This is a spread sheet format detailing a variety of abbreviated technical and restoration tips about the Hallicrafters S20R, an early circa 1930s era high frequency communications receiver. It is intended to present an orderly compilation of information offered and obtained during an actual restoration project done stage by stage- real time. Requests for edits or corrections will be promptly applied. Requests for deletions or additions will also be promptly applied. Care is taken to give credit to information sources, most of which were/are offered in Antique Radio Forum to help me with technical assistance. This compilation is presented with the permission of Antique Radio Forum and is not listed for general public access on the main page of this site.

The objective of restoration must remain true to the original design. Additional goals of this project are:

1.To restore full operating function to this fine old Hallicrafters receiver made during the first third of the last century, a time of Ford Tri-motor, The Great Depression, Man-o-war, and the Attack on Pearl Harbor.

2.To better understand the general operating design of the superhetrodyne receiver.

3.To define, in plain language, the design characteristics of the S20R.

4.To understand the technical evolution of the company's designs up to 1950.

5.To optimize receiver selectivity and performance using external filters or circuits available today.

6.To learn the basics of receiver alignment using a signal generator, multimeter, signal tracer and oscilloscope.

7.To encourage other skilled craftspersons to undertake the rescue and restoration of the many S20R and other Hallicrafters receivers that might otherwise be destined for the junk pile.

8.In achieving all of the above goals, gaining a better working and conversational knowldege of early and present radio design, performance and operation thus furthering the skill-art of Amateur Radio in general.

These tips are provided along with reminders to use safe practices during all restorations, use safety eye ware, employ proper grounding of receivers and test equipment. Good luck in your projects and stay tuned to Antique Radio Forum.

Mike Malsbary
K4CWN-ticket expired 1969,
1965-N0EFD-Chu Lai S.Vietnam,
1965-W6IAB-Camp Pendelton California,
1964-KR6MB-Camp Schwabb Okinawa
1967-MARS Station Marine Corps Base Eltoro
2017-W3PDR as of about 10 years ago.

"Restore it before it's gone forever."


Hallicrafters Communication Receiver #126721-circa 1934

Information Source Location Data Type
Its not necessary to turn the AGC (automatic gain control) off. Reducing the RF gain raises the AGC threshold as indicated by the S-meter position with no signals (or high level noise) present. This leaves the entire receiver in the LINEAR region where any increase or decrease in signal level results in a corresponding INSTANTANEOUS increase (or decrease) in audio level. The benefit of leaving the AGC turned ON, but with high threshold is that it will still act to protect your ears from BIG signals or static crashes, with the additional gain reduction associated with the AGC action and the associate time constant. For best reception of WEAK signals, it best always to leave the AGC FAST (off) positon so that the receiver gain recovers quickly from larger signals....Operators who use SLOW AGC (on) in pileups are not able to hear weak signals unde the pileup which explains much of the unruly behavior and continuous calling by less knowledgable operators. N4KG Design/operating characteristic
If it is necessary to take down the entire S20R cabinet and some component assemblies because of the condition of the unit- follow these steps. Begin by removing all tubes and packing them carefully away. Zip lock bags are good. Close the main tuning and band spread capacitors, viewing through the top lid, so that further handling will not damage the spacing alignment. Carefully move through the interior cabinet and remove all tiny bolt and screws fastening the cabinet. Pack in Zip Lock bad and ID the source. Once removed the chassis bolts are removed....freeing the chassis from the cabinet. Cut (and tag) the power leads to the dial light assemblies. Remove them from the tabs and store away. Moving the chassis around will damage the delicate and fragile orange dials, take care! The cabinet is comprised of tin mine were rusted or dust caked in place. A few drops of oil along the front top seam will loosen this joinery and allow the tabs to be lifted out of the slots. This action will expose the chassis, dial pulley assembly and circuitry for work and cleaning. IMPORTANT. To protect them, remove both main and band spread dials, place in zip lock bags an store away. These components can now be cleaned, polished and/or prepped and painted. Notes on S20R restoration now underway (10/30/06) MikeMalz hardware
Start by gathering schematics, service information and whatever manuals that may be available. Take notes, take pictures and draw diagrams of the part of the receiver you will be working on to help in the reassembly later. Disassemble the receiver to clean and check everything. Test tubes of course and start a list of parts and supplies needed for the restoration. Caps will be infamous wax paper variety; probably too leaky by now for reliable operation and will need to be replaced. Some are in more critical locations than others such as coupling caps and screen and plate bypasses. Resistors tend to drift upwards with age and those found to be over 20% away will need to be replaced too. Most will need one leg untied to accurately measure condition which is usually available while getting cap leads untied from the terminal strips (preferred method). A power desoldering gun comes in handy for this because it allows you to quickly and neatly suck all the old solder out of the way first so you don't end up burning parts trying to keep the solder molten while untangling leads off lugs. The little cheapie squeeze bulbs don't work well for this being better suited for printed circuit board work. There is an alternate quick and dirty method which makes me cringe whenever I see it but it still works none-the less and that is to solder the new cap to the pigtails of the old cap after clipping it out. Make some quigs by spiriling a piece of .020 wire around the end of a paper clip about 1/2" long to form a sleeve which is then soldered. Check your progress to make sure you still have working order after replacing every few parts. A good guide for diagnosing faults is to measure and compare tube pin voltages with the schematic. If the radio is not working voltage will be incorrect somewhere. GregG-Senior Member technique/wiring
Good news! I was just looking at the RADIODAZE catalog on line. They have replacement decals of he front panel lettering that you accidentally dissolved. The URL is N2GOX control panel lettering
Anyway, I like the Halli - a "real man's radio" - serviceable access to the chassis and topside, love those analog dials. The plan is to make (buy?) an outboard tuning eye along the lines of Phil Nelson's article at Heck, it's a great article alone for the S20R at Ted-guest< External Device
One thing I find helpfull is to document the underside of the chassis before I start. Make a digital photo or use a flat bed scanner to make a clear image. When you wonder where something came from you can look at the picture. I also have an S-20. It works very well for surfing. It is part of the home enertainment "stack". The antenna is a roof top 80' long wire connected to the set with coax. I did use proper grounding and lightning arrestors. Once you get the TV, VCR, SAT box, and all the rest, powered down the radio works great. Keep in mind almost everything in a modern house causes some sort of RFI. Scott Armstrong-Sr Member Document tag on S20R
You may already have it, but there are some files on the net of the owner's manual you can download. You may find them helpful. If not come on back and I can find the link. BAMA may have a copy of it. Gary Rabbit-Sr. Member Manual/schematic/parts list
How the signal generator connects will vary as per alignment instructions for a given set and may be connected differently depending on which band is being aligned. If connecting directly, use a .01uf disc cap to isolate the generator from the radio. I also take AC and DC meter readings between the probe tip and where I intend to connect it in case there's excessive voltage due to plugs being reversed on AC/DC sets, bad bypass caps or leakage to chassis in either the radio or the generator, etc. EdinSoDak Signal Generator
The circuits in the S-20R and the SX-28 are similar enough so that I would suspect that whatever your mentor did to his set could be applied to yours. I'm not sure what you mean by "conversion stage". He could have attached a converter ahead of the receiver (such as for 6 or 2 meters) or perhaps he rigged the set for double conversion by adding additional circuits. A bandpass filter (such as a crystal filter) would be a definite improvement for the S-20R but the SX-28 already has a crystal filter. So I'm not sure what kind of filter he may have installed on his receiver. Could it have been an audio bandpass filter like National used on some of it's receivers? Or maybe it was a preselector of some kind. Dave Doughty-Moderator external refinements
The old radio works well as designed. IMO 99% of Ham mods on old sets get undone to make them work properly. Get it running to original specs, if it does not preform find out what you did wrong. Trying to make it better almost never works out. IF you want something better, get something better. Any permanent mods will devalue the set. Let the old set have it's dignity. Restore it to original condition. Scot Armstrong-Sr Member restoration rule
Hey! That QF-1 was what let me use my S-20R as a communications receiver!!! That was the best thing that ever happened to that receiver!! The magic you could perform with the Q multiplier puts crystal filters to shame; peak at any place in the IF band, reduce bandwidth to 100cps, null out any kind of other signal. Those two were meant to be together, don't separate them if you want actual listening capability! mrutkaus Senior Member external signal enhancement
I prefer a Q-multiplier over crystal filtering. My Hammarlund HQ-160 has it built in. What I like most is you can favor one sideband over the other while keeping the carrier or BFO tuned to the middle of the IF passband. It seems to bring out the best in the desired signal under difficult receiving conditions and gives you much more control over how the radio performs. I'll bet a Q-multiplier would do wonders for an S-38. But I agree with w3jn...a Q-multiplier should not be placed next to an SX-24. Dave Doughty Moderator external signal enhancement
Peter Bertini wrote: quote: Originally posted by mrutkaus: Hey! That QF-1 was what let me use my S-20R as a communications receiver!!! That was the best thing that ever happened to that receiver!! The magic you could perform with the q multiplier puts crystal filters to shame; peak at any place in the IF band, reduce bandwith to 100cps, null out any kind of other signal. Those two were meant to be together, don't separate them if you want actual listening capability! Mike I agree with ya Mike! The QF-1 was a great HAM trinket that did wonders for simple receivers! I'm saving mine for the S20-R that's awaiting restoration here! OZ132HOME Senior Member external signal enhancement
Leaving aside the aesthetics (is that spelled right?) and/or the philosophical qualms about a Heathkit next to a Hallicrafters, I must agree with much that has been said. Skillful use of a Q-multiplier can and often does enable most of us to tailor selectivity much more closely than (most) crystal filters I have used. As Curt says, the crystal filters must be precisely and correctly aligned, and the Q-multiplier is more flexible. And, it's so cheap! Oh, I forgot, it does take more skill and patience to use the Q-multiplier effectively. OZ132HOME Senior Member external signal enhancement
"To overcome the selectivity problem, the superhetrodyne receiver was developed. This type of receiver has a high degree of selectivity because it is easier to build highly selective tuned circuits that operate at low frequencies. Reducing the input frequency to a lower one for amplification, detection, and processing results in a more stable and selective receiver. ...In the superhetrodyne receiver, the very high input signal frequency intercepted by the antenna is mixed with a lower , internally generated intermediate (IF) freqeucy. The IF frequency is detected, processed, and amplified to convert back to the same format as the original modulated signal in the transmitter. A fixed IF frequency is used as it allows the engineers to design receiver circuitry for maximum sensitivity, selectivity, and gain." F.L Gould F.L.Gould, Technical Guide to Electronic Communications, McGraw-Hill, New York, 1997 Technical Reference (The explanations in this book are clearly stated. Highly recommended.
"W3NQN Passive CW Filter: He reported that when he replaced his DSP filter with the passive CW filter that he assembled, he had the impression that the signals in the filter bandpass were amplified. In reality the noise floor appeared to drop one or two dB. When attempting to hear low level DX signals , the (builder) now prefers the passive CW filter to DSP!" An easy to build high performance passive CW Filter. ARRL Handbook, Published by ARRL, Newington, CT. 2005 Edition. p.12.31 Tech Reference
Summary Paint: Went to a body shop and an auto paint supplier to get info on custom color for the Hallicrafters S20R. The body shop said they would need to sand down into the finish to get the actual color, then would be able to provide a PINT for $50.00. The auto paint supplier did a spectrum analysis of the color-right at the counter- they said they could produce a paint very close to the color. I purchased a PINT for $12.00. Back at the shop I took the paint and reduced it with Nason acrylic reducer (10-20%) and put some in the air brush. The color came out quite green. Spec info (Green 55; H.S. Black 103; H.S White 144.1; Phthalo Blue 159.4; Binder 460.5) A test square on the cabinet just did not look right. So I've opted back to the Dupli Color and will finish out the cabinet work with that. Hope this paint adventure for my S20R helps others with similar cabinet restoration plans. Mike (note) One moderator offered that one restorer had a cabinet color analyzed and the supplier mixed a perfect match using an analyzer. The supplier then sold a pint for a more reasonable $6.00. Would be great if someone at ARF could generate contact info for this supplier. MikeMalz-new member
I have fooled around with this finishing enough, so this project is going to be Duplicolor acrylic, but future projects will defiantly get the new standard color, if it could be set up. MikeMalz
The cabinet restoration complete, its a good time to note some early circuit observations on my S20R. ARF's repeated reference to Phil L. Nelson's various articles on restoration are quite appropriate. Reading his article on the S20-R would be mandatory before beginning work. Notes from my early recon of the circuitry: WIRING-Some obvious previous work done- checked new wiring against schematic. Caution here since schematics can be interpreted various ways-and be correct!-don't rip out wiring if it isn't where you think it should be. Assume all wiring to be correct. New wiring seemed to suggest some audio transformer changes-noted in another ARF section. In my estimation, only some cloth insulated wiring was ragged and needed replacement. The wire colors were faded with age and required cutting the nice cable lace in order to trace the questionable wire. Technique noted in Phil's article; cut out from the terminal so evidence remains of correct location.(and in case you are interrupted) Once replacement wire is tined and ready to solder in, remove the stub-using established technique-and solder in the replacement wire. Thread the new wire along the cable length to the other end of the frayed wire. Cut similarly and remove the rotten wire. Attach the new lead after removing the final stub. Using a variety of colors in this process, restoration work will be easily spotted, and double checked. GAUGE Not sure if light 18-22 gauge wire common at Radio Shack is heavy enough for the higher voltages of this early 1930s receiver- so I used heavier gauge wire from an old motorcycle harness.COMPONENTS Components are mounted through two layers of sheet metal-the cabinet and the chassis beneath. To protect the cabinet finish, its good to mask the areas around the nuts to prevent scratching the finish-especially important with working loose the knurled dress nuts on the switches. A adjustable spanner works nicely with the nuts- very careful gripping and turning with a plier on the brass switch nuts will minimize damage.CAUTION!! Removing components for cleaning-On the S20R there is a variable inductor for BFO PITCH. There are two nuts (interior cabinet) that retain this control. Do not disassemble beyond this. The inductor is of a very fragile paper and wax construction and will crumble at the slightest pressure. Destruction at this stage, can jeopardize an entire project since replacements are rare and probably costly. 11-12-06 MikeMalz Project Notes
I'm in the middle of fixing up one of these, and re-capping is largely done. I wonder if anybody has experience with replacing the gimmicks with real capacitors, and how it affected performace. C30 couples the 6J5 BFO to 6SQ7. C57 and C58 couple the oscillator to the mixer on bands 3 and 4. This just seems like the cheap and dirty way of doing it. Also, how good is the original BFO? Is it steady enough to be good on SSB? I have an S20R with such an unstable BFO that I changed it to a 6SJ7 e.c.o. Somebody got his hands into this one and I have to do a bit of un-modifying. Comments are welcome. Thanks. tubemaster Senior Member
The gimmick capacitor is to LIGHTLY couple the BFO to the detector. If it's too heavily coupled (ie more BFO signal into the detector), the AVC will start to decrease the sensitivity of the radio. OTOH too light a coupling will require you to reduce the RF gain to obtain a decent beat note. One weakness of this design for SSB is completely inadequate BFO signal. If you're not too concerned about loss of sensitivity with the BFO on, replace the gimmick with a coupling cap of 10-20 pF or so. This will make for easier SSB copy. HOWEVER - too much coupling will load the BFO and make it drift according to the signal strength. Moreover, the BFO will try to phase lock to the incoming signal, and this will make for a growly sound on SSB signals. You can check for this by trying to "not quite" zero beat - if the BFO snaps in to zero beat rather than smoothly decreasing in tone, going to zero, then increasing as you tune the BFO thru its range, you have too much coupling capacitance. The same principle applies to the mixer. Too much coupling capacitance will pull the local oscillator as the mixer encounters strong signals. Hope this helps. w3jn Senior Member Technical Problems
Do you have a scope? You want at least ten times the BFO injection voltage as compared to the strongest signal voltage at the stage where they are mixed. More like 20 times is common. C.R. Moderator Technical Problem
I had read several references to electrolytic capacitor rebuild by cleaning out the can and inserting new metal electrolytics. Here's the results of my rebuild and hope it works. I removed the can from the S20R and with gloved hand and a half inch carbide drill bit commenced to drill it out. Nasty job but someone's gota to do it. Once clean, the interior was swabbed with carpenters glue and lined with a sandwich of masking tape and ordinary paper. Not cardboard- space is at a premium inside the can. The triple consisted of three capacitances at 400VDC. I ordered one 30uf and two 10uf at 450VDC- a third listing on the can is a 25uf at only 25 VDC. I had to settle for 30uf for the required 33uf and hope the 20% rule works for this. Important: The wires for the new capacitors all had to come out of the top of the can when sealed. I placed double insulation over all capacitor leads exiting the can. Heavy color code insulation from multi strand high amp wires (motorcycle harness) were slipped down each cap wire to the absolute wall of the capacitor. Black code from ground, red from positive. (yellow and green) marked the two 10uf positive leads. Total of six leads. Over all coded wires, black Radio Shack shrink wrap was installed. Next a layer of lid cut from a plastic coffee can with punched a hole large enough for all insulated leads was installed. It fit nicely into place since the circle was patterned from the cap can circle diameter. The caps are insulated from the metal can by the paper and masking tape as well as the double insulation. On top of the plastic seal a layer of plaster repair Krack Cote and fine nylon formed a nice hard surface when dry. Over this rather uneven surface a final layer of Bondo was smoothed out and topped with a layer of the nylon while wet. The three caps were inserted with the plus side toward the exit-the ground wire ran up the side of the cap and joined the six wire bundle. Bullet proof. The finish cap in the photo fit nicely back into its home on the S20R chassis and the little locking plate pulled it firmly to the chassis. Two machine screws neatly replaced the original rivets. When my order for other caps and a bag of terminal strips arrives from Antique Electronic Supply- the electrolytic rebuild task will be complete. Oh! The 25uf at 25VDC was dangling in jury rig fashion as a separate cap- it tested good so this will have to be mounted on the wall of the chassis somewhere-easy shot compared to the cap rebuild. MikeMalz

Its been over a month since the last entry. All caps and resistors have been replaced. Below deck work is FAIAP complete but still doing numerous QA checks of the point to point wiring as well as pin by pin checks of the tube sockets for proper components.(no power) Before moving on I did want to document the reassembly of the main tuning capacitor. The removal of this really dirty capacitor was essential to replace rotten wiring as well as several Texas dirt dobber mud nests. All pressure contact points on the rotating element of the capacitor were cleaned and checked for continuity with an ohm meter. Any hardware removed in this process was stored in a zip lock bag with notations to myself for correct reassembly. This includes the whole capacitor assembly. It is often weeks or months until reassembly can be done. The test comes at reassembly in this case when an inventory of washers, screws, insert washers (cleaned up on a wire wheel) and grommets allow a remount of the main tuning capacitor. (cap plates also realigned) At project launch the cap had to be cut out and new leads installed on the band switch below deck. Braided ground cables too were cut. I used solder braid to rebuild these leads and ready them for re-soldering. Note in the picture the old leads left on the three bottom lugs as a clue to where to reconnect. The old dry rotted grommets were replaced with new rubber ones from the hardware store. The four pictures show an easy reassembly rendering the very nicely designed Hallicrafters platform installed. Next comes re-threading the tuning cord and springs. I will hold off attaching the rare and fragile analog dials until just before the chassis goes back into the cabinet. I've already wrecked one and luckily found a replacement. Live and learn. MikeMalz<

Update July 4, 2007: To S20R restoration enthusiasts visiting this site hoping for a conclusion to the project, I apologize. The project was shelved a few months ago to prepare for my General Class Amateur Radio test, which I passed in May. I've been busy setting up my new rig, a Yaesu 857D with numerous digital computer programs- PSK31,MMRTTY,Ham Radio Deluxe, and the fabulous WSJT6 EME and Meteor Scatter weak signal programs. I expect to finish the S20R at some point but until then, best 73s and good luck on your S20R project. I'll listen for you on the air. Mike- KB3PDR<

Many months have passed without further work on the S20R. Suddenly this spring, some learned amateurs at a nearby radio club offered to look at my work and chart any corrections that may be necessary. Sure enough, I had made some fatal errors in re-wiring and a major error in the original S20R schematic was found which caused the 80 Rectifier Tube plates to glow red hot. Remember way back when all parts were put in zip log bags. Well good thing, when re-assembly time came these parts were well organized with necessary notes on the zip locks. Re-assembly was a breeze. Here's two tips. 1. Replace all of the brass locking screws on the control knobs with allen locking screws. My local hardware store has a good stock of screws. I'm there all the time. The allens allow good tightening on the old band and mode switches. The old brass screws will slip and even break at the screw slot. 2. The Radio Daze S20R decals are nice but clear coat paint will dissolve the white letters. Don't use clear coat paint to seal these lettering decals. (I put a spare decal on the bottom of the chassis-let it dry and tested before trying it on my work. Live and learn.) At power up, I was just in time to hear on AM the Philadelphia Eagles beat the Dallas Cowboys. A fitting conclusion to any project. MikeMalz Project Notes
READY NEXT ENTRY Source Location Data Type
Trouble Shooting and Alignment Part Two

Return To Antique Radio Forums