I have found a circuit diagram for the HP-21. I don't know if it is accurate but it is worth a look if you are interested.
http://www.cs.ubc.ca/~hilpert/eec/
Click on the calculators link at the top of the page, then click the HP21 link and then click TD23 Data: view
That should get you to hp21.gif
HP-21 Circuit Diagram Schematic
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