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HP Forum Archive 19

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HP75C Display (was zebra connector)
Message #1 Posted by Alberto Fenini on 3 Nov 2010, 5:29 p.m.

Dear all,

the pics below are the front side, back side and the section of an HP75C LCD display.
Since I'd like to clean the zebra connector, I'm wondering if anyone else has ever done it before.
Any good idea on how to remove the black iron shield ??

Thanks in advance for any suggestion you may have,

Re: HP75C zebra revealed
Message #2 Posted by Alberto Fenini on 3 Nov 2010, 6:49 p.m.,
in response to message #1 by Alberto Fenini

Zebra revealed !

By gently pushing the pcb, everything will slide out
I have cleaned the zebra and the pcb contacts with alcool
but it didn't get better.

However, it looks like one of the chip soldered on the back of the pcb was re-soldered ...
I'll try to swap the display unit with a LCD unit coming from a 75D which works fine and I'll post the results

Re: HP75C Bezel removed !
Message #3 Posted by Alberto Fenini on 3 Nov 2010, 6:56 p.m.,
in response to message #1 by Alberto Fenini

I'm posting this sequence, it may be of help to someone,
the 75C I'm trying to restore has also a ding close to the logo label
by using a blade and heating the surface I have removed the bezel
and discovered that it is a single piece including the keyboard layout

Tomorrow I'll polish the keys, and try to recover the ding.
If anyone has some ideas on how to smooth the ding, please leave your thoughts.

take care Alberto

Re: HP75C Display swap
Message #4 Posted by Alberto Fenini on 6 Nov 2010, 9:44 a.m.,
in response to message #3 by Alberto Fenini

Hello everyone,

for those of you interested in the project, hope to not bother too much the rest of the readers ...
I have opened the 75D and swapped the displays, which are fully compatible.
Well, it's not the disply the issue.
The display from the 75C works fine on the 75D and viceversa
the display known to work fne on the 75D has the same problem when installed in the 75C
What could it be the reason ??
Can the contrast be adjusted with a trimmer or a statement ??

thanks in advance and have a nice weekend !! Alberto

Re: HP75C Display (was zebra connector)
Message #5 Posted by geoff quickfall on 4 Nov 2010, 12:44 p.m.,
in response to message #1 by Alberto Fenini


I have been away on a trip to Bejing and for some reason the government screeners on the wifi network at the hotel have blocked HPMuseum and my outgoing email accounts! This happens from time to time.

First, remove the glue residue from the keyboard and the faceplate backing. Some type of citrus based adhesive remover and cotton rag should work. Then wash the faceplate in warm soapy water.

Place the faceplate between to thick sheets of clear vinyl, thicker then a freezer bag. I use clear to see the results.

With the faceplate sandwiched between the vinyl, the entire package placed on a VERY FLAT AND SMOOTH SURFACE (sheet of glass) and the printing visible (obverse) take a large soup spoon and rub over the ding. Flip the face plate over and rub some more from the (reverse) side. Continue this until the ding is removed.

Now with the entire face plate reverse side up rub the whole face plate taking care not to damage the reverse angle. This will impart a slight convex shape to the faceplate with the cup highest in the centre and lowest at the corners with the obverse side facing up.

This allows the faceplate to be glued in place (contact cement) with the corners fitting perfectly. Take a look at the following HPSolve article (HP42s) which is very similar to your 75.

June 2010

The second half of the restoration article explains the faceplate removal and glueing procedure. Also refer to the HP65 restoration piece, specifically how I smoothed the label out on it. There is a figure refering to label placement and the vinyl used.

Good work on the LCD panel also!

Cheers, Geoff

Edited: 4 Nov 2010, 12:46 p.m.

Re: HP75C Display (was zebra connector)
Message #6 Posted by Alberto Fenini on 4 Nov 2010, 5:32 p.m.,
in response to message #5 by geoff quickfall

Geoff !!
I was counting on your experience in removing labels and
making them look like new, I'm happy you liked my work,
honestly, it didn't seem to me such a great job, but I was in a situation where
I could'nt go back anymore ... if you know what I mean...
I'll work as suggested on the ding, and I'll post the results as soon as possible.

As far as for the display I need to test the display I have on the 75D before I can make any further assumption.
However, it seems that some of the chips have been re-soldered
Still, I'm wondering if all the pieces put together will make again an HP75C ....
Thanks for your help, and take care

BTW, how is the book project going ??
BTW (OT) how about the engine exploded on the A380 from Quantas ???

Re: HP75C Bezel - ding removed (?)
Message #7 Posted by Alberto Fenini on 5 Nov 2010, 7:24 p.m.,
in response to message #6 by Alberto Fenini

Hello everyone,

I have removed the ding, following Geoff's tips.

What would you ?
Geoff, can it get any better than this ??
Thanks for any idea or suggestion

take care Alberto

Re: HP75C Restore Project - Final conclusion
Message #8 Posted by Alberto Fenini on 6 Nov 2010, 4:29 p.m.,
in response to message #7 by Alberto Fenini

Hello everyone
as promised I had tried the display swap with a working display from an HP75D.
I have then discovered that the display are both working !!
Then I have swapped the memory boards. Since the memory board also host
the resistor for the contrast and the temperature based voltage regulation for the LCD contrast,
I wanted to be sure that also these were properly working, and so it was.
Then, I have reworked the base board, desolderin all of the connection pins
between this and the memory board, but it din't get any better.
Based on the service manual, thanks to Tony Duell for the hint, I have learned that while the columns are driven from some ICs solderd on the display PCB,
the rows and few other signals are driven from a chip that is on the logic board.
In case of bad display the service manual suggest to check all the soldering points, which I have done.

Does anyone has ever replaced a chip like this ???
take care and thanks for any suggestion ....

Cutting A Circle In Three Equal Pieces - Solution
Message #9 Posted by Thomas Klemm on 8 Nov 2010, 2:07 p.m.,
in response to message #5 by geoff quickfall

June 2010

In the same issue I found the solution of HP Solve Math Problem Challenge #1.

Is it only me who thinks that both solutions are unnecessarily complicated?

This is the drawing from the article:

Letís use a circle of unit diameter, D = 1.

Why not using a unit circle? It's much easier when r=1.

Sector S

Triangle R

Segment K

Now you can stop here and use the solver to find the solution or use the double-angle formula for sine:

Here we end up with a fixed point equation that can be solved by iteration.

First load the stack with:


Then repeat the following steps until you are satisfied with the accuracy of the result:


This will take some time but that could be a reason to deal with convergence acceleration.

The wrong Thomas

PS: Alberto, I'd like to beg your pardon for hijacking this thread but the King said I mustn't beg.

Edited: 8 Nov 2010, 3:16 p.m.

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