I recently bought a Westinghouse GP-7 transmitter in the swap meet at this year’s annual West Coast Military Radio Collectors Group meeting, held in San Luis Obispo, CA at the beginning of May. This transmitter was made for use in Navy aircraft, and it requires 120 VAC 800 Hz power like other Navy radios of its era. Aircraft commonly use AC power at higher frequencies than our common 60 Hz “wall power” so that their transformers and motors can be lighter. The higher power frequencies allow transformers and motors to use less massive iron cores without magnetic saturation. 400 Hz power is now commonly used in large aircraft that require AC power supplies, but this transmitter was made before 400 Hz power became the standard. Unfortunately, it can’t simply be plugged into 60 Hz power. That would saturate the transformer cores, and then they would release their magic smoke and stop working.
Military radio and wireline communications equipment, and related stuff.
Last weekend, the 18th annual meeting of the Military Radio Collectors Group was held in San Luis Obispo, California, at Camp San Luis Obispo’s NCO club. The event included equipment displays, presentations, field operations and a swap meet. I had a great time, and nearly every other comment I heard about this year’s meet was positive. I’m already looking forward to next year’s annual meeting, as well as the occasional field events we’ll probably have throughout the year.
The annual Military Radio Collectors Group meeting at Camp San Luis Obispo is almost here! This year, I’m pleased to announce that we’ll be conducting a joint crypto-related operation with the Maritime Radio Historical Society’s Coast Station KSM. Here’s their announcement of the operation.
Yesterday I was out at Fort MacArthur for an MRCG event. While I was there, I had a rare opportunity to ride in the fort’s half-track, complete with a quad .50 caliber antiaircraft gun in the back. I sat shotgun and operated the SCR-609 radio set. Some video may show up on YouTube someday. For now, here are a few pictures of the half-track.
Updated: Video of the event is embedded below.
For a long time, I’ve had a passing interest in radar antennas. Not so much the antennas themselves, but mostly their azimuth-elevation (az-el) drives. Every now and then, I’d search on eBay for any such az-el drives that looked both interesting and inexpensive, with or without antennas. I finally found one to buy about a month ago, and here’s a little bit about it.
I’ve just obtained a WW2-style packboard with an MT-702/U radio mount fastened to it. It’s similar to the FT-505 pack mount that I wrote about previously, and a BC-1335 transceiver will fit on it. However, unlike the FT-505 which has a wide spot for the BC-1335 and a narrow spot for a CH-191 battery box, this MT-702/U has two wide spots, each of which is the right size for a BC-1335.
I’m very happy to report that thanks to a NOS pair of Eimac 4-125A modulator tubes from Antique Electronic Supply, my T-368C is back on the air for the first time since I got it! I’ll still have plenty of tinkering to enjoy on it, and plenty of work integrating it into a full system with my R-390A receiver. I’m also happy that my antenna BALUN didn’t burst into flame upon encountering the hefty output of this small monster of a transmitter.
My T-368C transmitter uses three impressively large transmitting tubes. The power amplifier (PA) uses an Eimac 4-400A, while the modulator uses a pair of Eimac 4-125A tubes. These big tubes are beautiful in my opinion, especially when they’re operating with the plates glowing red. Sadly, they’re not normally visible in operation due to the transmitter’s opaque steel cabinet, studded with interlocks to keep folks away from the lethal high voltage that lurks inside. Even with the interlocks bypassed for debugging purposes (which is dangerous, and should be avoided when possible!), the big 4-400A tube is further obscured by an opaque metal chimney which ducts cooling air around it.
I’ve made some good progress on my T-368C transmitter already. The arcing problems have subsided on their own, though they may come back later. I traced down the modulator problem to a single resistor in the speech amplifier which failed open, thus removing power to the clipper tube’s plates and breaking the audio path. The transmitter is now working on CW and AM at full power into a dummy load! I also replaced another resistor in the same speech amplifier circuit, but I think it was actually OK and I just had a measurement error due to residual charge in the circuit.
There are still some kinks to work out…
I bought a T-368C HF transmitter project back in October, 2007 for $1575.42, and finally started seriously working on it a couple months or so ago. This evening, it got its first taste of power in many years! It’s semi-alive, but needs more work: