Mark's Green Pages

MT-702/U Radio Mount

Mark J. Blair, NF6X — Sat, 04 Feb 2012 08:21:12 +0000

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 don’t know what radio set(s) this mount goes with yet, or whether it’s from the WW2 era or later. It is constructed differently than the FT-505 that I borrowed and wrote about; that FT-505 was assembled from rails of cast (or possibly forged) aluminum riveted onto a stamped sheet aluminum base, while this MT-702/U is a single cast piece. I’ve heard from another FT-505 owner who says his mount is a single casting, though. “MT-702/U HUBBELL & MILLER CO” is cast into the base portion of the mount.

If you know what radio set(s) this MT-702/U goes with, please comment below.

Update:

Dennis Starks identified my new mount:

Your pack board is equipped with the later style Korean War vintage mount for use with a BC-1335 & it’s companion dry battery box.
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T-368C On the Air!

Mark J. Blair, NF6X — Sun, 22 Jan 2012 03:55:04 +0000

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.

I plan to experiment with outboard audio processing to see if I can improve the unit’s classic military audio sound, which is intelligible but not very clean-sounding. I am interested in experimenting with that stuff, but I would prefer to avoid modifying the transmitter’s innards as some other T-368 fans have done. So, my goal will be to see how much I can improve the stock audio using only outboard equipment. I’ll post any favorable results.

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T-368C Firebottles

Mark J. Blair, NF6X — Sun, 08 Jan 2012 06:15:17 +0000

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.

So, here are the tree tubes sitting on my workbench, at a nice, boring room temperature:

At this point, I have my T-368C working with the exception of low amplitude modulation depth. I replaced the large 50 watt modulator bias potentiometer on the front panel with a new one because the old one’s moving contact was badly worn, and not pressing against the resistance element tightly enough. This didn’t fix the modulation depth problem as I had hoped. More debugging shows that the audio appears to make it all the way to the modulator tube grids with ample amplitude, and the two 4-125A tubes are looking like the most likely culprits.

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T-368C Transmitter Progress!

Mark J. Blair, NF6X — Tue, 03 Jan 2012 01:29:37 +0000

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…

The modulator bias potentiometer looks bad when I measure it with a DMM, and I can’t seem to get quite enough bias on the modulator tubes for them to allow 100% modulation. I found what looks like a suitable replacement on eBay, and it’s on the way.

Two of the three panel meters are sticky, too. I need to rap on their glass to get them to respond to small changes. These are sealed, ruggedized meters, and I don’t know yet whether it’ll be practical for me to try to repair them.

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T-368C: First Smoke Test

Mark J. Blair, NF6X — Sat, 31 Dec 2011 12:31:19 +0000

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:

No actual smoke seen; just a healthy (?) amount of “hot dusty stuff” smell.
I can’t seem to get the PA to load up at 150mA in tuning mode as the manual directs; tuning for max RF output gives me about 100mA plate current and 65-70 watts of RF.
Full power mode is no-go: I hear some arcing sounds, then the overload relay trips.
Both blowers work, though the PA blower is quiet enough that I can’t hear it over the cabinet blower.
PA grid drive reads a bit high at 15mA.
Int. amp plate current reads 25mA.
In AM, I can only get the modulator bias current up to around 40mA. I don’t trust the bias potentiometer based on DMM testing earlier today. Whistling into the mac doesn’t swing the meter higher.
The filament voltage meter is a bit sticky; I need to thump on it to see small changes in voltage.
I don’t like having the back cover interlock defeated during testing, but those big tubes sure are pretty when they’re all lit up!

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Loading Paper Tape into the Fialka Cipher Machine

Mark J. Blair, NF6X — Mon, 26 Dec 2011 06:06:45 +0000

A collector who just bought a Fialka cipher machine asked me how to load the paper tape, which is tricky until you’ve done it a few times. Here’s how I load the wide paper tape (for printing and/or punching) into my Czechoslovakian M-125-3 machine. His machine is a Polish M-125, but hopefully the paper path is about the same (I’ve never handled the earlier model of Fialka, so I’m not sure about that). The paper tape path is a bit different when using the narrower print-only paper tape, but I don’t have any to demonstrate with.

First, here’s how the bottom edge of the paper reel holder fits onto the clips on the rear of the machine. There’s also a bracket (not shown) near the center of the tape holder which fits over a metal bit poking up from the rear of the machine.

Route the tape over the two rollers on the reel holder, and through the guide that’s between them.

Give the tape a twist as shown, and route it through the angled guide as shown. It’s OK if the tape falls out of the guide while you’re loading it; just slip it back in later. At this point, we’re just sticking it through the guide to make sure we twist it the right direction.

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FT-505 Pack Mount for the SCR-619

Mark J. Blair, NF6X — Sat, 17 Dec 2011 17:35:33 +0000

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.

Here’s the mount as I received it from Paul, screwed to a piece of wood for shipping.

Here are some detailed pictures that I took while measuring its dimensions. It’s made from cast aluminum rails which are riveted to a stamped aluminum base with solid aluminum rivets. Based on a verbal description from another FT-505 owner, I believe that there was at least one other FT-505 variant which was manufactured differently.

Here’s my SCR-619 mounted on the FT-505.

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Welcome to my New Blog!

Mark J. Blair, NF6X — Mon, 12 Dec 2011 23:08:05 +0000

I have had a static web page for many years, but I’ve just moved it over to a WordPress-based blog. The new format adds many features such as searching, tags, comments, and an RSS feed. I’ve ported over most of the content from my old web page with minor reformatting and revisions, and I maintained the original publishing date stamps for the old content. I may also go back and plug in things like pictures of past events that I never bothered to share before, so please forgive me for engaging in a little bit of revisionist history.

I hope that y’all like the new format!

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SRI-M550 Amplified Speaker

Mark J. Blair, NF6X — Tue, 05 Oct 2010 12:40:00 +0000

I bought this amplified communications speaker at a military surplus swap meet. It’s a nice little powered speaker which is intended for use with manpack radio sets (particularly ones with digital data capabilities). It is powered by a 9V battery which fits in a sealed compartment at the bottom of the unit. It is housed in a rugged aluminum chassis, and has a pair of clips on the back which can be used to hang the speaker from one of the “roll bar” handles which are common on these manpack sets.

The top of the speaker includes an audio accessory jack to which a handset may be connected, an on-off-volume knob, a toggle switch which selects analog or digital modes, and a tiny 6-pin auxiliary conenctor made by Hirose. The front of the unit has a speaker grille and a red/green LED. The LED appears to light up green when audio is over some threshold (i.e., functioning as a call light), and red when an attached handset’s PTT switch is pressed. White noise will light up the LED, so the radio’s built-in squelch function is still necessary to make the call light meaningful.

The permanently attached cable connects to a 5-pin or 6-pin audio accessory jack on the radio.

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Soviet M-125-3xx “Fialka” Cipher Machine

Mark J. Blair, NF6X — Wed, 28 Apr 2010 21:05:00 +0000

The “Fialka” is a cipher machine which was made and used by the USSR during the Cold War. I found this one on eBay, and it only required minor repairs to become fully operational. I’ll write more detailed information about it later; in the mean time, here are some pictures of my Fialka. You can also find more details about this kind of machine on Wikipedia.

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