My MX300R

[Zack08]

Hi guys,

I've been visiting this forum for almost a year now but I havn't felt the need to register until now...

I just recently got an MX300R for a price that I just couldn't refuse. I took it to my work bench and connected it to a 7.2V power supply (the radio didn't come with a battery). I turned it on and all I heard was white noise. I pressed the TX switch and the white noise didnt go away and the red light never came on. Even with the voulme knob turned all of the way up, the audio isn't very loud.

I opened it up and made sure that the contacts on the PROM were clean. I then left the PROM disconnected and the radio acted the same way. The row of contacts on top of the PROM are not attached to anything and I'm thinking they should be. I believe that the radio had the secure module in it but it was removed before I bought it, so I dont know if the missing module would have something to do with it. There are also no modules installed in between the "PRESEL" filter and the PROM, but I am unsure if the empty holes are only for optional modules.

By the way, the PROM is the one-time-program "A" module and it was programmed because the programming sticker on the inside indicates that it was set for 406.0000 and 406.17500 MHz.

Are there any simple ways to get this thing to TX and RX like normal or do I have a brick?

Thanks in advance

[Hoseman292]

I've had similar issues with a MX300R that I picked up a few years ago. The MX300R was released around 1984. I still remember picking up a brochure for the radio at a convention when Motorola was promoting it.

From what I understand many were used with SEAL teams for personal communications. It's really quite possible that one of the modules may have been removed to disable the radio, especially the transmitter. Regardless, it's still a neat vintage peice of Motorola history. If there ever were a battery pack that accepted AA batteries to use with the MX300-R I would consider putting some work and money into it to keep in my truck for hamfests. The thing would last forever. In reality the standard ruggedized OEM ni-cad battery alone would be far too expensive to make the project practical.

GOOD LUCK,
Tim

[RADIOMAN2002]

Usually if the radio doesn't tranmit, one of two things. The encryption module has been removed, or if the prom is still there, the radio is out of lock or the prom is blank. Sometimes the demilling process is nothing more than removing the master osc or tran osc, check for missing parts.

[Zack08]

Im pretty sure that the encryption module was removed, but I dont see how that would prevent it from transmitting in the clear.

The PROM is still there, though.

Where exactly would the frequency synthesizer be located? Maybe they removed that, too.

Edit: There is a small module in the upper part of the radio that says VCO, but it seems kind of small to be the master osc. And, like I said earlier, there are no modules in between the "PRESEL" filter and the PROM, but I dont know if those holes are only for optional modules.

I guess even if i cant get it to work, it's still a neat collectors item.

[dfc2]

The MX300-R was used by lots of Military units. AS an MP At Ft. Bragg in 1986-88. we used them. We had about 50 of them, all new, that replaced the older HT220/MT500's.

Althought they were great radios for the time, they were too damn heavy.... but they did have a few advantages... they ballanced the weight of the .45 we carried back then.. ( Garrison units switched to the beretta in late 1987).

Great radios...If I had a stack of promms and an R1801...I'd still be using them.....


DFC2

[1 Adam 12]

Hi guys,
I opened it up and made sure that the contacts on the PROM were clean. I then left the PROM disconnected and the radio acted the same way. The row of contacts on top of the PROM are not attached to anything and I'm thinking they should be. I believe that the radio had the secure module in it but it was removed before I bought it, so I dont know if the missing module would have something to do with it. There are also no modules installed in between the "PRESEL" filter and the PROM, but I am unsure if the empty holes are only for optional modules

Well, the radio won't function unless you have the secure module or a bypass, which you can wire in. The following modules go between the preselect and prom, PL processor and filter along with pl chip VCO module and there should be two offset modules, one marked 3.600 and the other 21.4. If you can post a pic, we can probably help you with more
I probably have most of the parts you need.
The down side is that that radio will not go far out of band, the up side is, unless all the zones are blown out, you can jumper the radio to work with another zone of the prom, giving you 12 channels you can program.

[Zack08]

None of the modules that you listed are still in the radio.

If I were to wire in a secure bypass, would that give me an idea wether or not the PROM is OK? I know it wont TX or RX like it should, but will it beep or do something when trying to TX or change channels? I would just like to figure out whether it would be worth buying the VCO and offset modules or just giving up. After all, this radio was from surplus and i dont know if anything else is broken.

I never had a specific use in mind for the radio and I don't what to put a whole lot more money into it just to find out that it still wont work.