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Mostly rechargeable batteries don’t survive decades.  The batteries in this PC-1500 were no exception.  Fortunately they were not leaking, but the capacity was zero. The photo shows the original battery containing 5 AA NiCad cells. I replaced them with NiMH cells with much higher capacity.  As a consequence the charging time is a lot longer.  

The yellow arrow in the left photo is pointing at a small plastic gear.  Plastic components shrunk around metal often show crack.  In this case the gear was not able anymore to rotate the connected larger gear.  Luckily I found a brass gear on the website of a German hobby shop. Given its measures and number of teeth it seemed almost identical to the existing plastic gear.  After widening the hole a bit I attached it to the metal shaft with a 2-component adhesive.  (See centre photo)

Also the pens needed replacement.  They are still available.

As you can see in the demo video it works well.