AT1 Heath kit (by N5AF)
When I received Heath Kit AT-1 number 2, I could hear glass tinkling around inside the box/cabinet. I got AT-1 number 1 going and on the air first.
I spread out a sheet of The Houston Chronicle ( Yesterday's News Tormorrow! ) and opened the case after placing the AT-1 on the paper. The 5U4 had either fallen out if it's socket or had never been there and was broken. After that mess was cleaned up, I gave it a deep cleaning, removed the old filter caps, replaced them and neatened it up a little.
The previous owner had mounted a big base with screws for the connecting wires to attach to and had an Ice Cube 120 relay mounted under the chassis for antenna changeover.
All that came out & I replaced the coax connector ( cheap plastic insulation that melted when I soldered to it. All of this in one day!
I hooked up a dummy load and a key, after finding a " new " 5U4. It tuned right up and makes plenty of power. Only problem was, the previous owner, probably to eliminate chirp, had wired the 6AG7 so that it runs full-time when B+ is on, which kinda defeats the purpose of my E.F. Johnson T.R. switch.
It will spend a week or so more on the workbench. I want to add fuse holders to the DX-20s and both AT-1s. Don't want any surprises while I have stepped out of the shack for a minute. When I was running the DX-20, I could hear the popping of the filter caps as it built up " speed " as the popping increased. The filter never blew, but it has got to be close.
One AT-1 will be parked on 3546 and the other on various cw xtal freqs that I have on 40 meters.
This one was an easy one, especially after doing the other AT-1 right before it.
Sam Neal N5AF
Real Radios Glow In The Dark (by N5AF)
I started collecting old World War II ARC-5 transmitters several years ago and now have almost a complete set of the transmitters and several receivers. Yesterday ( 3 Feb, 2010 ) I laid one of it's back and proceeded to start the conversion, as per the recent QST article in January 2009 QST magazine.
These were designed to be used as HF transmitters mainly in bomber type aircraft where there was a radio operator present and are light in weight, mostly aluminum construction. Each has a 1626 VFO tube and a pair of 1625s in the final, capable of 150 Watts input if it wasn't for the current fuse in the B+ line. Typically, they ran about 25 Watts or less. Each covers a particular part of the HF spectrum, with several of them covering ham bands without having to change the VFO circuit any at all.
The conversion process is relatively simple. The filament wiring needs to be changed so that the filaments will operate off of 12 Volts instead of 24 Volts as was the design Voltage. Each transmitter has a calibrator crystal in it and when the VFO is tuned accross the crystal frequency, the magic eye tube closes, giving an indication of calibration accuracy. The transmitter filaments must be operated off of direct current for this feature to operate.
I now have the filaments rewired for 12 Volts DC and the long ago fragerance of dust on hot glass is perfuming up my shack with long ago fragerances I had forgotten. I removed the two relays and am wiring the rig so that the VFO runs all the time ( while transmitting ) ( No QSK ) and also providing regulated voltage for the VFO to prevent drift. The filaments have been on for a couple of hours. I hope to have B+ on the VFO tomorrow.
I had one of these back in 1958, when I first passed my General, that covered the 80 meter band and had a great deal of fun with it with the exception of receiving a " Notice Of Out Of Band Operation " from the Canadian equilivent of the FCC. I had connected a long wire antenna directly to the plate caps with no final tank circuit to filter out harmonics and my 3700KC +- signal had a nice harmonic on 7400 KC+-.
The radios were designed for operation on aircraft that operated on a 24 Volt DC system. It is a simple matter to rewire the filaments in parallel to operate them off of more convenient 12 Volts AC or DC.
To minimize drift, it is recommended that these transmitters be operated with the VFO running all the time you are transmitting, and that they be operated off of a regulated high voltage power supply for the VFO.
The relays in the radios are taken out and thrown away and the power plug on the back is usually taken out and discarded and replaced with a more obtainable 8 pin octal plug.
Then, all that remains is to add a key jack to plug your key in to and key the cathodes of the 1625 tubes, which are basically 12 Volt 807 tubes, and take the little variable coil and antenna changeover switch out and discard them, mount a chassis mount coax connector where the push-button aircraft 25 foot ceramic post was at before. Total conversion time is about 2 and a half hours.
When #1 son drops by with his digital camera, I will have him make some photos for the SHARK website.
The one I am converting is for the 40 meter band. After I get it working, I will tackle the 80 meter one next. Supposedly over a million of these were manufactured during WW2. Details to follow!
Sam Neal N5AF