The Internal clock.
You can see my full build thread HERE
There are two awesome TTSH threads in Muffs.
2600 clone - Two Thousand Six Hundred (TTSH)
Two Thousand Six Hundred (TTSH) Project General Build Thread
The DSL-Man site has a very good TTSH thread
And of course the official build thread is here
The clock section in the ARP 2600 is integral to the Sample & Hold module.
In this section there is a sample & hold (on the right) and electronic switch (on the left).
I'm building the clock circuit first.
Here is a little on how it works.
THE internal clock is basically a LFO (square wave).
It's pulse width is determined by the variable resistor R385 (which is a linear 100K trim)..
The circuit uses the combination of Q45 (2N4870) & C115 (1µ5 x 1 Electrolytic cap) to make a relaxation oscillator. This produces a sawtooth wave which is then converted to a square by
A42 (a LM 301 comparator). It's output is pin 6 -- leads to the diode (CR47).
A relaxation oscillator is a non-linear circuit consisting of a feedback loop containing a switching device - in this case a transistor (Q45) that repetitively charges a capacitor (C115) through a resistance until it reaches a threshold level, then discharges it again. Relaxation oscillators are commonly used in car blinker lights to indicate which way you wish to turn.
This closeup shows the relaxation oscillator (including the 1.5uF cap & the 2n4870 trannie). At the base of the 47k resistor (R384 in the 2600 schematic) is your saw wave test point.
The square wave test point in the centre of pic.
It's at the base of the diode.