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I got my hands on pair of MOCOMA III radios:

enter image description here

While inspecting them, I noticed that channels are configured by inserting some components into designated slots:

Internals of the radio:

internals of the radio

Component closeup:

component closeup

As I want to make it work on multiple channels and ideally to be able to connect to other CB radios, I have some questions:

  1. What are those components? Are they oscillators?
  2. Why are they different, even if they control the same channel?
  3. What frequency will I transmit, if I use this radio now?
  4. Are those components avaliable now? What can I use to replace them?
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    \$\begingroup\$ Those are crystals, with different frequencies. \$\endgroup\$
    – ocrdu
    Mar 16 at 8:33
  • \$\begingroup\$ Unrelated to your question, the production quality of that device is horrible and brittle, it looks like it was hand-made in some hobbyist's garage. Meaning it might break at any point and also catch major EMC problems because of the messy wiring. This isn't a professional product by today's standards, but I guess it's from an era before professional electronics assembly was invented. \$\endgroup\$
    – Lundin
    Mar 16 at 11:36
  • \$\begingroup\$ @Lundin yes, those are quite old, from a pre-internet era, I do not intend to use them for anything critical though. \$\endgroup\$ Mar 16 at 12:24
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    \$\begingroup\$ @Lundin "This isn't a professional product by today's standards" True but it's what's expected for commercial construction of the era for CB radios. The use of a 455 kHz (first and only) IF I find eye-raising but that's also part of building down to a price. \$\endgroup\$
    – Graham Nye
    Mar 16 at 13:27
  • \$\begingroup\$ @Lundin Yet this was the standard production of those CB radio devices which flooded the market. And could be quite good and durable, tho the bakelite PCB are not durable as modern FR4... Industry have to smart somewhere you know ! \$\endgroup\$ Mar 16 at 13:51

2 Answers 2

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It's a 3-channel, crystal-controlled, 2-way CB radio and those components are quartz crystals.

The quartz crystal transmit / receive pair, seen in the photograph, pertains to channel frequency 27.205 MHz.

T 13.6025 is the transmit crystal for this channel.

With this channel selected, the crystal oscillator stage would output 13.6025 Mhz for doubling to 27.205 MHz and amplification in subsequent stages.

R 26.750 is the receive crystal for the channel. The received signal at 27.205 MHz would be mixed with 26.750 MHz to obtain an intermediate frequency of 455 kHz for detection and audio amplification (27.205 - 26.750 = 0.455).

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  • \$\begingroup\$ So, I would have to use a transmit crystal with half of desired frequency, and receive crystal 455 kHz less than desired frequency, right? Would I have to use receive crystal 10.7 MHz less, if I want to use CB FM frequences (radio does have an AM/FM switch)? \$\endgroup\$ Mar 16 at 12:30
  • \$\begingroup\$ @Lundin - Hi Lundin, Many thanks for pointing out the typo. It's been corrected. \$\endgroup\$
    – vu2nan
    Mar 16 at 12:42
  • \$\begingroup\$ Hi Revolver_Ocelot, You're right about crystal frequency selection . The intermediate frequency would be 455 kHz for FM also. \$\endgroup\$
    – vu2nan
    Mar 16 at 13:22
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What are those components? Are they oscilators?

They are crystals.

Why are they different, even if they control the same channel?

The choice of crystal will correspond to a certain carrier frequency. Which one depends on the radio design and modulation - some radios send and receive on the same frequency (semi-duplex) others use different frequencies for tx and rx.

We'd need to see a schematic to tell what those crystals do, it's not something we can tell by looking at pictures of PCBs. The easiest way to tell would however be to read the friendly manual.

What frequency will I transmit, if I use this radio now?

There is no way to tell with the info you have given.

Are those components avaliable now? What can I use to replace them?

Sure, you need to know their frequency and rated capacitance though. As for the mechanical package anything called HC49 through-hole should work. The size of the crystal package is unimportant, only the pin-to-pin measurement matters.

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