The Johnson 275W Matchbox Antenna Tuner

I had purchased a Johnson Matchbox from an estate a while back & decided that while I was home with the flu I would open it up and check on its condition.

The Johnson Matchbox is found most commonly in two versions, the smaller "275W" unit and the larger Kilowatt Matchbox. Why did I use quotation marks around 275W? Well, these units were manufactured back in the good old days when men were men and transmitting voice meant using AM, not single side band. The conservative rating of 275W of AM translates into roughly 800W of peak SSB  (Not really but close enough so you get the idea)

Unlike many who own a Matchbox I was hoping to keep it 100% original and that it would contain all its original components, including the antenna change-over relay and wiring for the high-impedance receiver antenna connections. I plan to use this Johnson Matchbox with a Heathkit AT-1 transmitter and Hallicrafters SX-25 receiver so the inclusion of an antenna change over relay and 300 Ohm receiver connections will make life MUCH easier. Something I didn't realize until I had the unit apart (There are a LOT of screws holding this thing together) is that there is also a receiver control contact on the relay to break HT and mute the receiver during transmit which will work with my SX-25.

An initial inspection showed that the only modification was a small piece of plastic wedged into the relay contacts that held the relay in the transmit position. It was easily removed and the relay coil and contacts tested for continuity. The contacts seem a bit dirty which, from the little I have read online, seems to be a common problem.

Once the relay contacts and band-switch are cleaned I will button the unit back up and connect it to the loop antenna I have recently run around the eaves of the house. The loop has been a huge improvement to the long-wire and magnetic antennas I have run in the past, at least as far as reception goes ... but that is a topic for another post.

The further adventures of the Heathkit AT-1

Work has been conspiring to eliminate my spare time but I was able to spend a few hours over the Easter holiday to clean up the shack and make space to put the Heathkit AT-1 on the desk again. I have been able to spend a little time going over parts that need to be replaced and making a list.

The Heathkit AT-1 chassis with case and VFO-1 behind.

There doesn't seem to be any show stoppers although the wafer of the meter switch has broken in two and will need to be repaired. If I'm not able to repair it then thankfully it is fairly simple and replacement rotary switch can be substituted.

This isn't going to be a museum quality restoration but the changes that were made to this transmitter in the past were sensible and if left in place are representative of period modifications. The original meter for example was not the highest quality and a Western or Simpson replacement would be an improvement. The original slide switches have been replaced with period snap-toggle switches which are also an improvement over the original.

The Heathkit VFO-1 however has been modified for grid-block keying which is a significant departure from the original and I plan to revert it back to cathode keying. Although a technical improvement it is not in keeping with the original design and needs to be undone. Everyone will have their own opinion but I think if I wanted modern circuits I'd get a more modern rig, so the VFO-1 will be returned to stock.

Hopefully I can carve out a bit of time here and there to work on this and slowly return it to working condition.

Heathkit's first amateur transmitter - Heathkit AT-1

The Heathkit AT-1 represents the commercial embodiment of the simple Master Oscillator Power Amplifier (MOPA) transmitter using a crystal controlled 6AG7 oscillator plus a 6L6 final output tube.

Although it was possible to design and build a simpler transmitter, the goals of output power and stability could become mutually exclusive when trying to operate with only one tube. For a novice class license holder of 1951 the Heathkit AT-1 represented a solid performing rig that would be relatively easy to construct and operate.

The Novice remained the primary entry license until the Morse code requirement was eliminated for Technician licenses in 1990. On HF it permitted code transmissions only, with a maximum power of 75 watts, (input to the transmitter's final amplifier stage) on limited segments of the 80, 40 and 15 meter bands.

For $29.50 and the loan of a few tools you could get some use out of that new novice license

The earlier MOPA circuit from the ARRL handbook of 1941 below shows a layout remarkably similar to the circuit of the AT-1 although it is designed for plug in coils rather than the band-switching arrangement of the later Heathkit transmitter.

MOPA transmitter using a 6L6 and an 807 as the power amplifier (ARRL Handbook 1941)

For a little added complexity MOPA transmitters generally offered better stability of frequency and keying waveform than single tube crystal controlled or self exited rigs. The straight forward design of the AT-1 should have looked familiar to novice class hams after studying the ARRL handbook or other radio publications.

Heathkit AT-1 Circuit diagram showing band-switching arrangement and link coupled output

Once the novice had upgraded his license the AT-1 could be expanded by the addition of the Heathkit VF-1 variable frequency oscillator to allow transmission on any frequency within the allowed band.

The Heathkit VF-1 Variable Frequency Oscillator

The VF-1 covered 160-80-40-20-15-11-10 meters and used an OA2 voltage regulator tube to provide a stable voltage for the oscillator. Ceramic coil forms, solid construction and high quality components were used to help increase stability.

It was recommended that to correctly couple an antenna to the AT-1 you would use an antenna coupler such as the Heathkit AC-1 which also included a low pass filter.

The Heathkit AC-1 Antenna Coupler. Designed to attach to a single wire by the insulated post on the front panel.

Heathkit AC-1 Antenna Coupler circuit diagram

Although Heathkit did not produce a AM modulator for the AC-1 there is provision for modulator connection on the rear panel. The earlier ARRL manuals have several suitable circuits for modulators that would work with the AC-1. Most functioned by driving a modulation transformer with the output from an audio power amplifier. The secondary of the modulation transformer would be carrying the DC plate supply for the power amplifier tube plus or minus the instantaneous voltage of the audio waveform. By changing the plate voltage to the final amplifier tube the radio frequency output would be controlled by the amplified audio frequency resulting in amplitude modulation.

An oscilloscope clock - Recycling vintage equipment without destroying it.

As a fan of vintage electronics, and vintage technology in general,  I love to see things being re-used rather than being dumped in the trash. This has the very desirable effect of preventing sometimes toxic chemicals from being released into the environment and reduces the demand for new parts that can also involve toxic chemicals during their manufacture.

The only downside to this recycling is that frequently the original function of the equipment is lost so I was pleased to see this creation from Bob Alexander. He has combined one of the oldest types of test equipment with some of the newest microprocessor based devices to bring us a Oscilloscope Clock that still functions as an oscillloscope!

Heathkit oscilloscope with AVR Oscilloscope Clock module

Bob ran into some problems while installing the AVR Oscilloscope Clock Module but was able to invert the horizontal output from the clock module before feeding it into the scope. To read about his build in detail have a look at : An Oscilloscope From the Trash

A small modification to the rear panel to allow setting the clock and switching between clock and scope.

Check the video below for a demonstration of the Sparkfun AVR Oscilloscope Clock Module in action on a significantly newer oscilloscope.

PACO C-25 Capacitor Tester

I recently attended the Houston Vintage Radio Association post holiday dinner and participated in the auction held after the meal. It was a great evening and if you have any interest in vintage radio I would suggest getting in touch with the HVRA and becoming a member.
Among other things I walked away with at the end of the evening was a PACO Model C-25 capacitor tester.

The PACO C-25 differs from my Healthkit IT-22b in that it tests both regular and electrolytic capacitors as well as using a 40Mc oscillator to enable a rough measure of capacitance using a bridge circuit.
Most of the time, with vintage vacuum tube equipment, capacitor values need only be "in the ballpark" to function perfectly so a high degree of accuracy is not required. For more accuracy I have a Heathkit impedance/capacitance bridge if required.
I have my doubts how accurate the tester would be when measuring capacitance "in circuit" but otherwise it looks to be a very useful bit of kit!
The users manual should be available here and the circuit diagram is shown below.

New project - Heathkit SB-101

I've recently been presented with a Heathkit SB-101 transceiver, Heathkit SB-600 speaker and Heathkit HP-23A power supply courtesy of Philip KJ5OW of the Katy Amateur Radio Society.

The SB-101 is a hollow state rig, the only solid state components being a few diodes. It uses the well known, and fortunately fairly common, 6146 output tubes in the finals.

The manual is available here : Heathkit_SB101_Manual

Heathkit SB-101 Transceiver

There looks to be a little work required before this rig is put on the air, a few resistors have cracked for reasons unknown and the rubber belts need replacement in the load and tune controls. It is something worth taking time on as the SB-101 is a well regarded rig that is still a strong performer today. There have been some modifications made to support an external linear master oscillator (VFO) which will need to be looked at but are probably fine as they stand.

I pulled my Heathkit HD-1410 keyer out of storage and was pleased to find it was the same color and style, matching the SB-101.

Heathkit SB-101 transceiver, SB-600 speaker, HP-23A power supply and HD-1410 electronic keyer.

Specifications:

Year of Manufacture: 1967-70 (Improved SB-100[1965-7])
Bands: 80 - 10 (500kc) (2 Mhz on 28-30 mhz)
Modes: CW & SSB
Power: 180W PEP SSB & 170 input CW - 6146 Output Tubes
Frequency Control: LMO- Linear Master Oscillator (Like PTO)
Selectivity: 6 Pole filter 2.1kc @ 6db down (Accessory CW filter 400hz)
Contstruction: Aluminum Chassis & Cabinet-Printed circuit board wiring
Keying: PTT and VOX voice operation, VOX with sidetone for CW
Power Supplies: AC or DC power supplies availiable
Microphone: High Impeadance Desk or Mobile Microphones availiable.
Frequency Control: Local LMO/Xtal, Remote VFO/SB-640 w/Mod

Accessories: SB-200 Linear Amplifier, SB-500 2mtr Xvtr, SB-600 Speaker, SB-610 Monitor Scope, SB-620 Panadapter, SB-630 Station Console, SB-640 Remote VFO & SB-650 Digital Read-out.

Features: Easy to operate, TX & RX circuits track, RX RF Preamp, Easy to calibrate and read dial calibration, Two-Tone Green wrinkle paint, Switched Metering, Construction Manual very comprehensive.

Heathkit Transceiver Resources:

• Heathkit HW & SB series parts : http://www.ultrawebb.com/OHP/index.htm
• Heathkit HW & SB series modifications : http://boatanchor.com/Heath/HW101MOD.PDF
• Digital Dials for Analog Rigs : http://www.aade.com/
• Dialcord replacement for o-ring drives : http://nrwilliamson.tripod.com/dialcord.jpg
• General HW & SB repair information : http://www.qsl.net/k5bcq/102/102.html