[MRCA] TBX-4a Coming alive

[Ray Fantini]

The old TBX was a marvel of technology! The receiver uses couple different voltages but I ran the one I had from a couple D cell batteries for the filament and lots of 9 volt batteries in series for the B+ and screens. Using batteries is way better then a AC supply for the radios older than the 8, they require a isolated A- return for the filament current display to work correctly and a C- for the gain control to work and also be aware that the TBX does not have a AVC circuit so it a net it will drive you up the wall constantly adjusting the gain up and down. I used one once in Dayton for the 3885 net and close in stations were painfully loud and all the BC -611 would be at different levels between loud and not there at all unless you worked the gain control. If you build the battery pack for the receiver you will find it last very long. The receiver draws less than ten mills from the B+ and two D cells have no issues providing the filaments. The original design had a hand crank generator for the transmitter that provides the two twelve volt sources, one for the filament of the 837 and one for microphone bias and the 500 volts for the tube. If you want to use it in the AM mode you will find that you need a microphone with tons of output. I put a signal transistor amplifier into a T-17 and found that had good audio. Trivia or Question: I think the TBX-4/5 or 6 were the only radios to ever see combat, famous picture of one at Tarawa comes to mind and the TBX-8 that replaced it was never deployed. Was this because the TBX family was such a poor radio in a combat situation that no one would use a TBX-8? Or that the modern SCR-300 family was so much better and HF field radios were thru by 43/44? In the accounts of Tarawa I have read that the marines dumped the TBX sets in favor of jeep mounted TCS sets, I like the old TBX but if it were important certainly would want the additional power and improved performance of the TCS myself.
Ray Fantini

-----Original Message-----
From: mrca-bounces at mailman.qth.net [mailto:mrca-bounces at mailman.qth.net] On Behalf Of chuck
Sent: Wednesday, January 19, 2011 10:02 AM
To: mrca at mailman.qth.net
Subject: [MRCA] TBX-4a Coming alive

I inherited a TBX-4a years ago, over the last few weeks I decided I'd like
to see it run. After building a power supply for the receiver
a kit I bought from antique electronic supply and reworking it slightly,
last night I got to hear it receive!!! I had been using a set
of headphones that were not working, advice from others said use a speaker,
which I did and its doing just fine.
Now for the TX section, this weekend if I can I'll put a PS together to run
the that side of it. I'm starting to enjoy seeing
this Military stuff run, (my wife will be thrilled, another collection part
of the hobby!) hi hi. I'll be needing cable for the
power supply, I'll build a battery box with both inside for aesthetics, what
did they use back then? and is it available still?
I need 7 conductors capable of 135v on one and 5 conductor for the transmit
power capable of 500v insulation..
 
 
thanks, maybe you'll hear me on the air with it soon!
 
Chuck -KA1MWP
 
 
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