HP-15C Zebra



#10

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.


#11

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

#12

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.


#13

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.


#14

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.

#15

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


#16

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.


#17

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.


#18

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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