[Boatanchors] Hallicrafters SX-71 drift

[Philip B Atchley]

Hi All,
This was also posted to the Hallicrafters reflector.

I 'think' I already know the answer but I thought that I'd ask some of
the best minds in all "Boatanchordom" ;-)

The primary (only right now) HF receiver in the shack is a Hallicrafters
SX-71 that was completely re-capped (Paper and 'lytics) and overhauled a
couple years back, and to all appearances is working properly in all
aspects EXCEPT stability.

For the most part, after a warm-up, it is fine for AM on all bands up
through 10 Meters, though it needs a slight occasional tweak on 21 MHz
up.  6 Meters tends to wander a lot when I'm listening to those
telephones <G>.

HOWEVER!  This is NOT a set one wants to use for CW or SSB work.  The
'slightest' change in room temperature OR line Voltage and it wanders all
over the place.  Yesterday I was trying to listen to the 40 Meter swap
net and finally gave up in disgust as I nearly had to keep one hand on
the tuning.  I tried to make a couple 15 Meter QSO's (using the Viking
Ranger).  In the short time I switched on the TX, sent an answer to a CQ
and then switched back to receive the station had drifted right off
frequency and I had to look for them 8^(20 Meters doesn't seem QUITE as
bad as 40, 15 & 10, perhaps because the closed tuning capacitor plates
add additional capacity,  BUT I still need to frequently retune on 20M
also.

The operating room conditions.  The receiver is operated with the lid
propped open about two inches as otherwise it gets very hot (especially
the power transformer).   I've tried operating the receiver with the lid
closed and a small fan placed on top of it, but that doesn't help any. 
It's still hyper sensitive to room temp (a couple degree change makes a
big difference).   

I'm in a mobile home and line Voltage takes momentary "hits" when the two
window air conditioners kick in, though it comes right back to "normal"
while they're running.  But I'm pretty certain the constant slow back and
forth "wandering" in frequency is Line related.  There are NO sudden
jumps as would take place with bad Mica caps etc.

As I said, the unit has been re-capped, I've tried replacing the LO and
BFO tubes as well as the regulator with no change.  

NOW that you know the set-up, here come the questions.

1.  An individual has told me that there 'has' to be a problem in the
SX-71 while I'm not convinced that there is (other than an old analog
design).  Is this "typical"?

2.  I've had a couple "old timers" tell me that back in the '50s the
drift was "expected" and that they just lived with it in "Olden times".  
That if the receiver drifted when they went to TX that they just re-tuned
when switching back to receive (seems awkward to me).  Of course not much
SSB was done in the '50s either.  What say ye??
73 from the "Beaconeers Lair".
Phil, KO6BB

Where DX begins at the noise floor!
Merced, Central California, 37.3N  120.48W  CM97sh

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