New HP-71 zebra strips?

[Jeff_Birt]

I found a relatively inexpensive HP-71B on fleabay a few minutes ago and snapped it up. Have been watching them for a while but this was the first sub $100 one I have seen. It was advertised as non-working but looks OK cosmetically. Half of the fun is in fixing old things anyhow.

A few months ago, when I posted about getting custom zebra strips made for the Casio RP-33 Redux RAM module some folks were talking about getting some made for the HP-71. Has anyone done so already?

I'm not sure if the unit I got used zebra strips or not but if it does and if nobody else has had zebra strips made, I'll look into it. I will need to order some more RP-33 zebra strips anyhow so I can add them to that order.

[Dave Frederickson]

Mark Fleming sent me a sample a couple of years ago, but it was unsuitable. I looked into zebra strips from Fujipoly a while back, too.
https://www.hpmuseum.org/forum/thread-10...l#pid91022

If your 71 is of the metal back variety, with screws on the back and spring connectors in the battery compartment, the two PCB's will be connected with a delicate ribbon cable. If your 71 is of the plastic back variety, with foam sprung contacts and a screw in the battery compartment then it will have a golden zebra connecting the boards. If there is any leakage in the battery compartment then it's practically guaranteed that if the 71 has a golden zebra then it's damaged.

Dave

[mfleming]

Flex ribbon cable can now be ordered in single quantities from Colorado-based Osh Park. I've used them in the past year and the quality is quite good. See below for details

https://docs.oshpark.com/services/flex/

[Jeff_Birt]

(06-11-2022 08:42 PM)Dave Frederickson Wrote:  If your 71 is of the metal back variety, with screws on the back and spring connectors in the battery compartment, the two PCB's will be connected with a delicate ribbon cable. If your 71 is of the plastic back variety, with foam sprung contacts and a screw in the battery compartment then it will have a golden zebra connecting the boards

Thanks. From the picture it looks like a plastic back to me.

It took me a few tries to get the size right on the first zebra strips I had made. Getting the height just right, to get the right amount of compression, is a bit of an educated guess. I'm guessing since you say 'golden zebra' it is of the type shown here: https://www.fujipoly.com/usa/products/ze...ctors.html. These seem to take less pressure to make a good contact than the silicone rubber type. I'm not sure the silicone type, which is easier to get made in small quantities, will work.

[Jeff_Birt]

(06-11-2022 09:01 PM)mfleming Wrote:  Flex ribbon cable can now be ordered in single quantities from Colorado-based Osh Park. I've used them in the past year and the quality is quite good. See below for details

https://docs.oshpark.com/services/flex/

Are you suggesting making a flex PCB to replace the original ribbon cable on the HP-71s that used that type of connection.

[Jeff_Birt]

After searching the forum here a bit more I was able to find some nice posts about the 'so called zebra strip' and some phots that had been shared. I understand better what the problem is. With the level of corrosion on the 'zebra' contact pads I don't think a direct replacement for the 'zebra' connector would work. I understand why folks use wire jumpers now.

I wonder if that is a standard footprint pitch where as a standard ribbon/flex connector could be installed on each side, perhaps an adapter PCB would be required. This would allow a ribbon/flex cable to be used.

Thanks everyone for the input.

[dmh]

Hi Jeff,

I raised it in your RP-33 post. I have a very clean 71b with only the zebra corroded so a replacement would be ideal.

The wire wrap solution doesn't appear to be very reliable as mine worked for a while with this solution and then stopped. It's a lot of wires and putting it back together they go everywhere and may get caught and damaged.

The original 71b zebra (models with plastic backs) also have alignment pins in the zebra housing so they sit perfectly (not sure if you've opened yours up yet to see).

I don't know what the development time and cost is but if you can estimate a cost I'm sure you'd get some preorders here (including me).

Hope this is possible :-)

(06-11-2022 11:58 PM)Jeff_Birt Wrote:  After searching the forum here a bit more I was able to find some nice posts about the 'so called zebra strip' and some phots that had been shared. I understand better what the problem is. With the level of corrosion on the 'zebra' contact pads I don't think a direct replacement for the 'zebra' connector would work. I understand why folks use wire jumpers now.

I wonder if that is a standard footprint pitch where as a standard ribbon/flex connector could be installed on each side, perhaps an adapter PCB would be required. This would allow a ribbon/flex cable to be used.

Thanks everyone for the input.

[mfleming]

(06-11-2022 10:04 PM)Jeff_Birt Wrote:  

(06-11-2022 09:01 PM)mfleming Wrote:  Flex ribbon cable can now be ordered in single quantities from Colorado-based Osh Park. I've used them in the past year and the quality is quite good. See below for details

https://docs.oshpark.com/services/flex/

Are you suggesting making a flex PCB to replace the original ribbon cable on the HP-71s that used that type of connection.

Yes indeed. There are a number of places where HP used a flex circuit for connections, including their thermal printers and the HP 97. The source of flex circuits I used prior to Osh Park required a minimum order of ~$80-100 for a batch of flex.

I made a flex to replace the zebra strip in the HP-41. That idea was greatly improved upon by The Calculator Store. That approach may not be needed now with the availability of real zebra strips, though I imagine the minimum order requirement remains.

~Mark

[Garth Wilson]

There seems to be a lot of confusion about what zebra strips are. They are not flex circuits similar to ribbon cable. They are compressible rectangular strips with very fine vertical conductive stripes that connect the rows of contacts along two parallel surfaces. I'm attaching scans of some industry magazine articles and ads here which I have processed for minimum file size.

[mfleming]

(06-13-2022 07:54 PM)Garth Wilson Wrote:  There seems to be a lot of confusion about what zebra strips are.

Not on my part, certainly! Anyone else?

Dave mentioned flex board2board connections for metal-backed 71's and zebra strips for plastic-backed 71's. Flex pressed against gold plated contacts was used to connect the port module assembly shell to the 41C main board and zebra strips to do the same for the CPU board. As mentioned, flex has been used as a substitute when zebra strips were unobtainable. Although not interchangeable, flex and zebra strips perform similar b2b functions.

Knowing now that there is a suitable source of custom zebra strips at hobbyist volume is great news for all those seemingly unrepairable calculators.

~Mark

[Garth Wilson]

(06-13-2022 09:25 PM)mfleming Wrote:  

(06-13-2022 07:54 PM)Garth Wilson Wrote:  There seems to be a lot of confusion about what zebra strips are.

Not on my part, certainly! Anyone else?

It keeps coming up in other parts of the forum where others make the mistake, and since I just today got these scanned, I thought I'd take the opportunity and post this here now since it doesn't merit a new topic just for this. I'll probably link to it in the future when it comes up again, rather than posting the same scans over and over.

[rprosperi]

Though I've too many scars to confuse these, I think it's a good idea to clarify this and I believe there is indeed confusion in the community, even if not among folks in the thread, and having clear explanations and photos of examples can only help. The 71B early vs. late models using different types of b2b connections no doubt has contributed to this confusion, at least among HP gear-heads.

Thanks for the post Garth, and I'd further say a post dedicated to just this topic with samples of each major type might well be a useful resource to refer back to.

[mfleming]

(06-13-2022 11:33 PM)rprosperi Wrote:  Thanks for the post Garth, and I'd further say a post dedicated to just this topic with samples of each major type might well be a useful resource to refer back to.

No criticism intended Garth. Actually, for any HP gear-head with time on their hands and a large collection, a list of the various interconnection methods used, organized by model or series, would be interesting. The early spring fork and plated hole approach was probably most reliable but most expensive. Seems like that went away with the Woodstocks. I've picked up and repaired half a dozen 48SX's at throwaway prices thanks to failing zebra strips

[Jeff_Birt]

I have seen in some cases where someone has used a flex PCB in place of an elastomeric 'zebra' strip and we have been talking about both in this thread which might be confusing so it seems Garth's interjection is well placed.

For future reference to other HP-71 newbies I'll attach an image of both types of interconnect HP used on the HP-71 which I gleaned from links elsewhere on the forum. The 'metal backed' type used a FFC soldered between the two halves. The 'plastic backed' used an odd zebra strip like connector in the middle. To me this looks more like a flex PCB rapped around an elastomeric cushion but I have not need one in the flesh as of yet. My HP-71 is not due for a few more days.

If a traditional elastomeric zebra strip were make to replace the original connector it would probably have a resistance of 1-2K per signal. I'm not certain how tall this connector actually is so this figure is a SWAG. I'm not sure if this added resistance would be detrimental or not.

   

   

   

[mfleming]

The metal-backed flex looks like flat wires sandwiched between two plastic sheets with wire ends protruding for soldering. A limited number can be found from Mouser. Regarding resistance of a traditional zebra strip, if it's just carrying keyboard matrix signals then the extra couple of K probably won't be noticed.

[Jeff_Birt]

Got the HP-71 in and unfortunately someone tried to fix it already. While trying to bodge a cap in they melted the plastic 'pseudo zebra' shroud and evidently threw the flex contact PCB away. Sigh... There is a bit of corrosion, but it is certainly not the worst corroded mess I have found.

Looking at the unit in person and understanding the layout a bit more I now understand that the display drivers and processor in on the top half and power supply, RAM and interconnects on the bottom. Given a 1mmx0.75 mm contact patch per contact and ~10mm height each there would be about 200ohms per line (estimating high here.) That 'might' not be a deal breaker.

The bottom PCB is quite thin and flexible but there does not seem to be a great deal of deflection in that area. Given the original 'pseudo zebra' used a silicon elastomer core the flex of the PCB might not be an issue.

Thinking about it this unit might not be a good one for testing the idea of using an elastomeric zebra strip as a replacement: A) I don't have the original flex PCB part, B) No way to test it without a working 'pseudo zebra'. I'll have to ponder what to do next.

   

   

[vassilisprevelakis]

(06-18-2022 12:48 AM)Jeff_Birt Wrote:  Got the HP-71 in and unfortunately someone tried to fix it already. While trying to bodge a cap in they melted the plastic 'pseudo zebra' shroud and evidently threw the flex contact PCB away. Sigh... There is a bit of corrosion, but it is certainly not the worst corroded mess I have found.

About 20 years ago, I had a problematic 71B with a totally ruined connection, so I soldered wire wrap wires between the two boards. After a brief panic when the "repaired" computer failed to show any signs of life, I realized I'd missed a connection, so adding this wire brought the 71B back to life.

Touch wood, it is still working, just fine.

**vp

http://www.series80.org