Turned out the trimmers were not bad but the bandswitch was.  I removed the coil for the broadcast band and hot-wired it into the circuit.  Radio worked perfectly and the RF tube was operational.  I suppose I could substitute the other coils one at a time and see if they work ok but I don't think I will bother.  I will use the radio on the broadcast band until I locate a replacement bandswitch assembly from a non-working S-20R.  If you know of one, please send me an e-mail.

73, Tom, W4OEQ

Figured out the B+ question.  Now, it appears that there is a problem with one or more coils or trimmers in the RF portion of the bandswitch assembly.  Do any of you have a junker Hallicrafters S-20R from which you can salvage the RF assembly (or the entire bandswitch and all three sets of coils)?  Thanks.  Tom, W4OEQ

Well, I am only partly successful with my rebuilding project. There is something wrong with the RF coil section, and I cannot find the problem. I completely dismounted the coils and band switch wafer. Cleaned everything. Replaced the one resistor in that section. Ran dummy voltages using a 6 volt battery. There appears to be a problem with one of the coils but I could not see any physical damage. I did see that there was not perfect continuity in one of the windings......there was a measurable resistance on one winding of one coil. The others did not show any resistance. The problem here is that the B+ feed comes into this winding on this particular coil. If I hook up the B+, it causes real problems, noises, short circuits, etc. I tried skipping that band coil and using the B+ on the other coils/bands. The radio did not like 300 vdc at that feed point but would tolerate 115 vdc, although it still was not functioning correctly on bands 2, 3, and 4. The strange part of this is that if I hook up the B+ where it should be, it sends the same voltage right to the stator of the tuning cap (through the switch wafer). I do not understand why this should happen.......unless there is a short in one of the coils. The wafer wipers seem to be working as they should.

I reassembled the radio without the B+ on the RF stage. The radio works ok on the broadcast band. I have not completed the alignment of the other bands. I may have to operate it without the band where the coil seems to be not functioning correctly.

In summary, I spent 4 days tracking this problem and never got it solved.

Anyone have a suggestion for something I am missing in my analysis? 73, Tom, W4OEQ

This is a duplicate posting that originated on the Antique Radios Forum web site.  I am posting it again here in the hope of finding an answer.

I am almost finished rebuilding my old Hallicrafters S-20R and have discovered that the RF amplifier 6SK7 has no B+ voltage.  It does have screen voltage.  The tube plate pin eight connects to the band switch, and I can verify this connection. The bottom end of the RF coil(s) connects to the B+ line. I cannot find this connection and believe it was lost when I rebuilt the radio. I probably disconnected a wire to get at some of the other components. The B+ comes directly from the power supply and connected somewhere near or on the bandswitch or one of the coils. The schematic diagram describes this as the bottom end of coil T13 and C45D. I have not been able to figure out which is T13 and where the connection point was (should be). Can anyone help me solve this problem? I have not been able to find a service manual for the S-20R. It would have the numbering system for the bandswitch pins and show where the B+ is to be connected. Thanks. 73, Tom, W4OEQ

What about the little "dog bone" resistors found in the area of the band switch?  These are so old the color coding cannot be read.  Are they any more unreliable than carbon resistors?

Unfortunately, the ones in my S-20R probably cannot be replaced without removing the switch assembly.

73, Tom, W4OEQ

I, too, am concerned about these few components that have not been changed out.  But, to fix them, I would have to remove the band switch and all associated wires and components.  I do not have adequate documentation of the radio.  No diagram of the band switch and coils.  No service manual.  Of course, I can take close-up photos and draw diagrams.  I may yet have to do this.  The radio does not have any reception on the highest band.  The others seem to be working ok.  I have DeOx'ed the band switch and resoldered as many connections as I can reach.  Before diving into this problem further, I am going to make certain the radio otherwise is working properly.  My major observation to date is that it really is not very responsive unless a long wire antenna is connected.  Six feet of wire on the workshop floor doesn't cut it.  I also note that tuning the osc trimmer is really touchy on the top two bands.  The trimmer screws probably have not been moved in 50 years. I wonder if I should clean each trimmer???  73, Tom, W4OEQ

If you are still watching this thread, here is an update.  The chassis has been cleaned.  All resistors and caps were replaced.  (Well, I left two that I could not reach under the bandswitch.)  Cleaned controls.  Re-soldered cold solder joints.  Did a stage by stage check tracing the signal from my signal generator all the way back to the RF amplifier.  Did a preliminary alignment and checked it out with a 20 foot hunk of wire antenna.  Radio seemed to be working as good as it could be at this stage of the game.  I temporarily installed an old Palomar Pre-selector and was impressed with how it improved the reception of the old radio.  I will do a second alignment tomorrow while waiting on an order for a replacement RF control.  I think I will replace the AF control as well.  Will also check the RF amp tube on my Hickok tube tester to see if it should be replaced.  Found a tear in the speaker paper and will patch it and hope for the best.  As a final step, I will tidy up the wiring and double-check all solder joints.  73, Tom, W4OEQ

Great suggestion!  I have some of that cleaner and will try it.  Fortunately, the case is in great shape, considering its age.  Repainting will not be needed.  Tom, W4OEQ

Great information!  I will follow up.  My dials are in good shape.  Just the plastic face plate is warped from too much heat in the storage facility where the radio sat for many years.  73, Tom, W4OEQ

Thanks for the suggestion.  I, too, have a Dremel tool and a Michael's store is in a nearby city.  I will pursue this idea.  To clarify, my receiver seems to be an S-20R.  It looks like the photographs that I find on the web.  However, mine does not have chrome decorative horizontal strips on the left and right sides.  Tom, W4OEQ

I'm still here and still working on the Hallicrafters S-20.  I called it an S-20A out of ignorance.  Until I found this one in a box at the bottom of a storage shed, I had never heard of one.  I have replaced all of the resistors and caps and have tested the tubes (as well as can be done with an old Knight tube tester).  I have ordered replacement tubes for about half of the set.  Still have not located a voltage chart but have borrowed one from the S-40 since some of the tubes are identical, although the circuits are not identical.  Have not yet checked out all of the references you folks kindly provided.  The audio output stage is working well but cannot proceed without the replacement tubes.  As a side note, it was a lot easier task when I rebuilt an old S-38.  I will need to replace the dial screen plastic.  One is warped badly.  Any ideas on replacement material?  Many thanks for the comments and suggestions.  73, Tom, W4OEQ

Need the voltage and resistance readings for each of the tube sockets for the S-20A.  I am restoring the radio and need this information to assist my troubleshooting.  The brief owner's manual provides a schematic drawing but not the voltage and resistance readings ordinarily found in the service manual.  Does anyone out there have this information?  Thanks and 73, Tom, W4OEQ

Thanks.  I ordered it from Manualman.  73, W4OEQ

Am repairing an old EMC Model 801 Resistance Capacitance Bridge.  Need a schematic diagram and parts list.  If anyone has this information or has a complete owner's manual, I would appreciate have a copy.  Thanks and 73, Tom, W4OEQ

Thanks for the explanation of the oxide.  I looked carefully at the tape where the brown coating had been stripped off, and all  I could see was the clear plastic.  (The strips that were torn off were about 1/4 inch long and 1/32 inch wide.) I assumed the clear plastic would not carry the sound and that stripping away a lot of the brown oxide would ruin the audio.  This particular tape was recorded on both sides (tracks).  The amount of the oxide that has been removed varies across the length of the tape and the two sides.  The audio is completely ruined in several places and only mumbles can be heard.  I am using Audacity filters to try to remove the low frequency hum, but thus far I have not been successful.  Later today, I will begin the effort to replace caps in the recorder and hopefully get rid of the hum in that way.  73, Tom, W4OEQ