You will definitely have to remove the bottom cover.
Simple, a few
screws. The S20R is a straight forward receiver and not
difficult to
work on. When restored they are quite good performers
and worth the
trouble.
What instruments do you have? To start out you must have
a VOM
(Volt-Ohm-Milliamp) meter. Even a very simple one will
do. Perfectly
adequate meters are available at places like Harbor
Tools for around
$15.00. Without one you can't do very much.
I also suggest looking at tubebooks.org at books on
trouble shooting.
A couple of first suggestions: look at the rec/send
switch to see if
its working. Use the ohm meter first. Secondly, check
the switch in the
headphone jack. If its not connecting the speaker you
won't hear
anything. Plugging into the jack a few times may clear
the contact. Note
that the headphones are connected to the audio driver
stage so the
phones must be high impedance to hear anything there.
Old fashioned
magnetic phones, of the sort used for crystal radios,
are required.
Do measure the resistance of the tube filaments. That
will at least
eliminate any complete duds but burned out filaments are
actually quite
rare.
Once you have a VOM do the socket resistance checks. A
variation of
20% is normal. It is helpful to follow the schematic
when doing the
measurements so you know what you are measuring. In some
places there
may be capacitors involved so that the measurements will
change with
time and perhaps polarity without indicating anything
wrong.
Then do the socket voltage checks. This requires
removing the bottom
cover. If you are not used to vacuum tube gear watch
your fingers, you
can get quite painful shocks.
For general trouble shooting always begin with the power
supply. If
the power supply is not working nothing else will work.
Again, in general, begin at one end and work toward the
other, from
input to output, or sometimes the other way. Do ONE
thing at a time so
you know what has changed.
Thoroughly clean all the switches before you begin.
Evidently the
power switch gave up since it appears to have been
bypassed. This is on
the tone control switch and is an add-on type switch. I
had to replace
the switch on my S-40A, which is almost identical to the
S-20R. It may
be the Send/receive switch is also kaput, look at its
terminals with the
ohm meter to see if its working. This is a plain Jane
toggle switch so
will be relatively easy to replace.
All the tubes in the S-20R were originally metal except
for the
rectifier. There are glass (GT) versions of all of them
but the metal
ones provide some additional shielding. If your tubes
turn out to be
good just leave them.
All the paper and electrolytic caps will have to be
replaced. An
original can type filter cap is available from Hayseed
Hamfest. He may
also have a kit of other caps. Modern plastic caps will
much improve the
performance of the receiver. There is one low inductance
cap in the RF
section, has a woven lead at one end. Just replace it
with a modern
plastic cap, they have much lower parasitic reactance
than the original
paper caps.
You will have to go over the resistors. Carbon
composition
resistors invariably increase in value with time. You
will probably find
that all resistors with values of 50K and above are way
out of
tolerance. Modern carbon film resistors are better than
the old ones
when new and will last forever. Probably any resistor
with value within
20% can be left in place.
Since most S-20R receivers were built in war time or
shortly
afterward you may find some "bult up" resistors
consisting of paralleled
resistors where the right value was not available.
Note that the original speaker is electrodynamic, that
is, the
magnet is an electromagnet and acts as a choke for the
power supply.
Mostly they are still good. If the speaker does have to
be replaced for
some reason a permanent magnet speaker can be used with
the field coil
replaced with a 1000 ohm about 5 Watt wire wound
resistor. See the
S-40A, which is nearly identical, to see how this is
done.
In my S-20R I replaced the detector and noise limiter
with the
circuit from the S-40A but the original works OK.
If you have questions I will try to answer them plus you
will get a
lot of help from others on this list.
There is an old book "Modern Radio Servicing" by Girardi
(not sure
of the spelling) which is helpful, I think its at
tubebooks.org
Good luck, neat receivers.
On 4/16/2024 4:39 AM, Bruce KX4AZ wrote:
On Sat, Mar 30, 2024 at 09:19 AM, Bruce
KX4AZ wrote:
....decided to take a chance on an Ebay sale of an "as
is" unit.
The cosmetic appearance didn't look too bad, and the
fact that it
powers on with at least /a /hiss?in the speaker, /and/
that it was
part of a ham estate,? gave me just enough hope to
make an offer
that was accepted.? But once I have it in hand I will
need to lean
heavily on the S20R experts here, since I have limited
skills
(truthfully, ZERO) with regard to troubleshooting
and/or restoring
tube radios of this type.
An update on this, I received the S20R boat anchor
last week, and I was
encouraged by the above average cosmetic appearance of
the cabinet, and
the fact that the power cord had previously been
replaced.? The unit
powers on as soon as plugged in (power switch
bypassed?), with dial
lights and tube filaments illuminated....at least the
tubes with a glass
enclosure.? But there is absolutely nothing detectable
from the speaker,
not even a hiss of any kind.? I have the schematic and
service manuals
in hand...but I am pondering what the next most
logical steps might be.
I am thinking that would be to open the chassis cover
to see if any of
the caps etc have already been replaced.? Would
checking for the
presence of B+ voltage be another key step? And I
suppose checking the
filaments on the non-glass tubes would make sense too.
--
Richard Knoppow
Los Angeles
WB6KBL
SKCC 19998
