HP71B plastic vs. metal back

[dbrothers]

I love my HP71B. It’s my go-to calc for everyday use. Noticing that under certain circumstances, part of the display was fading, I decided I needed a backup to fall back on, just in case. I managed to buy one (in somewhat rough shape) on EBay.

When it arrived, I noticed that my original has a plastic back, and the new one has a metal back, apparently held by three tiny screws.

What’s the difference?

[rprosperi]

The early 71B's had the metal back, but it was changed to the plastic back style some time in 1986, unrelated to other changes (e.g. ROM version) so it probably was simply a cost-savings industrial design change.

I know that some parts are not compatible between the older and newer design, however I'm not certain if the display components are affected, but other experts should comment within a day or so.

[KimH]

(08-22-2024 12:18 AM)dbrothers Wrote:  I love my HP71B. It’s my go-to calc for everyday use. Noticing that under certain circumstances, part of the display was fading, I decided I needed a backup to fall back on, just in case. I managed to buy one (in somewhat rough shape) on EBay.

When it arrived, I noticed that my original has a plastic back, and the new one has a metal back, apparently held by three tiny screws.

What’s the difference?

The main difference inside, is that the connection between top and bottom half is changed from a flat, thin flex cable mounted (soldered) on the right hand edges of the boards (my favorite) to a double “zebra” in the middle of the board (which tends to not work so well after 4 decades, especially if batteries leak). The metal back is part of the ESD protection. One of the screws connects the front and the back planes. On the newer models this is done with a piece of foil on the inside on the later (plastic-back).

Also - around the same time - the battery compartment with the 4 AAA, switched from rock-solid spring contacts (2 versions AFAIR) to a foam based, where the adhesive, keeping the contacts in place, simply lose their stickiness…