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sabato 8 agosto 2015

Moving an old CB walkie-talkie to 29 MHz AM - Part 1

Hello there, after quite a long pause I write again on this blog to describe my recent experience of moving an old CB walkie-talkie to 29 MHz AM in the 10 meter band.
I had found a nice pair of Inno-Hit RT923 talkies for 15 euros, while visiting a junk dealer shop near home, looking for something cheap and unusual for my personal collection of useless things. They looked in a very good condition and I though that probably there were still working too.
In the next few days, I sold the better-looking one on an on-line auction site for 10 euros and I decided to try moving the remaining talkie on 10 meters, 29 MHz AM. Here below a couple of photographs of the "star" of this post.

The Inno-Hit is a 3 channels, 2 watts, crystal-controlled handheld CB transceiver. The receiver is a single-conversion superetherodyne with 455 kHz IF. The tx frequency and the local oscillator frequency in the rx are controlled by two different crystals. The LO crystal is of course 455 kHz below the actual rx frequency.
The same rig was sold with several brands and some differencies (mainly cosmetic); for example Royce 1-402, Brilliant BT-1232, Finetone TNC-203, MacDonald CE-420, Universum BSG-3271 and maybe others. All those rigs came from the mid '70s and had a very robust steel case, with a small opening door on the front to access the battery pack. Some of them had external connectors for an external antenna, earphone, power supply or battery charger. Usually they were sold with only one channel actually equipped with crystals (for CB channel 11).

The astonishing successes of my good friend Franco I5KAP on 10 meters AM, with even very old and simple QRP rigs, inspired me.

A visit to Franco (who live 10 km or so from my QTH) convinced me to go ahead with my plan. Franco was very kind and helped me in deciding which modifications would have made sense to try.
First decision, was to leave the transceiver as much as possible in its original box and appearance. Given the very limited space inside the case, the only sensible modifications to do were to add external connectors for the antenna and power supply (or external battery) and of course to buy a new pair of crystals (for the rx and tx oscillators) for the 29 MHz AM band.
The crystals were ordered to a factory here in the north of Italy (unfortunately, they costed about 25 euros each, including taxes and shipment). Then I bought for 2 euros each some schematic diagrams of the walkie-talkies I mentioned above (in particular, those which had the antenna and power supply connectors) and I proceeded with my work.
I identified the better points in the circuit were to connect the external antenna and power supply, I prepared the small removable plastic panel that was inserted on the right side of the case (like in photographs below) and quickly checked that all was OK, both mechanically and electrically.

Now it was simply a matter of setting a small measurement bench and do some tweaking on the tx path and the rx path to ensure that the talkie could work properly at the new frequency. This will be the subject of next post. Thank you for reading.

5 commenti:

  1. Very useful article about Absolute great. Thanks.
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  2. I am hoping to do this with my Dyna-Com walkie talkies. The problem is getting the xtals! So many companies either no longer exist, or, like Bomar, do not make xtals anymore. Any suggestions where to get xtals are appreciated, and as well what xtal holders have replaced the HC-25/u that these old walkie talkies used. HC-50?

    -gary // WD4NKA

    1. Hello Gary, I know your great work with regen receivers (the regenerodyne architecture, in particular). I'm glad that you have found some interest in my experiments with an old talkie. As for xtals, I couldn't find any suitable rx/tx "matched pair" among surplus parts, so I ended up with asking a factory here in Italy for two brand new crystals. They took the old CB rx/tx xtals and made some measurements in order to apply similar specs to the new xtals. They also replicated the same size for case and pins, so to avoid any mechanical issues with existing sockets. Quite a costly solution for sure, but it worked.

  3. That's a great idea, send off the xtals! I'll see if ICM would be willing to do that. If not, perhaps the company you used might be willing to send overseas? Would you believe that one of the xtal manufactures here told me what I want would be a minimum $100.00? So....even if it was 30 bucks the pair, it could be, in the big picture, a pretty good deal! :) Thank you for your kind words regarding the Regnerodyne. I tell folks that the Regenerodyne found me! Merry Christmas! ( I forgot how to say that in Italian. )


    1. Hi Gary,
      If I remember correctly, I spent about 80 euros for a pair of xtals (29020 and 28565), including taxes and S&H. So not that far from 100 USD...
      Anyway, I suggest you to contact the factory via e-mail and ask for an approximate quote, all-inclusive. You will find their contact information here:
      Good luck with your projects and "Buon Natale" (this is how we say Merry Christmas :))