Clock signal generator using 7400
What you see here is a very straight-forward oscillator:

your NAND gates are just wired up to be inverters.

If you just take one inverter and connect its input to its output, it should theoretically start to oscillate infinitely fast. 

If you think about it, an inverter is just an amplifier with an exact phase shift of 180° and a high amplification.

In practice, its speed is limited, so that won't work reliable.

So, what you do instead is have two inverters in series, giving you a phase shift of 360° \$\pm\$something; since you can't tell 360° phase from zero phase – that's basically stable (if nothing excites them).

Then, you add phase shift – ideally, something that has a phase shift of 180° at but a single frequency. 

In this case, two high pass filters were built. High-pass filters have a frequency-dependent phase shift.

Your high-pass filters are RC filters:


C1ıR13
C2ı(R14+P)


due to P being adjustable, you can tune the frequency at which the oscillation stabilizes.

Lord knows the actual phase of these inverters – 1970's logic wasn't really fast or precise in analog terms. So, a theoretical analysis of this circuit will not yield useful data.

Since, with good chance, you don't have access to the same batch of suboptimal logic ICs that were used in the original design, but probably to logic ICs that are far better and faster, as soon as you don't sufficiently dampen that C2 filter, things stop to work.

Long story short, this only works with bad hardware of the past. Don't stick to the original schema; build your own oscillator. It's way easy these days (or really, since the invention of the NE555, which was exactly meant to solve this kind of problem. You are literally the first, and very likely the last person I meet for whom using a 555 would be a technical upgrade).