HP-71B Changing Lower and Upper Halves

[MikeSD]

I'm wondering if there are compatibility issues when changing a lower or upper half, on an HP-71B.

I'm not talking about going from metal to plastic back. But I have seen a couple versions of the plastic backs. There are at least 2 versions of the zebra strip connector pad arrangements. Very minor board layout change. But it made me wonder if top half must be paired to a specific lower half, or if if any plastic back lower can be used with any plastic back upper. Anyone know?

I tried hard wiring between the zebra connector pads on an intermittent connection but it wouldn't work at all.The flex cable on metal back is easy to bypass. Anyone have a good tip for bypassing the zebra strips? I just wired straight across. Seems pretty straight forward.

There is an unused pad near center that I just skipped.

[Dave Frederickson]

(03-18-2018 10:22 PM)MikeSD Wrote:  But I have seen a couple versions of the plastic backs. There are at least 2 versions of the zebra strip connector pad arrangements.

Didn't know that. Do you have pics?

[MikeSD]

(03-19-2018 01:43 AM)Dave Frederickson Wrote:  

(03-18-2018 10:22 PM)MikeSD Wrote:  But I have seen a couple versions of the plastic backs. There are at least 2 versions of the zebra strip connector pad arrangements.

Didn't know that. Do you have pics?

I'll see if I have both types and take a pic.

Here's an example

You can probably spot the differences.
* in the top one, there is an unconnected pad, right of center
* in the lower connector, the pad is omitted altogether.
* the fill area to the left of connector is different from top to bottom.
* the number of pads is different between top and bottom. In the connector on bottom, instead of a pad it's just the large area fill.

I've been looking at these to see if I miswired something. Hard to do since everything is straight across. But anything's possible. I've wired two and neither work. That seems odd, so I've been double checking I haven't got an offset.

[MikeSD]

I have a pristine HP-71B that is not working. it used to work but somewhere along the way, the zebra strip got damaged. At first it was intermittent. Now it doesn't work at all. I have seen some flickering of the display by messing with the strip but nothing makes it work.

So, I decided to bite the bullet and hardwire the connector. But there is no sign of life when I do that. I have several bottom halves and other top halves so I should be able to determine which half or if my connections are the problem. But with the differences I have noticed, I'm now wondering if a top needs a specific bottom. hmmm.

[Paul Berger (Canada)]

Well if you are concerned that there may be differences in the wiring I would suggest you ring out the connections on both sides and using the published schematic as a guide you should be able to determine what the connections are, and if there are differences between the versions.

Paul.

[MikeSD]

(03-19-2018 01:53 PM)Paul Berger (Canada) Wrote:  Well if you are concerned that there may be differences in the wiring I would suggest you ring out the connections on both sides and using the published schematic as a guide you should be able to determine what the connections are, and if there are differences between the versions.

Paul.

That's what I'm doing. Pretty hard to mess up the connection though. The zebra does the alignment and they have to be straight across, to every pad with a connection.

But I will check the connections later. I know the two halves I'm working with go together.

[MikeSD]

Here are two different plastic back boards. Much different.

[Dave Frederickson]

(03-23-2018 09:31 PM)MikeSD Wrote:  Here are two different plastic back boards. Much different.

Actually, some of the via patterns look similar. Looks like the top board is multilayer with a ground plane and probably a VCC plane. Can we see a pic of the other side?

Thanks,
Dave