Unusual HP21 Modfication?

[BobVA]

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.

[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

[BobVA]

(03-30-2016 08:18 PM)PANAMATIK Wrote:  What happens if you try to remove it ?

Since it's my only working 21, I'm hesitant to find out :-)

There appeared to be some rework on the other side of the board as well - the resistors next the the anode driver chip were different values than my other one.

I think this is an early 21 board - it has a fantastic looking ceramic and gold ACT chip. All my other Woodstocks have plastic ACT packages. Perhaps the diode was an engineering change for some imagined problem they later decided wasn't an issue?

[PANAMATIK]

(03-30-2016 09:05 PM)BobVA Wrote:  I think this is an early 21 board - it has a fantastic looking ceramic and gold ACT chip. All my other Woodstocks have plastic ACT packages. Perhaps the diode was an engineering change for some imagined problem they later decided wasn't an issue?

I also like the fantastic looking gold ACT chips, I have found one in my HP-27, but never saw one in an HP-21. Congratulations!

I didn't seriously want you to remove the modifications.

However I had to remove the ACT from my HP-27 for research, but it is still alive.

Bernhard