This is the only logic board in the HP-21. There are 3 integrated circuit chips. The largest one is the calculator ic chip. The two smaller ones are the LED display decoder and driver.
The charge circuit consist of a diode and the large resistor. The is a half wave rectifier that takes the 9.7V AC from the ac adapter. Very basic, but does the job of charging the NiCd battery of about 2.4V.
The other circuit is for voltage conversion to power the IC chips.
Note the protruding power pins for the battery, these design in important in keep the battery pack in place without the need of any screws or latch.
The printed circuit foil side seems to have be coated with a protective lacquer, but the component side do not have a protective lacquer. The print circuit wiring appears to be silver coated.
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