[GreenKeys] HAL ST-8000A Story

[William Wuttke]

I recently took a risk and purchased a rather shabby looking ST-8000A “for parts only” at a price I couldn’t refuse ($65 + shipping) on our favorite auction site (there was a lot of corrosion on the case, but the interior looked OK). I checked it out when I received it and found the demodulator to be working, but the modulator appeared to be stuck at 2048 Hz (failed bit test 6). Thanks to Jeff Angus and the helpful people at Barrett Communications USA, I got a number of copies of the Operator’s Manual, but schematics are apparently unobtainable. However, since the ST-8000A is a militarized/modified version of the ST-8000 (I have schematics for the ST-8000), I thought that ST-8000 schematics might at least point me in the right direction. I was really surprised to find that the circuitry was more similar than I had hoped. After further troubleshooting, I found that the modulator would actually have an output within a limited frequency range, but not what was dialed in. The ST-8000A modulator clock circuit is just a slightly modified version of the ST-8000 circuit. It didn’t take too much longer to find a bit stuck low, causing the 12-bit adder circuit (modulator clock synthesizer) to come up with the wrong sum. I received a replacement 74HCT374 today from Digi-Key and installed it. The modem is now back in operational condition! (I also used a lot of scouring powder and some sandpaper to remove a lot of white corrosion from the bottom lid and one side - it cleaned up nicely.)

73,
Bill
KE3BK