I built a prototype a few weeks ago, but couldn't get it to work.

There were two things wrong:

1. An incorrect wire
2. An incorrect IC.

I managed to short circuit the indicator LEDs when I put them in.  Had a simple brain fart, and wired two pins together that shouldn't be connected.

For the incorrect IC, I used a 74LS73 instead of the original 7473 - no LS or other designators in the middle.  I happened to have a single 74LS73 in a junk box in the attic.  When it didn't work, I ordered some real 7473s from eBay - I guess I got lucky, because they do work.

Here's the prototype showing a good diode connected the right way around:

Only the green LED is lit.

Here it is with the diode under test connected backwards:

Only the red LED is lit.

Here's a short circuit:

Both LEDs are lit.

The prototype works as it should.

With the 74LS73, the oscillator (the LM3909) mostly wouldn't start.  If I touched anything connected directly to the 3909, it would fire up.  That's (sort of) workable if you are just playing around, but definitely not something you want in a working tool.