Military radio and wireline communications equipment, and related stuff.
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:
The SCR-619 radio set could be deployed in several different configurations, including vehicular and man-portable. In a man-portable configuration, the BC-1335 transceiver and CH-191 battery box would be installed on a common pack frame with an FT-505 mount. These mounts seem to be quite rare, as I’ve only found about two or three collectors who have them or have ever seen one for sale. A few months before the 2010 West Coast Military Radio Collector’s Group annual meeting, Paul Thekan kindly loaned me his FT-505 so that I could photograph it, measure it, and display it with my SCR-619 set at the meeting. I didn’t just photograph it, though… I also created a 3D CAD model of it, with thoughts of someday fabricating a reproduction.
The KY-38 is the manpack variant of the NESTOR family of voice security devices. Used during the Vietnam War, this family included the KY-8 vehicular unit, the KY-28 aircraft unit, and the KY-38 manpack unit. These devices permitted secure voice communications over radio.
This particular unit has been demilitarized; that is, all of the cryptographic hardware has been removed from the unit before it was released as surplus, leaving only the case, power supply, interface circuitry, and an interesting electromechanical keying device. The battery box is also missing.
| © 2011 Mark J. Blair, NF6X | Suffusion theme by Sayontan Sinha |
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