I'm trying to restore and learn from a simple metronome assembled by my uncle in the 60's. It has a nice sounding beat and used to work until very recently — although unstable. I've made a schematic from tracing the board tracks which is included below without the power supply for simplicity.
Being a beginner myself, I cannot understand the disposition of Q1 and Q2. It looks like the SCR explanations I've seen around, but I'm not really sure if they are there to work as an SCR.
I assume that the rightmost part with Q3 and Q4 is just there to amplify a pulse generated by the leftmost part of the circuit, specifically coming from between R4 and C2. But how does this oscillator work at all?
The transistors are from RCA, with SK3020 being a silicon NPN and SK3005 actually a germanium PNP. I'm not sure if there are any particular characteristics of them that enable this functionality, and I could not find much about their differences to modern silicon transistors.
Any pointers or topics to read about in the direction of understanding how this circuit is creating pulses is greatly appreciated.
- Added the voltages I'm reading now throughout the circuit.
- The circuit was recognized by Spehro Pefhany as part of the RCA Hobby Circuits Manual, 1968, the original circuit is included in his answer.