[Hallicrafters] Re: S-38E BFO

[Mark Shaum]

Bill,

The circuit ground (noted as B- on the schematic) and chassis ground are 
different.  The grounds are connected together by the parallel resistor 
capacitor pair R22 (470K) and C24.  One side of the AC line is connected to 
the circuit ground, not the chassis directly.  There are differing symbols 
for the two grounds on the schematic.  This makes following grounds a bit 
confusing as some components connect to circuit ground (tube cathode 
resistors etc.) and others connect to the chassis ground (tuning capacitor 
rotors, some RF coils through capacitors).  Treat both grounds as the same 
for RF and chasing signal paths will make more sense, while ohmeter readings 
will remain confusing.

My S38E is not a MkII, but the BFO didn't work in it either.  "The S-38 Guy" 
(link to his excellent S-38 info is on the HCA web site) has some discussion 
about how most S-38E's likely had BFO's that didn't oscillate.  I had to add 
a tiny bit of additional capacitance between the grid and plate of the IF 
amplifier in my set.  I used a "gimmick" capacitor, just hookup wire twisted 
together for a turn or two.  I varied the twists (I think I ended up with 
two half inch pieces of wire twisted one turn, probably just a couple pf of 
capacity) until I could toggle the BFO on and off reliably with the front 
panel switch.  And yep, mine doesn't exactly match my schematic either. 
Sometimes I wonder if the Hallicrafters engineers were paid on a number of 
"circuit improvements" per week basis.

Now to get on with my S-38B that I can't get to STOP oscillating at the IF 
frequency even with the "BFO feedback wire" removed!  These simple circuit 
sets certainly can have their share of strange issues, but they are fun and 
a lot easier to struggle with than a 100 pound exciter or amplifier!

73! - Mark K9TR
http://www.qsl.net/k9tr

----- Original Message ----- 
From: "Bill Gerhold" <k2wh at optonline.net>
 For instance, I found (5) individual components (choke, caps and resistors)
 in the radio that the schematic shows connected to the chassis.  These (5)
 components are all tied together as they should on one end as they would be
 if connected to the chassis but there is no connection to the chassis as
 verified visually and with an ohm meter.  But, it works?????????  Go 
figure.

 So, while it works, I will button it up and go to the next project.