Started 23rd September 2021
I bought "UK MSF Time Atomic Radio Controlled Silent Wall Clock Quartz Movement Mechanism DIY Kit Replacement Set Accessories" which is a clock movement that takes its time from the MSF transmitter in the UK.
This worked well out of the box. It came with a pin in place locking the mechanism at 12 o'clock - any radio controlled clock must know where its fingers are.
Taking the front off revealed the insides, three blob chips, a couple of crystals, a ferrite rod antenna, and an interesting connector strip.
The board is marked "HD-1688MRCSD" which exists on the Google. I found a blog about it, the maker is "Sheng Bang", but no data sheet was forthcoming.
The edge connector is marked
REC REST SET VDD GND LG LR BA BR MU CDS
I can guess that the first three relate to the three buttons on the back of the clock, the connections to which are visible at the top of the photo, and that VDD and GND are power rails. First look with a 'scope shows both CDS connections providing the same output signal. Given three pulses the time between the first and the third is 1 second, but the second pulse is not in the middle. The signal is not present when the clock is receiving the radio data.
The pulses have three different durations - short (10668 μs), medium (10912 μs) and long (11157 μs). Each minute consists of a long pulse followed by seven short ones (4 seconds), then seven sequences of a medium pulse followed by fifteen short ones (seven times 8 seconds).
There are long and short gaps between the pulses. The long and medium pulses are always followed by a long gap.