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HP-15C Zebra
Message #1 Posted by aj04062 on 8 Dec 2011, 7:19 p.m.

I have a 1982 HP15C that seems to have a problematic zebra strip. I think I diagnosed this as the problem after powering up the display/cpu module disconnected from the keyboard. The symptoms are the same as when it is assembled in the unit.

I thought I had it solved when I substituted a strip from a similar vintage 12C I have. The 15C came to life, passed the keyboard test, etc. However, after sitting a bit, the same symptoms came back.
Does anyone else have experience curing issues like this? If it helps, I can post pictures.

Edited: 8 Dec 2011, 7:19 p.m.

      
Re: HP-15C Zebra
Message #2 Posted by John Robinson on 9 Dec 2011, 12:50 a.m.,
in response to message #1 by aj04062

Did you also clean the contacts on both the circuit board and LCD with a good quality circuit board cleaner ?. It could also come down to pressure, maybe you need a little more pressure to squish the zebra a little more - just a thought!!

Cheers, John

      
Re: HP-15C Zebra
Message #3 Posted by uhmgawa on 9 Dec 2011, 1:46 a.m.,
in response to message #1 by aj04062

Quote:
I thought I had it solved when I substituted a strip from a similar vintage 12C I have. The 15C came to life, passed the keyboard test, etc. However, after sitting a bit, the same symptoms came back.
Does anyone else have experience curing issues like this?

One of the problems with the legacy voyager lcd glass design is the lack of a vertical zebra retainer/stabilizer which makes (re)assembly a bit of a challenge. Unless the zebras are located near normal to the pcb before compression, they tend to distort and potentially buckle to varying extents which will lead to intermittent or even failed connections. This is aggravated by the fact the stainless compression frame fingers need to be sprung apart to seat into the pcb mounting holes. In the process of doing so the display tends to move around and the zebras along with it.

Eyeball the glass from either extreme side, sighting under the glass along the zebra length. If the zebra is flexed outward where it mates with the pcb, you can push it back normal by carefully pushing under the stainless frame along the length of the zebra, where and as needed. If it flexes inward that is much less fun, but you could try to pull it outward into alignment with a taut music wire or preferably a thin metallic strip which will have far greater rigidity in the needed direction.

If you need to fish under the display to coax the zebras into position, you'll also need to navigate around the two alignment studs which pass through the pcb.

            
Re: HP-15C Zebra
Message #4 Posted by aj04062 on 9 Dec 2011, 12:53 p.m.,
in response to message #3 by uhmgawa

When I initially fixed this unit, it was the little short zebra from the keyboard the LCD unit that seemed to be the problem.

I did clean with alcohol, but will look for other contact/pcb cleaner. Do you think a pencil eraser is good enough or should I use something else?

I'll look to see if I can insert something between the flex pcb and the zebra to get better contact.

Edited: 9 Dec 2011, 12:55 p.m.

                  
Re: HP-15C Zebra
Message #5 Posted by uhmgawa on 9 Dec 2011, 6:54 p.m.,
in response to message #4 by aj04062

Quote:
When I initially fixed this unit, it was the little short zebra from the keyboard the LCD unit that seemed to be the problem.

Ah, I see. I thought you were referring to the lcd zebras.

You can try buzzing out continuity between the key matrix and the CPU. I haven't done so but I'd expect somewhere in the few k-ohm range for a zebra contact pad-to-pad. The zebra resistance needs to be low enough to overcome the parasitic key matrix capacitance seen by the ~55KHz scan with enough noise margin for reliable operation. A scope on a row input of a pressed key position would confirm this.

It has been a while since I had an early voyager apart but IIRC the flex zebra contacts were gold plated. I'm not sure what type of foreign contamination may exist in your case but I'd avoid losing any of the gold plating as that will remove the copper pad oxidation barrier.

I was always a bit mystified by the construction of the early voyagers given a separate pcb for display and silicon requiring yet another zebra connect to the key matrix pcb. I usually cringe when the cost reduction madmen arrive with hacksaws. But in this case it is an absolute improvement by any measure.

      
Re: HP-15C Zebra
Message #6 Posted by Randy on 9 Dec 2011, 6:40 p.m.,
in response to message #1 by aj04062

It would be helpful to know what the actual problem is...

            
Re: HP-15C Zebra
Message #7 Posted by aj04062 on 9 Dec 2011, 7:21 p.m.,
in response to message #6 by Randy

Completely unresponsive calculator. It will display "c eef c".

Interestingly, I left the batteries out since I started this thread. I just installed to take a picture and it is working fine again. I ran keyboard and self test and they both pass.

I tried shorting the terminals before and that didn't cure it in the past, if you are thinking to discharge the capacitor.

                  
Re: HP-15C Zebra
Message #8 Posted by uhmgawa on 9 Dec 2011, 9:58 p.m.,
in response to message #7 by aj04062

Quote:
Completely unresponsive calculator. It will display "c eef c".

Hopefully it wont, but if it does get into that state again you might leave it be to determine if the ~10 minute display timeout occurs as that doesn't require CPU/program intervention.

Yet I'd suspect an intermittent joint on the display/cpu board as a cause of that symptom. Although admittedly I've never actually seen an early vintage 15c of that construction, but only a 16c and 12c. In those cases neither the board nor surrounding frame had provisions for a 15c's additional r2d2.

A picture would be interesting if you have reason to open it up again.

                        
Re: HP-15C Zebra
Message #9 Posted by Randy on 10 Dec 2011, 10:37 a.m.,
in response to message #8 by uhmgawa

Quote:
Yet I'd suspect an intermittent joint on the display/cpu board as a cause of that symptom. Although admittedly I've never actually seen an early vintage 15c of that construction, but only a 16c and 12c. In those cases neither the board nor surrounding frame had provisions for a 15c's additional r2d2.

+1 - as I have seen several flex circuit units with fractured soldering, the most recent, a 15C.


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