|Homebrew Transmitter and Receiver, ready for another|
go at Straight Key Night in 2014
Despite having an entire year to figure it out, I left fixing the keyer to the last minute. Back in the summer, I had modified the keyer to use a 2N3906 to key the gate of a BS170. However, when I hooked it up to the rig, it wouldn't key -- oscillator problems again. It went back to the project shelf to be figured out later.
At some point, I took the keyer back to Gwinnett county to try to troubleshoot it there. But as fate would have it, I didn't have time to figure it out. Right before Christmas, I brought it back to Floyd county.
A bit of work with the mini-scope proved that the oscillator wasn't firing at all. It took a while until I found the source of the problem: one of the wires to the speed potentiometer had broken off, so the RC circuit was broken. Without it, there's no phase delay, and therefore, no oscillation.
|Built in 1979, Mini-MOS keyer provides yeoman service again.|
Pulled a 2N7000 out of the junk box. The 2N7000 and BS170 are pretty much the same part, although the pin-outs are different. Hook it up and it seems to be working great. I can see the keying on the gate on the 'scope, and the transmitter is happy. Unplug the scope probe, put the lid on, and ... it's locking up again.
Hmm. Take the lid off, put the probe on, and it works again. On a hunch, I pulled a 100k resistor out of the junk box and put it across the gate to ground. Working -- even with the probe removed. Ah ha! That's the problem. The gate of the 2N7000 (or BS170) wasn't being pulled back down to ground, so the floating leakage from the 2N3906 was just barely enough to keep it turned on. Adding the scope probe brought in just enough resistance (a few megaohms), to overcome that leakage.
All ready to go. After dining out with a church group on New Year's Eve, I managed to work seven stations all before the ball fell. Of course, I told them that I was cheating, using a 35+ year old keyer instead of a straight key. No one seemed to mind. I did find that my 7061.1 kHz crystal stopped working. Worked one more person on New Year's Day for a total of eight.
A great way to bring in the new year. I guess I'll submit mine as a check log....