About a month ago, I found out that our club had a 60s-vintage Heathkit HD-10 Keyer that someone had donated. The only problem, according to our technical director, is that it didn’t work. He did, however, have the manual. Well, especially with the manual, I didn’t think it would be much of a problem to get it up and running, so I told him to bring it to a club meeting, and I’d take a look at it.
The first problem I encountered is that while it did indeed come with a manual, it wasn’t the right manual. The manual was for an HD-1410, not the HD-10. It looks like the HD-1410 is the HD-10’s younger brother. For one thing the HD-1410 has built-in iambic paddles, not the single lever of the HD-10.
So, my first task was to find a manual for the HD-10. This turned out to be a lot easier than expected. The manual is available from the Boat Anchor Manual Archive (BAMA). Actually since traffic on that server is limited, I downloaded it from the mirror site. I didn’t get the whole manual–more about that later–but what I did get included the theory of operation and a schematic.
The schematic of the HD-10 is, of course, much different than the schematic of the HD-1410. While the more modern keyer uses integrated circuits, the HD-10 circuit consists of 12 transistors, with the accompanying resistors and capacitors. In a way, that makes the HD-10 much more interesting to work on. I graduated from college in 1978, and by that time, very little time spent teaching how to build logic elements from transistors. Mostly, we concentrated on how to apply ICs, unless you took semiconductor design courses.
The HD-10 consists of a multivibrator to generate dots, a flip-flop used to generate either dots or dashes, and a driver to key the transmitter. There’s also a second multivibrator to generate the sidetone. A pot controls the frequency of the dot-generator multivibrator, which then control the character rate of the keyer.
One nice thing about this design is that the power supply voltage is not that critical, and Heathkit was therefore able to include a simple AC supply. It consists of a small transformer, a couple of diodes a resistor, and three electrolytics. It supplies voltages of +19 V and -16 V. The weight of these components helps anchor it when using the built-in single lever paddle.
I skimmed over the theory of operation and examined the schematic for 10 minutes or so, then dug into the keyer. I was never impressed with Heathkit’s packaging design, and my experience with the HD-10 did not change my mind at all. I removed the screws that held the front panel to the unit, but because of the way the cabling ran, I ended up scratching the case. The cables were not long enough to allow me to move it away far enough.
That exposed the printed circuit board, but the board was mounted upside down and there were no markings on the bottom of the board to identify the components. That meant I had to also remove the PC board, which was mounted on four long spacers. Once I removed the screws, I once again had to carefully arrange the front panel, PC board assembly, and base unit so that I could probe the circuit without shorting anything out.
Once I’d done all that, I started poking around. The well-written theory of operation made this a lot easier. It shows waveforms at various points and probing around, it looked to me like everything was working correctly. Why then, didn’t I get a sidetone?
It turns out that the HD-10 has a terminal strip on the back that lets you connect your receiver’s audio to the keyer, and then listen to it either with the keyer’s speaker or with headphones plugged into the keyer’s headphone jack. Why would you want to do this you ask? Well, back in the 60s–when this keyer was being sold–one normally used separate receivers and transmitters, and the transmitters usually did not provide a sidetone. This feature of the HD-10 allowed you to both listened to received signals and monitor the code your were sending.
The kicker is that if you were not going to use this feature, you had to ground the RCVR AUDIO terminal to ground. Once I did this, the sidetone was loud and clear. The end result was that it took me about an hour to disassemble the keyer and play around with it only to find out that I hadn’t configured the thing properly. Something that I could have possibly avoided had the manual on BAMA include operating instructions.
Reassembling the keyer took another half hour, mainly because I didn’t get it right the first time. The first time I tried it, I forgot to add nuts between the spacers and the PC board. Without the nuts, the board was not high enough, and something was shorting a connection on the PC board. I had to remove the board, add then nuts and then screw it down again. Finally, however, I got everything back together, and it seems to be working just fine.