Fixing a Kenwood TM-281 serial interface

I have a Kenwood TM-281 that I got second-hand from an unknown source, and have always programmed it by hand since it’s so simple. Recent efforts in the chirp py3 branch require a lot of re-testing of radios, so I decided to pull out my TM-281 for a quick verify. Come to find out, it was totally dead in terms of serial communications. Neither chirp nor Kenwood’s own program would talk to the radio, and a serial terminal confirmed no communication at all.

I whipped out the schematic and traced the two serial lines from the mic connector that are required to operate. The HOOK line is used as data RX for the radio, and has a zener diode (D902) between that line and ground, presumably to clamp RS-232 level voltage to the 5v it expects. I found this diode to be shorted, failing a diode direction test and registering about 4ohms to the chassis.

Since I’m using a proper 5v TTL cable, I decided to lift one side of the diode to break the short to see if that would allow communication. Luckily, it did and the radio immediately started responding to serial commands normally. Here’s a picture of the diode lifted:

Obviously the proper fix is to replace the zener with a suitable substitute, although it’s not clear to me what the right part is. The service manual shows it as “UDZW6.2(B) ZENER DIODE” which does not resolve to much for me. However, I suspect it’s just a 5V zener.

Category(s): Radio
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