[Hallicrafters] My ramblings about S38 grounds

[Mark Shaum]

Woke up this morning at 4 am when I realized one of my sump pumps is running 
continuously.  Strange as they haven't run since springtime due to drought 
conditions.  As access to my basement is from outside only and its 12 
degrees, I decided to defer troubleshooting until it gets light out and I 
can inspect the sump discharge line which might have split due to ice 
plugging the PVC pipes.  So I'm reading about the S-38 ground situation to 
pass the time.

Staring at schematics, it appears the S-38, S-38A and S-38B have the 
switched side of the AC line going to "chassis ground".  A capacitor, 
varying from 0.1 to 0.22 uf then connects "chassis ground" to "case".   This 
could be nasty, see below.

The C, D and E models have the switched side of the AC line connected to 
what is called common or circuit ground or  B- which is just a floating wire 
bus running around inside the chassis.  A parallel R-C combination then ties 
this to the chassis ground.  No connection is made to the outer case that I 
can see.

The antenna ground is through a capacitor to chassis in either case.

Since things like tuning capacitor rotors and pot shafts are directly 
connected to the chassis, the earlier models can indeed present the classic 
hot hot chassis zap danger if you manage to touch those shafts or any screws 
that are directly connected to the inner chassis.  I'm going to open up one 
of my earlier models to see if the .1 to .22 uf capacitor mentioned above 
actually connects to the outer case.  If so, and that cap shorts, you would 
have one side of the AC line on the cabinet.  Depending on how your AC hot 
and neutral is wired, that is a very deadly situation.  The later models 
appear to have the outer case isolated from the two grounds mentioned, so 
the only contact to the chassis is via control shafts.  Less of a danger as 
long as you keep the knobs on.

My preference is to use an isolation transformer when operating these 
receivers.  And I like to rewire the AC switch so that the neutral is 
connected to the inner ground (circuit or chassis as applicable) at all 
times, and the switch is in the hot side of the line.  Supposedly there is a 
chance of increased hum with this arrangement, but I haven't noticed any in 
the 38's I've modified for hot side switching.  And yes, I replace all of 
those ground to ground to chassis capacitors with good AC rated caps!

73! - Mark K9TR