Hallicrafters S-36A Rebuild



Picked this radio up on eBay late 2002. it's a Hallicrafters S-36A Ultra High Frequency Communications Receiver. They were first made in 1941 and some were used by the military during WWII. The receiver covers the 27.8 - 143MHz frequency range which at the time was called UHF but is now know as the lower half of the VHF frequency range. The front end uses 3 acorn tubes, a 954, 955, and 956. It has a total tube complement of 15 tubes and is capable of both AM and FM reception. This set sold for $415 in 1945.
Here's some pictures of the set as I received it prior to starting any rebuild. The set is aged and has rust spots. Painting will take care of the front panel and the case. Couple problems noted, S-Meter faceplate is broke and the meter seems to be twisted in its mounting. Band change knob is broken so the pointer part of it is busted off but actual band switching is still functional. The ANL switch is broke. The tuning knob is not original and is loose but works. Finally, the top lid has a broken latch.
Here's some before views of the guts. Very dirty but cleanable. At some time in its long life someone substituted 6AF4 for one of the acorn tubes. You can see it on the middle picture of the RF section. Other than that everything looks in tact and in good condition. All the screws for the back panel connections (Speaker/Ant) are missing. Somewhere along the line someone removed all of them from the connection posts.
Underside looks to be in good condition as well, even though most of the components will be replaced. I will probably re-use the original wiring harness as it  is laid out very well. All the transformers check good which was a relief as they would be the hardest items to replace if bad. The underside seems to be coated with wax everywhere, which seems to be typical of these old receivers. Not sure if it's just melted off from the caps etc. or if they intentionally coat everything with a waxy substance. Most of the tubes checked fair, only a couple showed low emission. I will probably end up replacing all them anyway.

The manual arrived and disassembly has begun

Front panel, case, and meter removed Here's a shot of two of the acorn tubes Dials removed. Can really see the gear mechanisms now, I think there's a bushing missing for the tuning shaft and that's why it loose and wobbly.
Chassis with the RF Assembly removed The RF Assembly out The two together, side by side
Underside shot with several of the transformers removed as well as the RF Assembly and the Ant trimmer Top side shot 

Slowly but surely it's coming apart

Top side view, nothing but the tube sockets left Under side view, nothing but the tube sockets left Tube sockets out, down to a bare chassis, they were the only thing riveted in, had to drill them out
Top side view of the bare chassis Top side view of the chassis all cleaned up, wired brushed, and ready for reassembly to begin Underside view of the cleaned up chassis

Reassembly has begun. I repainted all the transformers and the filter cap. So far I've reinstalled all the tube sockets, transformers, filter cap, fuse holder, AF gain control, AM/FM switch, BFO on/off sw, BFO pitch cap, and the headphones jack. Underside view of where I'm at
Reassembly continues. The power supply is all wired up, filaments are all wired, and the 1st and 2nd IF stages are complete. As you can see I also did not use the old wiring harness, I decided to rewire completely from scratch, it's more fun. Coming along, 3rd IF and FM Limiter stages complete. Most of the discriminator and detector stages are also complete.
As you can see I didn't replace all the components on the 3rd IF circuit board, mostly because I prefer to place decoupling caps right off the tube socket. Just about have the whole chassis wired now. Everything is wired all the way to the audio binding posts with the exception of the Tone Switch (where the wires are hanging out the front). I purchased an S-36 parts set off eBay and can now replace some of the items I needed (the ANL switch for starters and the tone switch).
Notice that I ended up adding some components to the 3Rd IF board. These were some wiring modifications of my own. I'm currently in the process of developing as built documentation as well. Everything's in the chassis now except the RF Assembly and the antenna binding posts. I ended up repairing the tone switch rather than replacing it with one from the spare set. On to the RF Assembly.

RF Assembly disassembly has begun. As you can see little by little it has come apart. The gear assembly has all the ball bearings missing and that's what was wrong with the tuning shaft. My spare RF assembly is slightly different however and the bearing assemblies are not removable on it. I will probably end up using the whole gear assembly from the spare set.
Here it's ALL the way apart. RF Assy base plate is all cleaned up. I completely disassembled the main tuning cap and cleaned it up. Looks great now.
Assembly continues, been kind of slow going. The parts are really crammed into the RF assembly. The Osc and some of the Mixer are nearly complete. The very first wafer section of the band switch was cracked, fortunately I was able use the one out of the parts set. It's really lookin good. I found that silver tarnish remover does a great job on the wafer sections and the trimmer caps. Getting there, the Osc and mixer are now complete as well as most of the RF amp. All that remains is the input to the RF amp and the Antenna section of the assembly.
Wiring of the RF Assembly is complete, look beautiful! Had a hell of a time getting the band switch shaft pushed in through all the wafer sections but with patience and persistence I got it in with out damaging any wafer sections. Ready to start cleaning up the final metal parts that will form the enclosure, then I'm ready to drip it in the main chassis. Ordered a new set of acorn tubes from Fair Radio Sales, should be here soon.

RF Assembly is complete and installed in the chassis. Still have to make the under chassis connections but getting real close to done. Gear Assembly and pilot lights installed. I was able to pop a bearing assembly off the parts set after all and fix the tuning knob problem. I also swiped the inertia wheel from the tuning knob on the parts set.

Dials are now on. Notice I ended up having to remove the inertia wheel from the tuning knob. The wheel was not allowing me to push the bandspread dial back far enough on the shaft and as a result it would rub on the front panel (wonder if this is originally how the dial got scored up). To bad, I really liked the feel of the tuning with the inertia wheel on the shaft. At this point I have also started getting new tubes for the set. One by one they are coming in, already got a whole new set of acorn tubes for the RF Assembly.

Front panel is on. Really came out good after repainting. Had all kinds of problems with the S-Meter. Had to put the guts into another meter case I had. Meter still doesn't work quite right, probably as a result of flopping around loose in the original case (which was also a substitute but not of the right size to properly anchor the guts. Next steps are to start alignment and troubleshooting problems that may arise. I still need to paint the cabinet yet as well.
For those that have been following this rebuild, I ran into some problems with the RF Assembly. It turns out to be a tricky little number. I was getting poooor sensitivity over the bands and found that wiggling the band switch for starters caused huge changes in sensitivity. I tore the whole RF Assembly  apart again and started the rebuild from scratch. For starters I discovered that a number of the wafer contacts had lifted and were intermittent. This as a result of pushing the shaft through all those wafer sections. I also ended up rewinding several of the coils in the RF assy. I did a little component rearrangement to try and minimize stray capacitance etc., and I soldered a lot of the ground connections to the chassis. I also discovered the band switch shaft was slightly bent, this was causing it to not line up and consequently would excessively push the wafer rotors causing the contactors to open up and make intermittent connection. I completely reassembled the RF assembly and the straightened shaft slide right through the wafer sections with no problem. The band switch now operates correctly. Next will be tuning up the RF amp, Mixer, and Osc stages of the assembly.
Finally finished. I purchased an R46 Speaker to go along with it, it has great sound with this speaker. I repainted the case of both the radio and the speaker. I'm still not totally satisfied with the sensitivity and will get back to fiddling with the RF section one of these days. But for now it's a beautiful piece of boatanchor equipment.



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