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Old 8th Nov 2020, 9:35 am   #86
Join Date: Apr 2018
Location: Newbury, Berkshire, UK.
Posts: 1,066
Default Re: I found it! A very sorry looking MK14.

According to the data sheet the 7442 has normal totem-pole outputs which have a short-circuit current of 18-55mA as apposed to the 7445's open collector outputs with 80mA sink capacity. This would imply that the digits will be limited to this level, which would cause the digits to vary in brightness depending on how many segments are lit. The 2 chips are pin compatible, but the design really needs the open collectot outputs of the 7445. That said, it should work to some degree and so the symptoms you are experiencing are probably something else. Do the chip legs just need a clean?
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