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Squelch replacement

A project log for Hand Held Control Head CB

Combining an old Motorola radio Hand Held Control Head with a Radio Shack CB radio

qbfreakQBFreak 06/08/2016 at 17:010 Comments

After a bit of troubleshooting the squelch pot has been replaced with a pair of MCP41050 50K ohm digital pots in series. After successfully testing the pots on with the Arduino and a multimeter on the breadboard, tried connecting them to the radio. Initially I there was no obvious change. The speaker would pop a little when I adjusted it each step but I couldn't unsquelch the radio. I had written a quick and dirty test program that let the control head up/down buttons cycle from 1-99 on the display and write the corresponding value to the pot. Taking a step back and trying to figure out what I was doing wrong, the answer came to me while I was chatting over on #Hacker Channel, I was assuming that minimum resistance was unsquelch, but what if maximum resistance (100K ohm) was unsquelch? A quick rearranging of wires on my breadboard confirmed this was in fact the case. Squelch is working properly!

Unfortunately I discovered something about the volume while I was playing with squelch and injecting a signal with the service monitor. Apparently the volume knob isn't really doing much. It seems to have a small effect in the middle of it's travel, but turning the volume down all the way does not silence the audio, it is still quite loud. Reversing the leads on the volume connection had no effect, it wasn't a case of confusing minimum and maximum as I initially thought. I'm going to have to dig a lot deeper into the volume control.

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