ICOM IC-229 squech circuit modification

by Karel Julis, OK1UHU, ok1uhu@qsl.net

This paper describes the modification of squelch circuitry of ICOM'S IC-229. This radio is very sensitive, has a good squelch performance, but, unfortunately, the squelch is missing a hysteresis. In a practical use, it leads to "stuttering" of weak signals, especially during mobile operation.

The described modification brings a hysteresis of about 3dB of input signal to the squelch. Thus, while operated nearby threshold level, the squelch does not stutter any more. Next, it brings a little delay to the closure of the squelch, which can bridge the short outages of mobile-effected signal. The overall sensitivity of the radio is not affected.

All the additional circuitry can be made on a small piece of thin PCB material (0.5mm in my case) using SOT23 transistor and 0806 SMD resistors. Only a couple of square millimeters is required. Next, solder some thin stranded wires to the additional board and cover it by thermal shrinking tube. Such a additional module can be glued, using warm glue, to the copper side of the main PCB of the radio.

The green wire only has to be pulled through some hole to the component side and taped to the IF module by a small tear of solder - thanks god, the required pin is located at the upper edge of the module, thus, the taping is very easy. All other wires can be connected to the appropriate pins of the IF module from a copper side of radio's main PCB without any problem.

Enjoy the much more comfortable squelch!