Unusual HP21 Modfication?

[PANAMATIK]

(03-30-2016 07:06 PM)BobVA Wrote:  I got to digging through my Woodstock scrap bin this weekend and, with some good advice from Bernhard, I brought two of the three of them back to life. In the process of doing so, I found an HP-21 with a diode added to the backside of the display driver section of the circuit board (photo attached) Anyone seen this before? And know what it's supposed to do?

There was also a paper sticker inside the back cover stamped "Jan 28 1976" with the pencil notation "K.B. + Logic" jotted on it, which seems to indicate it had been in for a repair at some point.

This component connects pin 5 of the anode LED driver to Vss battery minus. Pin 5 is 4 Volt supply for the driver chip. If it is a diode it could protect from overvoltage, or if zener it could limit the voltage. If it is a condensator it could stabilize the voltage. Perhaps the 4Volt from the power supply was noisy. I have never seen this before.

What happens if you try to remove it ?

Bernhard