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HP 71B broken
09-13-2017, 12:47 PM
Post: #1
HP 71B broken
Hi,
Have a broken 71B and need help!

Changed batts, disabled all external units.

Started to have problem after change of batteries. In the beginning before it was totally dead, bottom row from button "on" to "arrow down" was not working ok.

Anybody out there repairing a 71B?

Malte
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09-13-2017, 03:01 PM
Post: #2
RE: HP 71B broken
I usually have to wiggle the batteries in my 71B to make good contact. Also check again that the batteries are correctly oriented.
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09-13-2017, 03:36 PM
Post: #3
RE: HP 71B broken
Thanks Craig!
I have checked batteries. Measured 6.35 V on the metal going in to the calculator.
Malte
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09-13-2017, 03:38 PM
Post: #4
RE: HP 71B broken
If you have the version with a plastic back and the batteries have ever leaked, then there's a chance that the "golden zebra" connecting the two PCB's is contaminated. Open it up and gently separate the case halves to examine the connector.

If you have the version with a metal back then there's a risk of damaging the flex cable connecting the boards if the case is opened.

My current project is to repair a 71 by wiring the boards together. I'll post a pic later.
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09-13-2017, 05:16 PM
Post: #5
RE: HP 71B broken
Thanks Craig! I have checked the voltage at the 2 metal going into the calculator.
It is 6.35 VDC.

Dave! I have the one with metal case. s/n 2424A00215. Yes u are right, I could have damage flat connector cable inside. I had problems with bottom row was always upper case. Took calculator apart and cleaned with alcohol and than it was dead. I was really careful.

Malte
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09-13-2017, 05:24 PM (This post was last modified: 09-13-2017 07:44 PM by Dave Frederickson.)
Post: #6
RE: HP 71B broken
With the 71 open, measure the continuity in each trace of the flex cable. If there are any open traces then you can solder a small wire (30 AWG) in parallel with the open trace. Consider replacing the cable with discrete wires, which was HP's solution. I also have a metal-backed 71 with a cable replacement. I can post a pic later.

Dave
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09-13-2017, 08:18 PM
Post: #7
RE: HP 71B broken
(09-13-2017 03:38 PM)Dave Frederickson Wrote:  My current project is to repair a 71 by wiring the boards together.

I've done this some time ago, in fact the HP-71B I use most often is the one I fixed this way.

The zebra connector was gone and I used thin wires (used for wire wrapping) to connect the two boards. When I finished I powered up the 71B and got nothing. Thinking I fried the calculator I left it on the bench. Later I want back and found out I'd missed a wire. When I added that, it worked fine and has been working since.

You'll need either good eye sight or a big magnifying lens to do the repair.

**vp
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10-02-2017, 02:40 AM
Post: #8
RE: HP 71B broken
The damaged strip could look like this:

[Image: image-1.jpg]

[Image: image-2.jpg]
Soldering thin wires to reestablish contacts:
[Image: image-6.jpg]

Wires should be long enough to allow easy opening and laying flat the two half cases.

E.
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