Luis is absolutely right. I am pretty sure that the design of the DC503 was before 4000 series of CMOS was invented.
I did find a referenceto the MC4035P. It was described as a quad 'D' type latch with open collector outputs.
I looked at the schematics of the later DC50xA counters and they do not use these chips.
I think the best source is another DC503. They are selling pretty cheap on eBay.
Regards,
John
Correct,
The MC4035 is NOT a CD4035
(the MC14035 is a CD4035).
luis c.
ct1dmk
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