[Hallicrafters] S-72 Continued (long)

[David L. Thompson]

I put the S-72 on the bench and started working on it as I got some spare
tubes and a new leather handle.

First off I got both a S-72 manual and a Sams PhotoFact from Alvin N4IQ.  My
receiver is much closer to the Hallicrafters manual than the Sams.   The
Sams has a toggle switch for ANL in the upper right corner and the
bandspread knob doubles as a tone control.   Mine has neither.  I got into
mine and I also feel that my model never had the battery connection or cable
even though the switch (battery or AC/DC) is there.  There does not appear
to be solder on the connections and where the battery cable could come out
the top of the chassis there is a solid metal plug.   I was told that even
with the low voltage (1.4V heaters) tubes that drain was still excessive.  I
guess this model just left the battery out!

In looking through the schematic, I was told that one defect with the S-72
is that some models had one IF and uses coupling to replace a possible
second IF.  Well this model has a second IF and IF transformer. Funny they
don't identify the transformer except by number but I think its 1.8 mcs.
They don't identify the first IF buts the specs say 455 kcs.

All of the tubes check out except for a V5, a 1U5, which serves as a diode
detector and audio amp. I noticed that the diode detector portion only reads
mid scale of the bad on my Heath TC-2 and reads questionable on a small B&K
Tester I borrowed.   The tube I got from Antique Electronic Supply read good
on all testers and was put in the circuit.  Well, the receiver did not
detect signals as well.  I had to hold my finger to the antenna connection
and then only loud broadcast stations were heard and nothing above 10 mcs.
Tuning standard broadcast stations was painful as the audio was loud even
with the volume on minimum and the BFO/Sensitivity seemed to not work very
well.   I switched the 1U5's around but finally put the one with the poor
diode detector back in and signals were again received.   The BFO operated
the same no matter which of the 1U5's were installed.  I did look at some
notes I got from a W2 and he comments on the BFO needing to be aligned per
the manual (1000 kcs)
to work properly.  I had done an alignment but missed the BFO setting and
adjustment.

Not having the signal generator cables handy I turned on my 23 year old
Yaesu and checked calibration.  I was surprised at how close it was.  I did
find the setting for the bandspread to be backwards as according to the
manual you need to set the bandspread to zero and then tune the bandspread
back down ( zero to 100 is down??) to cover the band you wish to spread out.
Using just the whip, I was able to hear signals on 40 and 20 and the
bandspread covered 40 from zero to 50 and 20 from zero to 45 once I set the
main tuning per the signal from the Yaesu.  The Yaesu has a modulated AM
signal so I used that in lieu of the BFO and was able to tune in SSB and CW
signals on both bands.  I listened to a pair of W0's on 40 SSB and two
Spanish (EA3's) in the Dx portion of the band.  A VE3 was found CQing on CW.
I heard two W6's on 20 SSB and the Yaesu AM signal (I turned down the drive
to zero and put the signal into a dummy load) tuned in SSB and CW easily and
I was amazed at the stability of the receiver.  Now to fix the BFO and
attach a real antenna.   The separate wire for attaching an external antenna
is missing so I will solder one on to the antenna load connection or do what
sandy W5TVW advised to attach the antenna to the whip.   It will probably be
too hot to handle!

The leather handle arrived from AES and its almost an exact match.  Its AES
part # P-H263.  Its a handle for a guitar case and sells for $7.95.   The
problem (pointed out to me by two others) is that Hallicrafters used rivets
to hold the handle.  Now I have to figure how to replace the rivets without
damaging the case.   Suggestions are welcome!  I used neutral shoe spray to
clean up the leather case and plan on using brown polish to bring back the
color and cover the scratches.

Pardon the length of this message and any typing errors.  I had a stroke in
late 1998 and have vision problems (blurred vision and random spots).  Don't
think I would have the nerve that W2BBK had in the early 50's to take the
S-72 as my sole receiver to VP2.  Working several hundred CW stations on 40
plus a dozen on 20 AM is still amazing from what looks like a AC/DC
broadcast receiver at first glance
(without the Gonset converter HI).

73 Dave K4JRB