This clock is based on the Single Z570M design, but uses an adapter board to fit the larger IN18.
The PCB is only 51mm x 51mm (2" x 2"). Further details.
With only one tube, the time is displayed in a sequence of 4 numbers. eg, 10:25 is displayed as 1-0-2-5.
There is a small pause between each 3 second display cycle, so you can recognize the start of the cycle.
Click on a picture to open it in a Popup.
Single Z570M and IN18 side-by-side:
The IN18 likes a bit more current than the ZM570M, so R1 on the clock board must be changed.
For the Z570M R1 is 22K, but for the IN18 a 10K resistor provides the required current.