In rec.radio.amateur.homebrew Dick wrote:
[previous paragraph deleted]

Particularly interested in knowing where HP mounted the ink
reservoir, was it on the moving carriage that carried the pen or is it
on the fixed body of the recorder ? Maybe someone has had experience of
using this equipment and can describe the arrangement they saw when
refilling the ink reservoir. Or did HP use disposable ink cartridges ?

There were several variants of this chart recorder, 680,681,682,683. The
one I remember used Esterbrook pen cartridges, HP part numbers 1530-0095
(blue), 1530-0703 (red), 1530-0704 (green), 1530-0984 (purple), and
1530-0905 (black). The cartridge was mounted on the inside back wall of
cabinet, and attached to the pen stylus with a capillary tube about 8" long.
You might still be able to obtain the cartridges from an office supply store.
Manuals are available at http://www.manualsplus.com

I particularly recall one incident where a technician was trying to clear
a clogged capillary tube. He disconnected the tube at the stylus end and
fitted a 10CC hypodermic syringe to the loose end. He then shoved the
plunger with great gusto and the next sound I heard was a loud "POP!!!".
The cartridge exploded, covering his face and shirt with red ink dots.

Don K6LTS
email address at the QRZ website.