Wednesday, January 9, 2013

Straight Key NOT!

Since I was thinking about Straight Key Night before, you might anticipate I would have an extensive operating report of the Homebrew 40m Transmitter and Receiver. Well, not exactly....

I did go and review the rules. There are no prohibitions against using electronic keyers. Indeed, the guidelines state "This 24-hour event is not a contest; rather it is a day dedicated to celebrating our CW heritage." I certainly consider my nearly 34-year-old Mini-MOS keyer part of my CW heritage.

So, that was my plan -- to cheat on Straight Key Night and use my keyer. 

I was busy during New Year's Eve, so I went out on the morning of New Year's Day and hooked up the keyer to the 40m Transmitter. With the spotting function, it appeared to work. The keying was a little soft, but I was able to find my transmitting frequency with ease. I tried two or three crystals, and then I noticed the oscillator wasn't stopping. If I unplugged the keyer and plugged it back in, I could send one element, and then it would stick on. Then it stopped working altogether.

At this point, I was sort of in a panic. I pulled the cover off the keyer and looked inside. What I saw next set me back a moment -- there was no battery in the keyer! I had been using the keyer solely on the stored power in the electrolytic capacitor for about five minutes. Now that is a low-power station accessory!

I scrounged up a 9 volt battery and verified that I had not fried the 30+ year old CMOS chips. However, it appeared that the CMOS gates had enough leakage that it would not reliably cut off the 6CL6 oscillator. 

The original Mini-MOS design keyed through a PNP transistor in order to work with grid-block keyed rigs -- with a negative keying voltage. That worked great with my SB-401 and later with a DX-60B and the FT-101E that I borrowed. When I bought a Kenwood TS-430S in 1985, it had a positive keying voltage. I simply removed the PNP transistor and connected the keying line directly to the CMOS 74C02. Since it worked, I kept using it.

I think the original problem was there are no free gates in the seven chips that make up the keyer. In order to drive a NPN transistor or enhancement-mode FET to key the transmitter, the keying sense has to be reversed -- instead of bringing a positive voltage to ground, we need to go from ground to a positive voltage for the base / gate of the transistor. In retrospect, it would have been easier to have left the PNP transistor in place, use it to drive an NPN transistor or  FET.

A quick look in the junk box revealed some 2N7000s -- an excellent enhancement-mode FET to key the transmitter. (The K1EL keyers use this same device) I just need to dig up a PNP and some resistors. Ah, a project for another day.

Oh, and what about Straight Key Night? Well, since I had just gotten the K3 going, I decided to play with it instead of SKN. I ended up working DX spots on 12 and 17m, including working VP2MRV on RTTY using the K3's FSK D CW to RTTY mode. Cool.

SKN? Maybe next year....

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