MOS MCS2529 math chip

Brent Hilpert bhilpert at
Fri Jul 20 13:40:22 CDT 2018

On 2018-Jul-20, at 10:18 AM, Jules Richardson via cctalk wrote:
> Anyone got pinout/spec information for a MOS MCS2529? In particular, I'm curious about operating voltage. I acquired a Melcor SC-635 calculator yesterday and there seems to be some uncertainty about the output voltage of its (rechargeable) battery pack; some places say 2.4V, i.e. the pack is a pair of 1.2V cells, but others say 9V.
> 2.4V seems a little low to me for typical logic, but on the other hand I've seen a period ad which says that the external PSU was 9V - and so the rechargeable battery must have been somewhat less than that.

Rechargeable and 1.2V/cell would correlate to 2 NiCd cells, a not-unusual configuration for calculators of that period.

Such units would typically use a simple built-in switching power supply to boost the battery voltage up to levels adequate for the logic and/or display.
In the pic of the PCB board here:
the chunky box component 'below' the IC is probably a switching PS module.

It was also common to use simple resistive current limiting in the charge circuit for NiCds.
In consequence, the voltage supplied by the external AC charger may be quite a bit higher than the battery voltage.
It's possible that's where the 9V external spec comes from, if not just a mistake.
Sometimes the current limiting R is in the external charger, sometimes it's in the calculator.

Further, such designs also tended to rely on the battery to provide AC filtering & voltage regulation (limiting) of the charger V down to the battery V.
If the battery/cells have been removed or are in really bad condition, operating the calc from the original external charger
can result in too high a voltage being applied to the electronics.

My usual procedure for such calcs is to cut out the NiCd cells (there is ~0 probability they are any good), noting the polarity.
For testing, clip on a bench supply to substitute for the batteries, set of course to the appropriate V for the battery ( # of cells * V/cell ).

If the batteries have already been removed and there are no polarity markings, let me know if you'd like some assistance trying to figure it out.

If you want to power it through the charger jack, then you need to assess whether there is any internal charging circuitry (rectification, aforementioned current-limiting R, etc.) sitting between the jack and the cells.

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