A replacement for the HP-41 zebra connector

[Hans Brueggemann]

although i very much appreciate the effort to find a solution to this ubiquituous "zebra" issue on the HP-41 series, i'm still not convinced by the kind of a makeshift approach that we take to solve it. the kapton wrap-around looks good, but all flex-ribbon ideas (cable tubing, silcone tubing,...) to "cushion" the flex-strip do lack of a material characterization, which makes it kind of va-banque when looking at the reliability.
hence, i'd suggest that we also look into something that is called Oriented Wire Shield
i've seen this stuff before on prototyping sockets(!) for microprocessors that come in a BGA package.
what do you think?

[AndiGer]

Today I took the time to give the flex a try.

Put it in a 41CV that did not work before. Calc had one broken now glued lower post. The flex did not work correctly, only displays looking like the upper half of starburst over all digits. The same as with copper zebra in. But: For a part of a second I could see all annunciators on and all starburst. Could not reproduce.
Later I remarked that the repaired post is 0.8 mm higher than the other one and the case did not close correctly at this side. Saw it when I had removed the case separator very clearly.
I will try to shorten the post and try again later. Maybe that's the real problem, no good contact at the zebra/flex.

Put in a second one, the one in the post Mark first time mentioned his flex. With jewing gum zebra. After installing I got all @ (looks like that) in all digits. Had that also with the original zebra. Later got weird display.

In both calcs I re-used the black separator and cable wiring (2.4 mm diameter) for giving pressure.

Conclusion: Didn't bring up both calcs back to life but could see the same results as with original zebras.

Andi

[mfleming]

(03-28-2017 07:43 PM)anetzer Wrote:  So...

Silicone tubing seems to be what most think to be a good choice.

... text removed ...

Question to those who tried: What size would you go for in terms of total, compacted silicone thickness? Is 2 mm enough or should one look to 4, 5mm even?

Thanks!

I've run through some tests with the solid core O-ring material from McMasters, and one size works well. The cord diameter is listed as 3/32" (2.4mm) but in reality measures 0.103" (2.6mm). While the cord works well it is still a bit too thick. I can close the case but there is a small hairline gap at the joint (see Sylvain's pictures). The solid core cord really doesn't compress to any real degree, so there is likely to be a trade-off between fit and functionality.

A hollow core cord or thick tubing might be a viable alternative. Something slightly large (2.6mm - 2.9mm) with an inner diameter >= 1mm might do the trick. Keep in mind the grey hollow core cord was 1/8" (3.2mm) and slightly too big for many.

Lastly, here's a flex that was in one calculator about three weeks. the dimpling is pretty noticeable, but the flex seems to stretch and mold itself to solder blob protrusions. While I was preparing this design, I feared I might have to cut away some of the flex material but that no longer seems the case.

   

~ Mark

[anetzer]

(04-02-2017 03:02 PM)Hans Brueggemann Wrote:  what do you think?

Weird thing nobody took up on this so far. It seems a great idea.

Only: How would anyone get a chance to test several samples and try out what dimension and material mix works?

And it would especially interesting if we then could get a willing person to order a larger batch of that material and batch-cut it to appropriate measures on a more or less automated cutter bench?

A.

BTW: Did anybody ever have the same problem on an HP-71 or are those machines immune to zebra strip problems? If so, the HP-41 posts seem to be a deep-rooted menace indeed...

[anetzer]

(04-05-2017 06:43 PM)mfleming Wrote:  A hollow core cord or thick tubing might be a viable alternative. Something slightly large (2.6mm - 2.9mm) with an inner diameter >= 1mm might do the trick. Keep in mind the grey hollow core cord was 1/8" (3.2mm) and slightly too big for many.

Thank you so much!

Still waiting on my first test order for silicone tubing...

[Dave Frederickson]

(04-05-2017 06:57 PM)anetzer Wrote:  BTW: Did anybody ever have the same problem on an HP-71 or are those machines immune to zebra strip problems? If so, the HP-41 posts seem to be a deep-rooted menace indeed...

Yes. I've had several 71's with signs of battery corrosion and they all had corroded zebra connectors. Note that the zebra connector for the 71 is about 3/8 inch tall.

[mfleming]

(04-05-2017 07:22 PM)Dave Frederickson Wrote:  

(04-05-2017 06:57 PM)anetzer Wrote:  BTW: Did anybody ever have the same problem on an HP-71 or are those machines immune to zebra strip problems? If so, the HP-41 posts seem to be a deep-rooted menace indeed...

Yes. I've had several 71's with signs of battery corrosion and they all had corroded zebra connectors. Note that the zebra connector for the 71 is about 3/8 inch tall.

By any chance is the 72 connector geometry similar enough to the 41 to use a flex circuit solution? (chin rub)

anetzer Wrote:Weird thing nobody took up on this so far. It seems a great idea.

Only: How would anyone get a chance to test several samples and try out what dimension and material mix works?

I did look at the suggested Z-axis material a few days ago. A search turned up three U.S. companies offering a similar material, all three of them saying "Contact for Quote." In my experience, a company specifying that condition for trade is similar to a restaurant with a menu that has no prices.

I did find two material items from one of those companies (only one item in stock) at Mouser. It was a sheet 18" x 18" x 0.62" (457mm x 457mm x 1.6mm) for $375, quantity one. I lost interest at that point, given that is nearly thrice my own total R&D investment so far

I'll wait for the OP to follow up on their own search & test results!

~Mark

[Dave Frederickson]

(04-05-2017 10:59 PM)mfleming Wrote:  

(04-05-2017 07:22 PM)Dave Frederickson Wrote:  Yes. I've had several 71's with signs of battery corrosion and they all had corroded zebra connectors. Note that the zebra connector for the 71 is about 3/8 inch tall.

By any chance is the 72 connector geometry similar enough to the 41 to use a flex circuit solution? (chin rub)

The 71's connector geometry is very different from the 41. There's a picture on p.18 of this manual: http://www.hpmuseum.org/forum/attachment.php?aid=2637

The zebra is wrapped around a polymer core, then slid into the black plastic housing which has guide pins. From the pic you can see that the connector is about 0.7" long, 0.15" wide, and as I mentioned, about 0.4" high. The housing and polymer core can be salvaged, so all that's needed is a thin flex like this:



Dave

[mfleming]

(04-05-2017 11:35 PM)Dave Frederickson Wrote:  From the pic you can see that the connector is about 0.7" long, 0.15" wide, and as I mentioned, about 0.4" high. The housing and polymer core can be salvaged, so all that's needed is a thin flex like this:

Looks easy enough. Could you provide overall flex dimensions, contact width, pitch, and number? Perhaps a pic of one, stretched out for view? The housing and core of the HP-71 connector sounds an awful lot like a general solution for the HP-41 (two needed?) Perhaps save the idea for iteration #2 if there is enough interest in this current solution.

~Mark

[Dave Frederickson]

Hi Mark,

The overall flex dimensions are 1.00" x 0.57". The length of the conductors is 0.52".
The elastomer dimensions are 1.00" x 0.40" x 0.07".

The conductor specs are about the same as the Fujipoly "B" wire size.
http://www.fujipoly.com/usa/products/zeb...ctors.html

71B Top PCB contacts


Elastomer and housing


Flex


Dave

[mfleming]

(04-06-2017 01:58 AM)Dave Frederickson Wrote:  Hi Mark,

The overall flex dimensions are 1.00" x 0.57". The length of the conductors is 0.52".
The elastomer dimensions are 1.00" x 0.40" x 0.07".

Hi Dave,

I could see a flex circuit with contacts at each end that match the width and pitch of the corresponding PCB contacts. A large keepout area around the contacts would allow them to wrap around the ends of the elastomer.

The flex circuit is a laminate of two sheets, and the conductor carrier sheet is actually pretty flexible. It should bend without much resistance. A little laminate left between conductive pads would provide a stiffer outward bow if needed. Think of the HP-41 I/O block flex and the way it connects to the keyboard PCB as an analogy.

It would be easier to visualize this with a flex at hand, so PM an address and I'll send you a sample. You might try cutting out a section of the contact area to test the fit and fold. If it looks feasible we'll see where it goes from there!

~Mark

[Dave Frederickson]

(04-06-2017 02:52 PM)mfleming Wrote:  It would be easier to visualize this with a flex at hand, so PM an address and I'll send you a sample.

Sure. PM sent.

BTW, what does anybody know about a service manual or schematic for this version of the 71B?

Dave

[Dave Frederickson]

Here's the flex in the 71B's zebra housing.







I trimmed the flex to 1" x 0.8".

I think this could work.

Dave

[mfleming]

(04-15-2017 03:30 AM)Dave Frederickson Wrote:  ...
I trimmed the flex to 1" x 0.8".

I think this could work.

Dave

Interesting! Based on your dimensions, I'll see if I can work up a design this weekend and get a rough quote for a short prototype run. I assume the 0.8" dimension is sufficient to wrap around the interior piece?

On the 41 flex replacement, I'm reconsidering the idea of routing outside conductors between pads for a more compact design. I'm now thinking of removing all protective overlay material in the area around the contact pads so there is no standoff effect. Not sure that's necessary though, given how thin the material is in relation to pad width.

I'll let you know when I have some numbers.

~Mark

[Dave Frederickson]

Hi Mark,

Here's a view from the end, showing the completeness of the wrap. It looks like there're some small gaps that could be fixed with a little preforming. It could be a little longer.

[aurelio]

sorry for completely missing this post, I would be happy to test on my side this nice your solution!!!
I would like to solve a bad contact problem that sometimes worries my CL
let me know please if still in time I'll send you what is needed
Thank-you

[Dave Frederickson]

(04-15-2017 03:13 PM)Dave Frederickson Wrote:  It looks like there're some small gaps that could be fixed with a little preforming.

Now that I look at it a little longer, the bulges may be desirable as they would flatten out and increase the contact area.

Dave

[mfleming]

(04-15-2017 06:43 PM)Dave Frederickson Wrote:  Now that I look at it a little longer, the bulges may be desirable as they would flatten out and increase the contact area.

The gap should certainly stay, as it would take up any slack or flex in the distance between contact ends. Looking at your photos, I'm having another thought on the 71B flex design. The contacts are at a 0.50" pitch, with each contact slightly wider than the space between contacts. If I were to try and lay down contacts and conductive traces on the flex circuit along those same dimensions, it would be pretty sensitive to misalignment.

An alternative might be to replicate the wound gold wire arrangement using a flex circuit. I could lay down tracks that are 0.010" wide spaced 0.020" apart (sorry for all the Imperial measures here guys, I'll probably have to translate to metric for the PCB house later anyway). No trace would be wide enough to bridge two PCB contacts, and you're likely to get two flex traces per PCB contact. If those two wide PCB contacts at the right end of the photo carry power, then they would get proportionally more current-carrying traces.

I'll see what that looks like by guesstimating PCB contact dimensions and seeing how they would line up with parallel traces. I'll also check minimum trace dimensions with the fab.

aurelio Wrote:I would like to solve a bad contact problem that sometimes worries my CL
let me know please if still in time I'll send you what is needed

Sorry, I sent the last sample to another member about a week ago! However, I'm hoping to finish up and order a larger supply in the next couple of weeks. I've been trying to figure out, with the help of others, the most reliable spacer material to use with the flex so I could provide all needed materials. Given a variety of fairly cheap and available materials will serve the purpose, I wonder if I should just provide the flex by itself (along with suitable instructions and suggested supplies). Naturally, all at material + labor + postage (plus a little lagniappe to recover the R&D investment )

We'll see soon I hope,
~Mark

[mfleming]

Dave,

A flex circuit replacement for the wire-wound conductor looks viable. This picture shows a design with 0.01" conductors spaced 0.02" apart. The green rectangles are what I think are the dimensions of your PCB contacts. The hatched areas of the flex circuit are free of the protective overlay.

   

Have a look at the dimensions and see if I have the overall shape correct.

~Mark

[Dave Frederickson]

Hi Mark,

The dimensions look really good. I measured the distance between contact areas, the plus signs in the pic, and measured 0.46". Your pic looks like 0.50".

Do the shaded areas at the top and bottom represent anything? Was your plan to not have a coverlay to reduce cost? Do you think there's any advantage to a narrower trace/space, like 5/5?

Dave