UP to exhaust the heat.  The fan, running MUCH slower than it's 
intended to is totally quiet, I can't hear it even if I put my ear to the 
top of the receiver.  The fan was secured using solid hookup wire through 
opposite corners of the fan and the top lid, no holes drilled to deface the 
receiver, nearly instantly reversible.

HOW DOES IT WORK?  MUCH better than it ever did.  With the slow fan it runs 
a fair amount warmer than it did with the 12 Volt fan sitting on the top, 
but cooler than no fan at all.  It seems to be MUCH, MUCH more stable than 
it was before, with or without the fan.  I actually listened to the West 
Coast Swap Net on 7240 on BOTH Saturday and Sunday (3 hours each day) with 
almost no drift.  I just had to tweak the tuning very slightly a couple 
times during the first hour (set was turned on about 45 minutes early).

I suspect that this is a case where the slight lowering of the temperature, 
combined with "stirring the air around" in a steady pattern is better than 
"too much" room air being pulled through the set, or the set being allowed 
to "cook" at full temperature.

73 de Phil,  KO6BB
http://users.mercednet.com/beaconeer/
Merced, Central California, 37.3N  120.48W  CM97sh

***************************************************
* RECEIVERS:                                      *
*  #1. Homebrewed "Mitch Lee" Design              *
*  #2. Hallicrafters SX-71 (1955)& VLF Converter  *
* ANTENNAS:                                       *
*  #1. 20 Meter long "loaded" Sloper oriented E/W *
*  #2. Half Sized G5RV Inverted Vee oriented N/S  *
*  #3. Roof mounted Hustler 6BTV 6 band Vertical  *
*  #4. 15 Meter-band Inverted Vee oriented NE/SW  *
*                                                 *
* Number of different NDB's heard: 974            *
***************************************************