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HALLICRAFTERS HT-20 TRANSMITTER REBUILD




 
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Author Topic: HALLICRAFTERS HT-20 TRANSMITTER REBUILD  (Read 678 times)
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wa2pjp
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« on: June 25, 2019, 09:08:04 PM »

About a year and a half ago I purchased a Hallicrafters SX-88 and did a complete restoration to it (In the receiver section you will see the project SX-88). I decided I would like to have the HT-20 to go along with it and found one in Mass. about a year ago before I had gone to Nearfest (First picture). I felt very lucky to find one as they are pretty rare. I had it in the back of my truck and brought it home. The condition seemed to be in OK shape but after a close inspection, some of the shafts were frozen, rack rash on the front panel, some rust peeking through the front panel, the cabinet needed to be repainted and the chassis showed signs of corrosion and rust and the cooling fan was frozen. So, I decided this would be a chassis up restoration just as I did with the SX-88. The second picture is the transmitter on the bench and a shot of the chassis before being stripped.


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wa2pjp
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« Reply #1 on: June 25, 2019, 09:10:29 PM »

This is the chassis with the transformers removed and top of the chassis pretty much stripped. the second pictures is the stripped chassis and the wiring harness removed.


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wa2pjp
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« Reply #2 on: June 25, 2019, 09:14:19 PM »

Here are the parts on the bench for cleaning and rebuild. Coil section out and some of the coils to be rewound. The chassis along with all brackets, shafts, and covers were sent out to be chrome plated. The last picture is the chassis back from the plating shop.


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wa2pjp
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« Reply #3 on: June 25, 2019, 09:21:04 PM »

All tubes sockets were replaced with all new tube sockets because some of the old sockets had bad pin tensions. All new terminal strips installed along with new fuse holders and screw terminals. The second picture is of the wiring harness reinstalled and cleaned up. Some of the leads in the harness were frayed so the lacing was undone and new leads run through the harness. Also, I found it strange that the harness had extra leads that were never used and still had the factory tinning  on the ends. I used the extra leads for some mods I had done. The roller inductor was disassembled cleaned and rebuilt.


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wa2pjp
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« Reply #4 on: June 25, 2019, 09:25:14 PM »

Here is the capacitor part of the tank circuit also unassembled cleaned  and rebuilt. I found that this was the same unit that was used in the Johnson Viking transmitters.Top of the chassis from the front and back with the transformers installed. All Transformers were sent out to be rewound for 120 VAC primaries along with the modulation transformer. If you noticed, I polished the plate cap for the 4D32. The output of the tank circuit which ran through the antenna match switch was a piece of number 12 bare wire about 11 inches long that ran from the switch down through the chassis and connected to the change over link on the bottom of the chassis. This link either routed the output through the low pass filter or to a direct output SO-239 connector. This bare wire running under the chassis meant that the output also radiated inside the chassis and RF could get into other areas that it shouldn't, like the audio section. I removed the bare wire and replaced the run with a length of good quality coax to shield the RF from the antenna match switch to the change over link to prevent any RF from getting into places it shouldn't. I have no clue why Hallicrafters didn't run shielded cable...........


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wa2pjp
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« Reply #5 on: June 25, 2019, 09:32:10 PM »

Another side shot of the chassis showing all the major components reinstalled. Bottom of the chassis showing all wiring redone, and some mods that included a 24 VDC PTT system and 120 VAC brought out to the rear octal plug to control the antenna relay. If you look close at the lower right of the underside picture, you will see a small bracket I installed with the relay and power supply for the PTT system. Also I decided to make the modulator bias supply variable and you can see a small square bracket holding a pot above the bias rectifier tube with a screw driver adjustment. The last picture is the chassis with the front panel reinstalled. If you look closely at the final tank circuit, you will notice I changed the plate blocking cap to a door knob cap and made a small bracket to support it. The reason the mica blocking cap was just under rated for the job and I wanted to prevent a failure. Also I replaced the toggle switches with NOS Cutler Hammer switches. Also the audio cables going to the audio pot from the modulator was replaced with cables I had made with Belden double braid and number 20 stranded line to keep hum down. The original shielded line was some real cheap mic cable that had a wrapped shield which you could see the center conductor through.......how much did they save doing things that that? The craftsmanship was very poor in general with leads on component to long or poorly soldered, some burnt wiring where someone in assembly was careless with an iron.........this was at the factory because no one could get to some of these areas with pulling the rig apart.


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wa2pjp
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« Reply #6 on: June 25, 2019, 09:44:24 PM »

Here is the transmitter on the operating bench along side the SX-88 I had reworked last year. In the rebuild all hardware was replaced with all stainless screws and nuts.......I did not used any of the old hardware. The chassis was a two piece chassis with a base and removable top plate, I was surprised to find that the self tapping screws used that held the top plate were of three different type all mixed and matched around the chassis. Some were Phillips, some were hex drive and some plated and some not......I thought to myself "was Hallicrafters sweeping the screws up off the floor because they ran out?" these were from the factory, not changed by someone later on.....if you look close to pictures of the top chassis, you can see the different screw types. I had re-taped most of the screws hole for 6-32 stainless hardware and they were all Phillips head. I also noticed that there was no easy way to zero beat was the basically put the transmitter on the air, so Since the "key" terminal on the back of the chassis also appeared at the octal socket and the back of the VFO, I took the screw terminal and brought the low voltage supply out to them and connected a momentary push button switch in a small mini box for easy zero beating. That id the box with the black push bottom in fron of the transmitter. The last picture is of the old parts and the container full of the old screws. The Heathkit VF-1 VFO was an unassembled  kit I had in my stash and built just to be used with the HT-20.
When I had gotten this transmitter, ten crystals were installed that were all MARS frequencies and some of the low band aircraft frequencies. The chassis never had a name plate installed on the back and no stamping on the chassis that you see on any other production units which I though to be very strange. No silk screening on the chassis which I have seen on other HT-20's.  Also the VFO input was disabled so if you plugged a VFO in, it would never work. There was some remote type wiring installed, but I couldn't figure out what they were trying to do and it was all factory done because of how the screws were tapped and the extra wiring in the harness that was not used. I think this transmitter may have been a special order for a military MARS or CAP post at one time. I had used some of these extra lead in the harness for the new PTT system and for the VFO spot mod so no extra holes were drilled and all mods can easily be reversed. The transmitter is on the air and I have gotten great reports with it on 75 and 40 meters. Using it with the SX-88 is a real joy and I get alot of "wow" comments from people.


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w8khk
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« Reply #7 on: June 25, 2019, 10:24:05 PM »

That is an absolutely beautiful restoration!  Attention to detail paid off and you can be very proud of it.  You should be rewarded with many years of reliable operation.  Thanks for sharing all the details and photo story.
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Rick / W8KHK  ex WB2HKX, WB4GNR
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« Reply #8 on: June 26, 2019, 01:11:41 AM »

Joe,

This is yet another example of the fine craftsmanship you put into all of your restorations. I even hesitate to call your work just a 'restoration', because you certainly take it to another level. This is a 'born again' HT-20, and it doesn't just have the Hallicrafters name on it, it's got your name on it!

Thanks for documenting its rebirth in photographs, and posting it here. It was a pleasure to work you on-air and hear that fine transmitter for myself, and now, to see it too.


Don
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KL7OF
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« Reply #9 on: June 26, 2019, 08:53:22 AM »

SHINY......Very nice job
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W2PHL
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« Reply #10 on: June 26, 2019, 09:00:25 AM »

She's a beauty! Tremendous restoration Joe...looking forward to hearing it on the air!!!

Phil
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W3GMS
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« Reply #11 on: June 26, 2019, 05:23:30 PM »

Over the top rebuild Joe.  Beautiful work as usual.

Joe-GMS
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kc4umo
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« Reply #12 on: June 28, 2019, 07:45:20 AM »

Words alone cannot explain the beauty of that restoration.
Very well done!
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ka1bwo
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« Reply #13 on: June 28, 2019, 06:38:53 PM »

Absolutely incredible restoration  Joe
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KE5YTV
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« Reply #14 on: June 28, 2019, 07:33:57 PM »

Wow Joe! What a beautiful job. That's truly a "show" transmitter. You have certainly raised the bar on restorations.
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Mike
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w3jn
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« Reply #15 on: June 29, 2019, 02:12:03 PM »

Great job as usual, Joe.

We need to have an dual SX-88/HT-20 QSO.

As I recall my HT-20 has coax running inside it to the ant terminals (of which there are two - 50 ohm and a 25-450(?) ohm).

What did you do to install PTT?  Relay across the xmit switch?
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« Reply #16 on: June 29, 2019, 08:11:32 PM »

Here is the transmitter on the operating bench along side the SX-88 I had reworked last year. In the rebuild all hardware was replaced with all stainless screws and nuts.......I did not used any of the old hardware. The chassis was a two piece chassis with a base and removable top plate, I was surprised to find that the self tapping screws used that held the top plate were of three different type all mixed and matched around the chassis. Some were Phillips, some were hex drive and some plated and some not......I thought to myself "was Hallicrafters sweeping the screws up off the floor because they ran out?" these were from the factory, not changed by someone later on.....if you look close to pictures of the top chassis, you can see the different screw types. I had re-taped most of the screws hole for 6-32 stainless hardware and they were all Phillips head. I also noticed that there was no easy way to zero beat was the basically put the transmitter on the air, so Since the "key" terminal on the back of the chassis also appeared at the octal socket and the back of the VFO, I took the screw terminal and brought the low voltage supply out to them and connected a momentary push button switch in a small mini box for easy zero beating. That id the box with the black push bottom in fron of the transmitter. The last picture is of the old parts and the container full of the old screws. The Heathkit VF-1 VFO was an unassembled  kit I had in my stash and built just to be used with the HT-20.
When I had gotten this transmitter, ten crystals were installed that were all MARS frequencies and some of the low band aircraft frequencies. The chassis never had a name plate installed on the back and no stamping on the chassis that you see on any other production units which I though to be very strange. No silk screening on the chassis which I have seen on other HT-20's.  Also the VFO input was disabled so if you plugged a VFO in, it would never work. There was some remote type wiring installed, but I couldn't figure out what they were trying to do and it was all factory done because of how the screws were tapped and the extra wiring in the harness that was not used. I think this transmitter may have been a special order for a military MARS or CAP post at one time. I had used some of these extra lead in the harness for the new PTT system and for the VFO spot mod so no extra holes were drilled and all mods can easily be reversed. The transmitter is on the air and I have gotten great reports with it on 75 and 40 meters. Using it with the SX-88 is a real joy and I get alot of "wow" comments from people.


Wow, beautiful work! You are a real radio craftsman. Can't wait to hear it on the air on my SDR.

73,

The recently-married WB4AIO.
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