Rebuilding The Heathit AT-1 Transmitter - Testing and Wrapup

I did further testing of the restored unit, measuring voltages against the manual, and they all looked okay.

The unit was speced at operating from 105 to 125 VAC. At the time it was released line voltages in North America tended to be lower than today (typical 120 VAC or more now). For testing purposes I found that an input voltage of 117 VAC gave heater (6.3 VAC) and B+ voltages that were close to what was listed in the manual, so I did my testing with a Variac set to that level.

I measured the RF output on each band both with a rather inaccurate SWR/power meter and by measuring the peak to peak output voltage on an oscilloscope across a 50 Ohm dummy load.

    80M: 13 Watts
    40M: 12 Watts
    20M:6.3 Watts
    10M: 1 Watt

Power on 10 meter is quite low, but is apparently normal as the output tube has very low gain at the higher frequency. I captured the output on different bands on my oscilloscope:

Since the power supply is not regulated, there is quite a drop in the B+ on key down, from 512 to 430 volts. Note that there is also 139 VDC at the code key contacts, so you want to avoid touching them!

Here is the oscilloscope output of keying:

And using the spectrum analyzer mode of my Rigol scope:

It is interesting compare some of circuitry before:

and after restoration:

This project was a lot of fun - I got to recreate the experience of building from a kit. I was also fascinating think that someone built this in the late 50s or early 60s and obviously used it quite heavily over the years.

I will have to try it on the air once I get an new antenna up. The power level qualifies it as a QRP rig.

I'm still looking to find the VF-1 matching VFO if I can get one at a reasonable price.

Rebuilding The Heathit AT-1 Transmitter - It's Alive!

Initial powerup tests are good and indicate that it is producing output on all four bands. I powered is up slowly with a Variac and then did some testing with an input voltage of about 110 VAC. I'm seeing output on all four bands, close to 10 Watts of output power depending on the band.

I find it is easier to adjust the driver and output controls by looking for maximum power output rather than the built in meter (the usual procedure is to adjust the driver control for a dip in the grid current and then adjust the output control for a peak in plate current - except for 80 meters where the drive
input is not tuned). The meter is an old iron vane type that is not damped and bounces all over the place until it stabilizes - which is not very good as you need to quickly adjust the controls to minimize the chances of damaging the output tube.

I still need to make some more voltage and power output measurements and look at the output waveform while keying.

I'm still also waiting for some octal plugs - one is used for an optional modulator (and needs to short two pins when not present) and one is for an optional VFO.

Heathkit sold the VF-1 VFO or Variable Frequency Oscillator that allowed it to transmitt at frequencies set by the VFO rather than fixed with crystals. I hope to acquire one of these some day - they show up on eBay quite often.

The unit could also transmit using AM voice, with an external modulator. Heathkit never offered a modulator kit, but various circuits were published at the time and in fact Heathkit published a suggested circuit in their 1955 sales flyer. It would make for an interesting project to build one - it used 5 tubes (one dual), making it a little more complex than the AT-1 transmitter.

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 - 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 - Overview

I decided to start a restoration project on this Heathkit AT-1 transmitter. This was Heathkit's first amateur radio kit, and was quite successful, with many built and used in the late 1950s and 60s and even today for fun and nostalgia reasons.

I bought this unit at a hamfest a few years ago and did some restoration to get it working, including painting the case. I made a YouTube video that describes it in detail.

Now I would like to more completely restore it. The copper plated chassis is badly corroded and the construction quality is poor. Many of these units were built by beginners, often young people with limited experience in electronics. No doubt many repairs and mods were also done to this unit in the over 50 years since it was first built. This one has had a modification done to replace the final tube with a 6E26 to get more output - it was described in an article in Popular Electronics magazine in 1958, as well as a few other mods.

My plan is to totally strip it down to the bare chassis, removing all components. Then I will rebuild it from scratch using some of the existing parts and new ones, where possible. I will also undo the mods and revert it back to the original circuit.

Not all parts will be original, so it won't be "museum quality". It is a tradeoff between keeping the original circuit but using new parts so it works as well or better than new. If I was planning to use on a regular basis, I would make some additional changes to make it a little safer: at minimum a grounded cord, fuse, and some safety caps.

A total rebuilt like this is sometimes done for collectable radios. The AT-1 is somewhat collectable, but not particularly valuable. I have always wanted to do a complete rebuild of a Heathkit, and this will be a lot less effort than for a more complex transceiver like a Heathkit HW-101, for example.

Part of the fun will be the opportunity to build a Heathkit from scratch by following the steps in the manual.

Low-Cost Pixie Transceiver Kit

The Pixie transceiver is a simple QRP amateur radio transmitter design. I earlier blogged about and made a YouTube video covering QRPME.com's version of this design, the Lil Squall.

I recently picked up one of the inexpensive Pixie kits that are sold on eBay. This one was only US$6.80 with free shipping, so I thought I would take a chance and buy one.

After a few weeks waiting for shipping, it arrived. The kit includes a nice silk-screened and solder masked PCB and all parts and connectors including a 7.026 MHz crystal and a 2 Watt 47 ohm resistor for use as a dummy load.

It needs headphones, a code key or keyer, and antenna and power source. It runs on 9-12 VDC and puts out about 600mW of RF power.

It went together in less than an hour and worked when I powered it up. It uses one IC, two transistors, and fixed inductors (no toroids to wind). The parts are all through-hole. The board is a little tight with some parts mounted vertically. It came with two sheets of instructions - basically a parts lists and a schematic - which was adequate to figure out how to put it together for someone with some experience with electronics.

On transmit I measured 240 mW at 9VDC and slightly less than 600mW of RF power at 12VDC into a 50 ohm dummy load.

The transmit frequency is fixed by the crystal, but the receive frequency can be adjusted slightly using a potentiometer. You could use different crystals but the circuit is only intended for the 40 meter band with the parts supplied.

As is typical with this design, this is not an easy rig to make contacts with due to the lower power and fixed frequency. The receiver is also not very sensitive and can suffer from AM broadcast band station overloading. A preamplifier, like this Ameco unit, can help a lot.

The one I bought was listed on eBay as "HAM RADIO 40M CW Shortwave Transmitter Receiver 7.023-7.026MHz QRP Pixie Kit DIY".

All in all, a fun little kit for the price.

Heathkit RX-1 Recapping

I finished "recapping", i.e. replacing the paper capacitors in the Heathkit RX-1. Some were a little tricky to get at when the unit was assembled.

Here are some of the original caps:

And here is an image after recapping (the news ones are all yellow):



I didn't notice any significant difference after recapping, but it is still a good idea to do. Depending on the circuit location, a leaky capacitor can damage a tube or other expensive component. Below is all the old caps after removal: