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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.