P31-DDC 30.660Mhz FM "Handie Talkie"

[w9sl]

I was given 3 of these radios but cannot find any data on them. They seem to be working although the batteries are all bad. They are single channel FM portables on 30.660Mhz with base loaded whip antennas.
I'm interested in what their intended use was, why would one want portable low power communications on 30Mhz? What were these good for?
Could they be converted to the 10M ham band?
Where might I find schematics and specs for these antiques? The donator of these radios indicated they went back to the 70s and he was licensed to use them as KA2750.
Any help in answering these questions would be much appreciated.
Thanks in advance.

[k2hz]

That would be from the early 60's. The only info I can find on it is it uses a Motorola Type AB2 TX xtal and type YM45 RX xtal.

It appears to be a predecessor to the PT200 series P31DDN from the mid 60s. That is as far back as any of my references go other than the xtal cross reference I mentioned above. I don't know if DDC was an early production PT200 or an earlier model packset.

http://www.repeater-builder.com/motorol ... 2a45-c.pdf

The low end of low band had a lot of industrial and public safety use back then. The PT radios were common for forestry and petroleum (drilling and pipeline) users. 30.66 was a "petroleum radio service" frequency.

[w9sl]

Thanks much, that pdf is for exactly what I have and it has all the info I could hope for.
I'm curious as to why they they chose 30Mhz for portable low power field communication, was it because they didn't have transistors that would develop significant power at VHF frequencies?
Any thoughts on that?

[k2hz]

UHF technology was rather primitive in 1960. The general practice was Low Band in rural areas and VHF in more urban areas. Portable radios were just starting to become really practical in the early 60s since earlier units still used miniature tubes. Your unit may be one of the first that was all solid state.

Systems like forestry and petroleum were Low Band with high power base and mobiles for wide area coverage out in the boondocks. The packset portables were the solution for crews that had to work away from their vehicles to at least be able to talk back to a nearby vehicle.

I got involved with 2-way in 1962 and UHF repeaters were just begining to come into use in metro areas. There was a sales rep in the area that loved Low Band and was selling a lot of small business sytems on 30.800 with 100W mobiles to somebody like a plumber with 3 or 4 mobiles just to talk around town. My first experience with UHF was around 1965 when we put in a repeater for the local Pepsi distributor and we were afraid it would only cover a few mile radius. It turned out to work better than expected around town but not like low band when you got out in rural areas with hills and foliage.

[w9sl]

I didn't realize these units went back to the 1960s and that explains the technology available at the time.
I got out of school with an EE degree in 1958 and at that time the transistor was new and the state of the art was a Raytheon CK722 germanium audio transistor.
I'm not sure what I will do with these units. I thought I might move them down a Mhz to the ham band, but I'm not sure it's worth the effort.
Thanks again for the valuable info you have provided.
73,
Don, W9SL

[k2hz]

It might tune down to the high end of 10M. Worst case, some circuits may need to be padded with additional capacitance. The cost of the xtals would be the biggest risk to see if it will retune down to 10M.

One possible negative of a unit that old is it could be real "wide band" (+/-15 kHz deviation - 36F3) with poor (40 kHz) RX selectivity for today's word. That was an option on the newer DDN series and could have been standard when the DDC was built. Since the DDN is 1966 or earlier that is why I am guessing the DDC could be from about 1962-1964. If it is solid state, it probably is germanium like the early Motracs.

73,
Dick K2HZ

[w9sl]

According to the specs in the pdf, the dev is +/-5khz and the selectivity 20khz so it would be compatible with today's band plan, and the main thing that seems to change going from 25-54Mhz is primarily the xtal freqs. One could probably add capacitance to the oscillators and bring it down a mhz from 30.6 to 29.6 or thereabouts. Might be fun to play with if one has time to kill. Maybe I'll take them to a hamfest or put them on ebay for someone who wants to play.
Thanks again for your help.

[WB6NVH]

Just to clarify:

P31DDC is a PT300 with a dry battery base as shipped. That means D cells, alkaline. P31DDN is the version with the nicad battery (which is diamond shaped, inside the shorter battery housing.) C is dry cells, N is nicad. They are otherwise the same in vintage.

The PT200 and PT300 radios were made from approx. 1964-1974 with the majority being in 1966-68. They are narrow band, as in the traditional meaning of it, +/- 5 kHz. Basically the HT200 boards with a power amplifier. Germanium transistors abound so be careful. You should be able to get 5-6 Watts out. P21DDC would be the PT200.

They work just fine on 10 Meters with no mods needed if they are the 30-36 split. I have several on 10 now which were that model. The 25-30 split also needs no mods. 36-42 needs work and is almost useless. 42-50 will pull up to 6 Meters fine. Crystals are expensive.

Forestry and power utilities loved pack sets. More power than an HT and huge battery life. I have talked over 100 miles on my 6 Meter PT300 from a hilltop. Low band hand-helds don't work worth a darn because of antenna radiation inefficiency. But you put the pack set on the hood of a vehicle, or the roof, and they work great.

If you have the nicad battery box, some careful watching of eBay should turn up the alkaline D cell battery box, which in today's prices needs a king's ransom of D cells but is actually more practical. There is also a railroad battery box which uses three spring top 6 Volt lantern batteries, and an AC supply to make a portable base station.

You cannot move the radios very far outside the original bandsplit. A 30-36 will NOT tune to 6 Meters and a 42-50 will NOT tune to 29 MHz. A full alignment is required to move these in frequency more than a couple hundred kHz, except that the receiver IF can be left alone.

[k2hz]

Thanks for the clarification.

I have some literature from around 1967 that refers to the "DDC and DDN" series accessories being the same and the service manual that just lists the DDN. A crystal cross reference from about 1965 just lists the DCC which I was I suspected it was earlier production.

While I have never seen a "wide" (36F3) version, it was listed as an option for the PT200/300 which is why I suspected that could be the DDC.

International Crystal does a good job with crystals to Motorla specs but they were $20 each last time I ordered any.

When buying PT battery boxes from eBay, beware of the condition. The D cell boxes were very subject to corrosion and the terminals may be totally eaten away. The lantern battery versions seem to hold up better. I have 2 PT500s still in service at a RR museum. The D cell packs are lower profile but a real pain compared to the lantern battery version.

[DJP126]

You might also take note (and I'm surprised that nobody mentioned it) that these radios are POSITIVE ground.