A replacement for the HP-41 zebra connector

[mfleming]

(04-19-2017 02:06 AM)Dave Frederickson Wrote:  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

Hi Dave,
That's good information about contact spacing to have. I did assume 0.05" between contacts from a quick glance at the image you posted. Much better to have an actual measurement of the distance. Could you also measure the width of the contacts and the gap between them? I was estimating the gap to be around 0.02" and that gap distance is important in terms of trace width.

The flex contact width obviously must be less than the gap between PCB contacts to avoid shorting two PCB contacts together. I was making the flex width about half of the estimated gap in case there was any skew in the flex with respect to the PCB. I assume the axis of the flex will be a little off angle with respect to the PCB axis since you're assembling and mounting the connector by hand. As to making the traces narrower, I'm close to what the PCB house can handle, but could probably get the traces a little smaller.

The hatched areas at top and bottom are the exposed and plated flex contacts that make contact with each PCB. The exposed contacts can be gold plated on request (adds to the price, of course). Normally every PCB is coated with a protective layer, usually green, but can also be blue or red. Contacts and solder points that need to be left exposed are indicated by a "keepout" layer, which is the hatching you see. A flex circuit bonds a protective plastic layer over the base layer and the copper traces instead of the coating used on a PCB.

I went ahead and put a keepout around the entire contact area for two reasons. One was because the remaining cover area between traces was so small I was certain a reviewer would ask to have them removed. The other reason is that the cover material between traces would act like a standoff, keeping the flex contact just above the PCB contact it is supposed to touch. Not a big deal in the 41 flex design, given the flex contact is 0.1" wide and the cover material is about one-hundredth that thickness. The exposed flex contact will easily bow downward under pressure to touch the PCB contact. Not so with yours, the ratio being ten or less. Best to have no interference if possible.

I did do a quick online estimate as opposed to a formal quote for the 71B flex. As expected, there is a minimum charge. The piece count per cost increment is proportional to the flex area though, so you wouldn't bump up to the next cost level until more than 100 units are ordered (wish my design were that way!)

This brings up the same question I'm posed with; Are there enough people interested in a repair option like this that you can recover some or all of your minimum investment by selling your excess pieces to them? (I assume you don't need to fix 100+ 71B's!) I'm thinking the next step would be to do a poll and see how many people might actually want one of these - 71B flex or 41C flex.

Thoughts?
~Mark

[Dave Frederickson]

Hi Mark,

I don't have a very good means of measuring the spacing between the pads. From the below pic it looks like about 25 mils. Contact width also looks like 25 mils.



There are a lot more 41's out there than 71's and of that this flex would be only for the later version. Of that, how many folks are doing repairs caused by leaking batteries? Not very many.

What's the minimum? Is there any savings in eliminating the coverlay, the polyimide protective layer, or because of the minimum charge is that a moot point?

Dave

[mfleming]

(04-20-2017 01:56 AM)Dave Frederickson Wrote:  Hi Mark,

<snip>

There are a lot more 41's out there than 71's and of that this flex would be only for the later version. Of that, how many folks are doing repairs caused by leaking batteries? Not very many.

What's the minimum? Is there any savings in eliminating the coverlay, the polyimide protective layer, or because of the minimum charge is that a moot point?

Dave

You bring up a good point on audience size. As to leaking battery repair, I've bought several 41 I/O flex from Diego. To be honest, I've only bought a couple of working calculators, preferring instead to adopt a "shelter calc" for parts or repair. Seems silly to pay $$$ for a registered breed when a mutt will do. Also nice to save one of the little guys from recycling, but then maybe most people have better things to do with their time

I can check on minimum details, but for now figure on about $80 plus extra for gold plating, shipping, etc. I would guess minimum would be tied to labor more than material, but eliminating process steps (like overlay) might help a little.

I think only a poll would likely gauge the depth of interest among members. Perhaps tomorrow...
~Mark

[Dave Frederickson]

(04-20-2017 02:54 AM)mfleming Wrote:  I can check on minimum details, but for now figure on about $80 plus extra for gold plating, shipping, etc.

To be honest, I wouldn't pay $80 for a flex cable.

[Geoff Quickfall]

Back to the 41C wire wrapped connector:




The silicone tube is 2mm and the wire wrap adds another .04 mm. This is easily compressed to 1.7mm before resisting further compression. It also springs back to 2.04mm when the conpression force is removed.

Geoff

[mfleming]

(04-22-2017 06:32 AM)Geoff Quickfall Wrote:  Back to the 41C wire wrapped connector:

The silicone tube is 2mm and the wire wrap adds another .04 mm. This is easily compressed to 1.7mm before resisting further compression. It also springs back to 2.04mm when the compression force is removed.

Geoff

Your measurements confirm my experience with differing materials. A 1/16" (1.6mm) solid cord was a bit too small for a reliable connection, while a 3/32" (actually 0.104" or 2.6mm) was too large to get a complete case closure. The closest compressible sample was a gray hollow-core cord with a nominal 1/8" (3.2mm) diameter that would compress to 0.07" (1.8mm). That material however did require a fair amount of force to compress to that height, and did not recover from compression very quickly. It had to be left alone to recover for a couple of days before it would work again.

To get something that springs back the way you've observed, I'd probably need either a solid cord with a Soft or Medium Durometer rating, or a relatively thick-walled tubing. The tubing would likely need a Medium or Hard rating in order to spring back quickly.

At this point I've heard results from about half of those who have tried the flex circuit. Assuming the others haven't encountered any show-stopping problems, I think it is worth seeing if anyone else would be interested in a replacement for the HP-41 CPU connector. I'll post a poll shortly asking for feedback.

My thanks to everyone for your comments and suggestions,
~Mark

[isanchez]

Hi,
I do agree with your conclusions. From my experience, the smaller one is too small and the bigger does not allow to close the calculator properly.

I am willing to check now with a "thick" double-side tape by 3M. It is around 2mm thick and can be fitted in place very easily. I am now trying to see the best way to implement it. I shall come back to you with the results.
Kind regards

Ignacio

[mfleming]

I've finished up what I think is the last round of material tests, and have found a sample that works perfectly as an offset pressing the flex contacts against their corresponding PCB contacts.

I tested three Buna N O-ring material samples; one 2mm square with a medium hardness rating, one 2mm round with a hard hardness rating, and one 2mm round with a medium hardness rating. I had tested square material in the past without success, but thought it worth a try once more with the more accurate height dimension. The square material failed with two connectors in two calculators. At best, I could squeeze the bottom of the case and get some display segments to flicker.

Both round materials worked flawlessly, the only difference being with case closure. The medium hardness material felt just like an original CPU connector was being used, providing a tight closure with only a light tightening of the screws. The hard hardness rating material on the other hand left a paper thin gap and a sense that the case wasn't quite closed. Again though, both of the round sample materials resulted in an immediate MEMORY LOST display when the battery pack was inserted. No wobbles, no reset, no pop-it-out, pop-it-back-in.

So the medium material appears to be the winner. The material yields enough to pressure to completely seal the case. The round shape concentrates the compression force along a thin line through the connector contacts to provide a dependable connection.

Case closed (sorry, pun intended!)
~Mark

[Massimo Gnerucci]

(05-04-2017 10:24 PM)mfleming Wrote:  Case closed (sorry, pun intended!)

:D

Thank you Mark!

[rprosperi]

(05-04-2017 10:24 PM)mfleming Wrote:  Case closed (sorry, pun intended!)
~Mark

A suspicious man might suspect this full thread was faked, just to be able to successfully pull off the best closing line in many years, but I've seen enough cases to know that's not the case.

Well done!

[mfleming]

Hi all,

A full write-up on the whole connector replacement project is still in the works but for those who have received their flex order, here are some quick instructions on how to employ them (mind you, it ain't Rocket Science).

Make an initial crease in the flex circuit. Fold the flex over with the contacts on the outside, and then place the holes over the lower posts. Using the fingers of one hand to hold the flex down at the posts, use a fingernail to smooth down the bend in the flex to make a crease. No need to fold it to the finished position just yet.

I've included about twice as much of the medium 2mm O-ring offset material needed for your flex, along with a short length of the hard material to play around with. Cut two pieces of material to a length of about 1.65" to 1.75" (42 or 44mm) so that it will run the length of a row of contacts with a bit extended beyond each end of the contact row.

Place a strip of Scotch brand scrapbooking tape to hold the offset material in place while the flex is folded and the CPU board placed on top. The tape and offsets will be on the back of the flex just above the two rows of contacts at one end of the flex. An alternative to tape would be to use a glue stick and lay down a tacky layer to hold the offset material. Align each offset with the contact pads beneath. Since the cord uses the compression force of the closed case to press the flex contacts against the PCB contacts, you’ll want to line the cord up pretty much with the centerline of the contacts.

Place the flex in the case. I’ve always placed it with the crease towards the bottom and the bottom end of the flex tucked under the protective cover over the keyboard PCB traces. This would be the time to give the flex crease a few more passes with your finger while keeping the cords aligned with the contacts. Since the flex will spring back up when you want to place the CPU board on the posts, I’ll sometimes put a small piece of tape on the upper flex to hold it down over the posts.

Replace the CPU and close the case. Once closed, replace the battery pack. If all is well, you’ll be greeted with the MEMORY LOST message!

~Mark

[isanchez]

Hi,

maybe can be of interest, you can find elastomeric zebra connectors for HP41 series at thecalculatorstore.com .
I find that José has obtained some stock from somewhere. They are not original, but do work. I have checked a pair and they work fair in a 41CV.

Regards

Ignacio

[Divasson]

    From Ignacio's last email (and thanks to his help for the development), we have put together a number of HP41c repair tools and pieces:

- zebra connectors: for full nut calculators.

- zebra holder: in the case of the membrane zebra connector destroyed.
   

The above is valid for full nut 41c, but we haven't tested it with nut-screwed circuit, only with pressure held circuits.


- Back case screw post repair piece: For this one we do have a repair instructions page. This is valid for all generations of HP41c, and quite useful, I think.
   

- Flex circuit assembly: using a base of our design, we paste Diego's flex circuit. After doing a number of these, we have decided not to use the original base anymore, since the adherence of the flex circuit is much worse than with the new design, which was designed with the adhesive circuit in mind, with more adherence areas and a better surface quality for the task. It has also a tighter fit.
   


The base can be found also in Shapeways.com, should you want to do it yourselves.

We are preparing (Ignacio mainly!) a number of other repairs, in particular a definitive upper and lower post repair, as well as side ports and battery holders.

[twoweims]

Is there any chance that these are still available for purchase?

[dingebre]

(09-17-2019 10:42 PM)twoweims Wrote:  Is there any chance that these are still available for purchase?

Try here:
https://www.thecalculatorstore.com/epage...P41_corner

David

[Artur - Brasil]

Please,
Does anyone have the dimensions of flex-pcb used in HP-41 for connecting batteries and modules to main board?
I know Diego has a replacement for selling, but they are expensive to me and I'll still have to import them, which may inccur in taxes. So, I'll try some pcb industry by here in Brazil.

I got one flex in a 41C, but it is so destroyed. I couldn't take the measures from it.

Best wishes
Artur