The display part of the board is very simple. There is one input to a set of daisy-chained decade counters which feed a set of latches. The counters merely count pulses until the latches are triggered, at which time the output of the latches mirror the inputs, and the count is displayed. Not an efficient way to display arbitrary numbers, but it makes sense for a frequency counter (since all you have to do is trigger the latches once per millisecond and you have an automatic KHz measurement, for example).
Decimal points seem to be hardwired to a rotary switch (no logic involved) which grounds one, two, or none of the decimal point cathodes. I will probably need to change this to use a transistor driver so that I can connect it to some low-voltage logic.
I will probably leave the latches in place, remove the counters, and feed my inputs into the latches. I'll feed the latch enable input as well -- my current design concept does not require it (because I'll be feeding serial-in, parallel-out latches anyway), but it'll be more flexible in the future to have that available.
These are closeups of the "count" test mode. The rate of the counting is
about 20Hz. My shutter speed is slower than that -- so you can see
multiple lit-up digits.
If you look closely you can see that some repairs have been done.
This shows the nice compact layout. The discrete components at
this edge of the board seem to be all power-supply related.
Close-up of the display ICs. From left to right, the SN7490 is
a decade counter, feeding the SN7475N latch, feeding the
SN7441 decoder/driver. This is the least-significant digit.
This part of the circuit is the counter itself.
This shows how nicely the board is labeled with available voltages
and test points. The empty spaces are for the optional additional
digits.