On Sat, 10 Mar 2012 21:28:25 -0600, DaveW wrote:

I've begun the restoration of an S-38B. Just started to replace the
filters an I already have a question. The old multi-section cap ground
is connected to pin 2 of the 35Z5 rather than directly to the chassis
ground as shown in the schematic he
http://i1206.photobucket.com/albums/...watcher32/S38-
BFilamentStringFilterCap.png
Pin2 is connected to the chassis ground through the filament string,
which has a 125 ohm resistance when cold. Is this a problem or were all
S38's wired in this manner?

The filter cap appears to be original but you can't always be certain.
It's clear other repairs have been made during its lifetime.

If anyone can recall recapping an S-38 I'd be interested in your
observation regarding the filter cap ground wire location.

Thanks,

DaveW

I have never worked on an S-38 but I don't see how it could work wired
the way you described. Perhaps someone botched a recap job, couldn't
figure out why a new capacitor caused hum as bad or worse than the old
cap, and put it away for some future troubleshooting.

Are you sure you counted the pins correctly and that you are, indeed,
looking at the 35Z5 socket?

I checked a tube manual and your schematic shows the correct connections
for a 35Z5. The physical location of the capacitor's negative lead
should be where the wire from the switch connects to the chassis although
some other places are likely to work as well. The idea is to keep the
ripple current flowing through the capacitor from getting into other
circuits. The chassis has resistance and inductance and this current
causes voltage drops that can inject hum into sensitive circuits.

As others have mentioned, it's not the safest way to design a radio, but
it did work.

Good luck,

--
Jim Mueller

To get my real email address, replace wrongname with dadoheadman.
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