I have collected over the years several hundred MX and MX-S radios from auctions. Everything from a 2 channel 340, anumer of black faced S model 340's for the Secret Service, to a 48 channel with TT, MDC-600, scan, multi PL and DES from a DEA auction. I also have about 1/2 a dozen MX-300-S porta-mobiles on various freqs. I also have 2 different suitcase repeaters in Zero cases that use 2 portables strapped into a frame.
They were the best radios of the time, and I carried one for nearly 10 years (the actual same radio) while working NYCEMS and it never failed working 24/7 between all our crews. The plastic frames were a problem which continued even after an early mod to not have to totally disassemble the radio to change the frame (A model circa 1977) out from the battery breaking it at the bottom when sat on. I found that in the S model you had to have the grounding clip tight inside the radio for it to work properly. I converted a few of the MX suitcase repeaters to S models using some brand new boards I picked up, and I still use it as my frequencies are in the T band. Even if the radio has a one time prom you can jumper it to one of the other 3 12 channel zones or 16 if programmed that way.
I set up about 60 units for my son's CAP unit on their VHF channels. I felt it was much better to have a 20 year old MX broken or lost then a current radio at that time while they were EPIRB chasing in the woods of upstate NY at Fort Drum. We also used the UHF ones while at Boy Scout camp in the Adirondacks, and I also had rental units on one of my repeaters and used many on GMRS.
I bet you never heard of a MX mobile. It was made in Germany and was just a MX chassis in a box with a funky looking mic. I can remember installing only one in the Hackensack N.J.Police Chief's car in the glove box. I don't think it was a big hit due to it's low power output and small speaker.
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I noticed with a UHF "S" model that simply tapping the end of the antenna while transmitting would cause loss of lock. The RF shielding and bypassing was not so good on those radios.
That usually was a bad solder connection or dirty Converta Comm connection for the antenna. When you pull the radio out of the holster by the antenna everyday you eventually break the wire connection to the T/R relay. If I wan't using the radio in a CC I would jump out the external antenna.
I have been looking for years for the mid band H42 model radios, along with a Railroad Micor for my collection.