Well, I finally got around to hacking the red ring to support 24 LEDs, and the hour hand on my Bulbdial now displays half-hour increments. I think it helps in reading the clock, especially as it approaches the hour.

I used pins 6, 7, 8, and 9 to drive the extra LEDs. For fun, I updated the schematic. I hacked the board layout for the extra LEDs, and then printed it on a transparency to use as a drilling guide. I found some old copper stick-on pads in my mother's basement, and used those for soldering the LEDs and wires.

The pads are kind of clunky, and dodging existing traces is a little difficult near the board jumpers, so it's fairly ugly-looking (ok, ok - *very* ugly-looking), but, apart from a couple of bad solder joints, it came right up.

Modding the SW was quite straightforward. Mostly just changing 11's and 12's to 23's and 24's, and repopulating the hours lookup table. You have to be careful when crossing :00 and :30 in time-set and time-display modes.

I had hoped to do this almost completely non-invasively. I found some 4-conductor ribbon-cable and some 4-pin Berg-style sockets. My thought was to squeeze the sockets into place between the green and red rings, and then simply plug the LEDs directly into the pin holes. That would have worked great, but the Bergs were just too big. So I had to resort to doing violence on the circuit board.

Next, I'm thinking about working up a new layout and having a few boards made for some friends.

Nice, I think it should also be doable by just using 7,8,9 and 10. Line 6 has only 2 times three combinations (with 7,8,9) to be used.
This might be also more local given the positions of the 1-10 lines on the red ring.
Did you able to do your hack with a single-sided board ?

The problem with using line 10 is the added resistance you need on lines 7, 8, & 9. For the six combinations of those with line 10, you don't want to add any resistance to them. But you do want to increase their resistance for the six combinations of 7, 8, & 9.

I decided that it would be cleaner to forget about using 10, and use 6 instead. I added a 22 ohm resistor to each line as I brought them up from the green ring. For 7, 8, & 9, I simply clipped out those sections of existing zero-ohm jumper, and replaced them with 22 ohm resistors. Since there are already (10,6) and (6,10) LEDs on the red ring, I left in the line 6 zero-ohm jumper, and then split a separate line 6 circuit with the 22ohm resistor on the red ring itself.

I drilled out my existing red ring for the extra LEDs and soldered in point-to-point wiring. Looks *really* ugly, but it works, and you can't see the mess from the outside of the clock.

I wanted to use 4-conductor ribbon cable with 4-pin sockets. I was just going to insert the LEDs into the sockets. I prototyped it and it worked great. It would have been a much cleaner mod, but I couldn't squeeze in the sockets where they needed to go.