[SiriusHardware]

Did you get more than one IC?

There is a small chance that the new IC's QD output has been damaged by some as yet undiscovered problem on the CLK1 line. If you have a spare, try fitting it but with pin 7 sidelined right from the start so that the output can't be damaged, and see if that IC has 1MHz on its pin 7. If you don't have a spare we'll proceed for now on the assumption that your replacement IC is still OK and the cause is elsewhere.

Where, though? At the moment I don't know, I will have to browse the diagrams for a while. Any thoughts, anyone? When last checked the 'load' input (pin 11) on UG5 appeared to have a steady high (5V) on it, high being the 'inactive' state for the load input. If it was taken low momentarily, it would reset the counter to zero.

Colin measured for frequency on that pin as well, so if the counter is being periodically reset that should have showed up as a frequency with a high duty cycle,

One thing you can do, Colin, is to remove UG5 and (just working on the chip, and with the black lead on pin 8) measure the resistances from pins 2, 3, 6 and 7 to pin 8. Then with the red lead on pin 16, measure the resistances from pins 2, 3, 6 and 7 to pin 16.

As all of these pins of the device have the same function, I would expect the resistances from those pins to pin 8 to all be about the same value as each other. The resistance from the pins to pin 16 will be a different value but again, it should be about the same for all four pins. If one pin looks significantly different to the other three, that would suggest that pin was faulty. By the way, be careful not to measure yourself when taking these readings. If you have an experimenter's 'breadboard' (a white slab with holes to plug components into) plug the IC into that to stop it from skating around while you are trying to take measurements.

There is a way we can find out whether the chip is or is not counting all the way up to 15 (1111) but let's see what you get from the above first.