Recently [Ken Shirriff] got handed a mystery IC by [CuriousMarc] who had found said IC in a box of various old chips. Labelled only with a date code of the 47th week of 1985 and a mysterious F1-10-5 alongside a logo that didn’t lead to any hits in the typical IC databases, it was up to [Ken] to figure out what lurks inside this CDIP chip.
As it turns out, it’s actually a pretty interesting IC with an easy to identify die once the ceramic package had been cracked open with a surgical hammer-and-chisel strike.
With the die under a microscope it was identified as a Harris HF10, itself a variant of the National Semiconductor MF10. This is a dual switched capacitor filter, with the modern version being e.g. the TI MF10-N in both PDIP and SOIC, but sadly no CDIP any more. Availability seems scarce, so the MF10 may not be long for this world.
These filter ICs can tune their capacitors to act as a low-, band- or high-pass filter. How this works is explained by [Ken], with switched capacitors replacing the resistors of a typical filter circuit. The main reasons for doing this over just using a resistor include how large resistors can get compared to the relatively small capacitor, not to mention their fixed value versus the adjustable resistance from the switchable capacitor.
The remainder of the circuit is mostly there to enable the switching and overall operation, providing a fascinating glimpse at a chip that would have been incredibly useful in the 1980s, including in synthesizers where this adjustable filter feature was most convenient.
When you see a listing for an Ethernet switch whose specification list is in effect ‘yes’, with a four-digit price tag when new, and with the seller asking for less than 10% of said $3,000 asking price in an ‘untested’ condition, the only rational thing to do is of course to mash that ‘buy’ button. This is what [This Does Not Compute] 
