Today, we begin the construction of the MST-13 timers as if we were Ulrich Lumpert who just learned from his boss that he had to build a few of those in a hurry.
Obviously, we need a circuit diagram. Here is the diagram that Lumpert produced on 7 August 1985.
Now, to build a timer, you need to produce a pulse at a given frequency (Let us say 1 Hz i.e. one pulse per second) and then you just count 60 of them to make a minute and repeat this to produce an hour.
The last two steps are rather easy and of course exact operations (assuming you do it right). But building a 1Hz pulse is not that simple, at least if you intend to build a very accurate timer. We will now focus on this specific part of the diagram.
What we see here is the “heart of the timer”. There is a quartz that beats at 4.194304 MHz (2^22 Hz) and which is connected to a 24-stage frequency divider and oscillator (HEF 4521BT). The output “14” of the “4521” divides the signal by 2^22 and thus produces a 1 Hz pulse. Pretty simple, isn’t it?
You can see the quartz and the “4521” on these pictures of the Togo timer.
Let us jump a bit in time and look at the design of K1 as sketched by the CIA (John Orkin*) in his ANALYSIS OF DIGITAL TIMER 88SP006 Report.
Let us now compare the original design made by Lumpert the and CIA diagram of K1 (The Togo timer). You will notice a small but significant difference. While the original diagram had two fixed values capacitor (33 and 27 pF), the real timer has one fixed value capacitor and a variable capacitor. Why?
Well, life is never really easy, specially if you have to build something like this in a hurry. I suspect that Lumpert realized that the timer was badly inaccurate. 
So, he decided to “pull” the frequency by tuning the “load capacitance” applied to the quartz. But of course, the Thuring boards he had ordered had NOT been designed for this…
To be continued.
1. Actually, this information can be found in a BKA report. But the translation in English is so bad that I could not make any sense of it until I decided to build such a timer myself.