Rebuilding The Heathit AT-1 Transmitter - Coil Wiring

The last stage of assembly is the wiring of the oscillator and bandswitch coils on the top of the chassis. Most of this is done with heavy solid wire. I used #14 solid copper house wire with the insulation removed. The oscillator coil, four bandswitch coils, two tuning capacitors, and bandswitches are all connected. It is important to follow the order shown in the manual and double check against making wiring errors that will be hard to change later during assembly.

Soldering the heavy wire is beyond the heat capacity of my small Weller soldering station. I dug out my old 140 Watt (also Weller) soldering gun and it was ideal for the job.

I am quite pleased with the results, expecially when comparing it to the photos I took of the original assembly. The last step is to connect the power cord.

I am now ready to power up and test the unit, after carefully checking the wiring one last time.

Rebuilding The Heathit AT-1 Transmitter - Final Amplifier Tube Wiring

I finished up the next set of assembly instructions  - the wiring around the final amplifier tube.

The last toroid is another large one that is too heavy to stand in the air by its leads. so I sat it on the chassis. As this has some high voltage (close to 1000 volts), I wrapped it in electrical tape to better insulate it from the chassis.

Next up is the assembly of the coils, much of which will use heavy bus wire and be on the top of the chassis.

Rebuilding The Heathit AT-1 Transmitter - Oscillator Stage

I completed the wiring of the oscillator stage. The two RF chokes were replaced with toroids. Once is larger, so I mounted it against the chassis using a rubber grommet. The other is small enought to hang in the air by its leads.

I powered up the unit (slowly with a Variac) with the rectifier and oscillator tubes and a crystal installed and verified that I see an RF signal at the output of the stage, so it is working. Now to move on to the final amplifier stage.

Rebuilding The Heathit AT-1 Transmitter - Power Supply Wiring

I finished up the first section of wiring assembly - the power supply.

I inserted the 5U4G tube, temporarily hooked it up to power, and slowly turned the voltage up using a Variac. I confirmed getting over 350 VDC from the power supply output, so the power supply is working,

Rebuilding The Heathit AT-1 Transmitter - FT-243 Crystal Adaptor

While I was waiting for the tube sockets to arrive so I could get started with assembly, I took a little diversion.

The AT-1 has a socket to accept a crystal in the FT-243 form factor, which was popular at the time it was sold. Since then crystals have generally become smaller, and FT-243 crystals are no longer available. In fact, the demand for radio crystals has gone down significantly in recent years. One of the most well known crystal manufactuers for radio use, International Crystals, founded in 1951, shut down in 2017.

Some FT-243 type crystals are shown here, along with a more modern type meant to be soldered directly into a PCB.

As a little 3D printing project, I spent an evening using openSCAD to design a plastic case that allows a small crystal to be mounted inside, and using a couple of pins from an old octal vacuum tube for ins, turns it into the FT-243 form factor. Here is an example of my prototype.

I put the 3D printer model files here and made them freely available in case anyone else wants to use them.

Rebuilding The Heathit AT-1 Transmitter - Starting Assembly

Finally the tube sockets arrived, so I could get started with the actual assembly.

The new sockets were a little smaller than the originals and the mounting holes did not quite line up with the existing holes in the chassis. I cut the holes in the sockets into slots using a Dremel tool and cutoff wheel. That made it fit well enough.

As well as the tube sockets and terminal strips, I remounted the transformer and choke, front panel controls and panel.

I'm now ready to start wiring and installing the components.

Rebuilding The Heathit AT-1 Transmitter - Winding Inductors

As mentioned earlier, I will reuse the original band switch and oscillator coils, power transformer and filter choke. I decided to wind new RF chokes using toroidal cores.

It calls for two 1.1 mH chokes and one 90 uH. I used a little toroid design application I wrote myself and came up with these designs:

   1.1 mH: FT-82-43 core, 44 turns, #28 wire.
     90 uH: FT-50-43 core, 13 turns, #28 wire.

I wound them and measured the values using my AADE L/C meter. They read a little low so I added a few turns.

One other part I didn't mention earlier - I will replace the line cord with a new one using a brown two-wire extension cord that is similar to the original.