This’ll be a quick one. The 111 is a pulse generator with ~ 500pS risetime. it has a pre-trigger output, which is adjustable from ~30nS to ~250nS. This makes it very easy to use an external trigger, and get the pulse centered on the screen.

Mine had 2 problems:
1) the pre-trigger delay wasn’t adjustable
2) the rate wasn’t adjustable, being fixed at ~200kHz or so.

Always check the rails first. The -15V line was down around -9V. I did a quick test of the Zener diode, which had to be done with a power supply, out of circuit.

The multivibrator portion of the circuit was adjusting, but the avalanche section that generates the sharp pulse* pre-trigger generator was oscillating in free-run, not taking it’s cue from that multivibrator.

Next I decided to check Q40, which turned out to be bad, showing up on my chinesium tester as 2 diodes.

I decided to just jam some basic bitch PNP transistor in there to see if it would work. To my surprise, it did!

* Edit: Q40 is not running in avalanche, that’s Q84. It would have been very lucky if some random transistor was capable of generating a sharp, predictable avalanche pulse.

Now I’ve got adjustable pre-triggering at 500pS, which has been useful for testing out some of my sampling plugins.