The AM Forum

THE AM BULLETIN BOARD => Technical Forum => Topic started by: WU2D on October 01, 2006, 05:24:28 PM

Title: MAB - Marine Airborne
Post by: WU2D on October 01, 2006, 05:24:28 PM
How about a complete AM radio station that you can jump out of an airplane with?

I picked up an MAB more than 15 years ago. The MAB is a one channel walkie talkie that was used by Marine Paratroopers in the Island Campaigns of the South Pacific. The idea was that it could be used to call in artillery from ships or support runs from aircraft, once the troops were dropped in.  What is interesting about this radio is that it is basically weather tight, with a fibercloth case and gasketed cables and even a waterproof on off switch. It takes a 6 V bias battery, a 1.5V fil battery and a tapped 135V supply to run it. Half of the package is XCVR and half is for the batterries. I managed to crystal it for 3885 (it covers 2.3 - 4.5 MC). I easily made up a battery pack consisting of 3 C cells in parallel (A Supply) 15 9V batteries (B Supply)  and 4 AA cells (C Supply) and it worked like a champ. I used it a a couple of Hosstraders.

Then something bad happened - it went dead  :P. This radio was made to be used once and discarded once the battery pack went dead in 1945 and it is miniature contruction in every sense of the word. I did not look forward to attempting a repair! In fact I have procrastinated for 10 years! The entire tranciever box is 6 X 3 X 1 Inch and it weighs less than 14 oz! The whole transciever is made of 7 brand new miniature battery tubes which were very new at the time it was designed in 1941.

More Below

Mike WU2D

Title: Re: MAB - Marine Airborne
Post by: WU2D on October 01, 2006, 05:45:22 PM
Long story short - She had a bad 1S5 Detector first Audio Tube (which I replaced with a 1U5 which required swapping three pins on the socket) and a bad headphone element and three suspect caps which were changed out. But she is ready for Hosstraders once I get a fresh battery pack made up! ;D


Title: Re: MAB - Marine Airborne
Post by: The Slab Bacon on October 01, 2006, 08:56:28 PM
Pretty cool little rig! Military tactical radios of that era were usually designed with fairly sensitive receivers and extremely pissweak transmitters. (so a distant enemy could not hear them). I love stuff with those old 1.5v filament tubes. I built some of my first radios as a very young jn using them. Many 1, 2, and 3 tube trf's and regen sets. Those tubes offer very good sensitivity at very low plate voltages.

Stuff like that is getting very hard to come by these days. We have a few guys in the Mid Atlantic Antique Radio Club that really go nutso over that kind of gear. Some of it is very interesting.
                                                                             The Slab Bacon

Title: Re: MAB - Marine Airborne
Post by: WU2D on October 02, 2006, 09:42:28 PM
Well Slab,

The beast had one more demon. The RX section seemed to overload on fairly weak signals (like GFZ's? 8)). It turned out to be the  problem that most old receivers have; a leaky AGC capacitor. This rig used a .003 uF AGC cap against a high value resistor. All of the resistors are embarassingly high values, designed to save battery current. Imagine if you used all of the highest values that a tube circuit could tolerate. Anyway, the typical resistors are all between 2 Meg and 6 Meg and they all have gone up in value as you could imagine..but it still works.

Tomorrow night I will put the beast together. Notice the battery pile. The D Cell is the fills. The 4 AA cells are the Bias (way overkill) and the 15!!! Ultralife Lithium Batteries are the B+ and B++. Now 15 of these batteries would set you back some big$ but they were cast offs from the test department at work. The advantage of lithium is that the supply will not degrade with age and theoretically will be just as good next year.

See you at Hosstraders!

Mike WU2D

Title: Re: MAB - Marine Airborne
Post by: WA1LGQ on October 09, 2006, 01:31:10 PM
Mike I worked you on that thing at Hosstraders, but I guess your batts died quickly or something. I was parked about 300 feet North of the W1E setup. Where were you on the grounds at that time? I think you said you were near WA1GFZ. Were you near where he was parked? How much power does it run? Yeah it sounded restricted, but quite understandable...........Larry

Title: Re: MAB - Marine Airborne
Post by: WA1GFZ on October 09, 2006, 02:17:50 PM
Oh that was you Larry?
Were you talking through your avatar?

Title: Re: MAB - Marine Airborne
Post by: WA1LGQ on October 10, 2006, 05:39:44 AM
Yeah Frank, as usual.............

Title: Re: MAB - Marine Airborne
Post by: WU2D on October 11, 2006, 05:58:27 PM
Yeah Larry,

I even put Frank on the microphone but he was not yelling loud enough. I was hanging out on that side and fired the thing up a couple of times. No problems with the batteries.

Mike WU2D

Title: Re: MAB - Marine Airborne
Post by: wa2zdy on October 15, 2006, 07:11:38 PM
And here I thought I had something unusual going with my Motorola HT200 Handie Talkie on 29.600 FM at festers.

Nice rig there, glad you've got it squared away.  Enjoy it!

Title: Re: MAB - Marine Airborne
Post by: WU2D on October 15, 2006, 11:03:36 PM
Thanks Chris,

Holy Cow - an HT-200. For heavens sake don't drop it on your foot - Hi. When I worked at Mot in th early 80's we had a couple HT-100's and already they were considered rare. I think they put out 100 mW and they were certainly small compared to the HT-200 BRICK.

Great fun playing with "handys" - troubleshooting thiose cordwood construction boards is not so fun.

Mike WU2D
AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands