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27S display - a show stopper?
Message #1 Posted by Thomas Radtke on 11 Oct 2011, 3:35 p.m.

A colleague sold me this quite beaten up 27S today (for a very reasonable price):

While it has interesting features, I found the display to be not only problematic, but an imposition. Is it just this package with the protective cover making it almost unreadable? I know this kind of LCDs from the 18C/19B, but this one tops it all. For any viewing angle, a certain adjustment of contrast between black background and slightly readable characters has to be found.

What were your thoughts when you bought your sample (when it was new)?

      
Re: 27S display - a show stopper?
Message #2 Posted by BruceH on 11 Oct 2011, 3:54 p.m.,
in response to message #1 by Thomas Radtke

Mine is fine and perfectly readable so it sounds like yours has been damaged. If you can post a picture of the screen then the knowledgeable folks here might be able to make useful suggestions.

            
Re: 27S display - a show stopper?
Message #3 Posted by Thomas Radtke on 11 Oct 2011, 4:03 p.m.,
in response to message #2 by BruceH

Thanks for your answer, Bruce.

Two angles, same contrast. The camera got the contrast in the top image much better than my eyes do.

[Edit: Used a flash light - under very bright light the problem appears to be less visible]

Edited: 11 Oct 2011, 4:35 p.m.

                  
Re: 27S display - a show stopper?
Message #4 Posted by Neil Hamilton (Ottawa) on 11 Oct 2011, 5:01 p.m.,
in response to message #3 by Thomas Radtke

Hi Thomas,

My circa '93 17bii from Singapore has this same "feature" while my 2 other 17bii's (both Indonsesia, later in the decade) do not. The later ones have a much wider field of view before "blacking out".

Interestingly, the indicators (shift, print, etc.) do not seem to be affected to nearly the same extent as the dot matrix area.

What is the date and source of the 27S?

                        
Re: 27S display - a show stopper?
Message #5 Posted by Eric Smith on 11 Oct 2011, 5:42 p.m.,
in response to message #4 by Neil Hamilton (Ottawa)

Quote:
Interestingly, the indicators (shift, print, etc.) do not seem to be affected to nearly the same extent as the dot matrix area.

That's because the dot matrix area has a much higher multiplex factor than the annunciators.

                              
Re: 27S display - a show stopper?
Message #6 Posted by bill platt on 11 Oct 2011, 6:33 p.m.,
in response to message #5 by Eric Smith

What does that mean? (or what is the multiplexer?)

                                    
Re: 27S display - a show stopper?
Message #7 Posted by Eric Smith on 11 Oct 2011, 8:55 p.m.,
in response to message #6 by bill platt

Segments on a dot-matrix display generally aren't driven continuously. Some fraction of them are driven at any given time. For the dot-matrix area of the 17B/17BII/27S/42S display, one row is driven at a time, which is 16:1 multiplex. The annunciators are generally driven at a lower multiplex rate, though I don't know specifically for those calculators what the rate is.

                        
Re: 27S display - a show stopper?
Message #8 Posted by uhmgawa on 11 Oct 2011, 6:13 p.m.,
in response to message #4 by Neil Hamilton (Ottawa)

Quote:
My circa '93 17bii from Singapore has this same "feature" while my 2 other 17bii's (both Indonsesia, later in the decade) do not. The later ones have a much wider field of view before "blacking out".

IIRC Hosoda-san (www.finetune.jp) suggested this was due to a weak/broken boost regulator for the lcd bias resulting in a low drive voltage. IME I've seen this result from too high of a drive voltage so I'm not quite certain. I think in part the lower contrast and viewing angle dependent contrast/depth is a characteristic of the lcd and controller in use. But certainly an out of spec drive voltage can be aggravating the situation. Some of my Saturn units as well have rather poor contrast and I'd been reminding myself to experiment with tightening it up.

Quote:
Interestingly, the indicators (shift, print, etc.) do not seem to be affected to nearly the same extent as the dot matrix area.

The annunciators might be multiplexed via a different rate/scheme such that the average segment "off" voltage is well below the lcd polarization threshold. Or maybe there is just less crosstalk between them given the size/spacing geometry difference relative to pixels in the matrix field.

                              
Re: 27S display - a show stopper?
Message #9 Posted by Lyuka on 11 Oct 2011, 7:59 p.m.,
in response to message #8 by uhmgawa

Quote:
broken boost regulator for the lcd

for more information about it, take a look at
'A work-around for sticking-to-low-contrast LCD problem of the HP-42S'
and previous thread 'An HP-42S fix success, and a contrast problem'.

Regards,
Lyuka

P.S. The PCB and the LCD of the 27S is identical to that of the 42S.

Edited: 11 Oct 2011, 8:02 p.m.

                                    
Re: 27S display - a show stopper?
Message #10 Posted by uhmgawa on 12 Oct 2011, 1:00 a.m.,
in response to message #9 by Lyuka

Quote:

for more information about it, take a look at
'A work-around for sticking-to-low-contrast LCD problem of the HP-42S'
and previous thread 'An HP-42S fix success, and a contrast problem'.

Ah, I think I see what my confusion was above. You were describing low contrast due to insufficient lcd polarization drive resulting in a faint display vs. the low contrast inherent in this series of lcds under normal operation.

The latter problem is due to the intended "off" pixels receiving an average multiplex voltage just over their polarization potential which partially enables them, thereby reducing the contrast ratio due to saturation. AFAICT it really looks to be an issue of the controller either logically in terms of excessive multiplex ratio (or less likely analog drive) either of which results in RMS voltages (and optical polarization states) for on vs. off being too close causing readjustment of the lcd drive rail to be needed as the viewing angle changes.

This appears unrelated to a glass quality issue as the polarization level doesn't seem to visibly ghost due to crosstalk and the partial polarization is uniform. While these displays perform rather poorly by modern standards I seem to recall this being typical of matrix displays around the time these calculators were developed.

Quote:
The PCB and the LCD of the 27S is identical to that of the 42S.

The 17B I peeled open was very close except for what appeared to be a few cosmetic layout changes. Unsure whether that was model specific or a uniform change made to all models relative to manufacturing date (probably the latter).

                                    
Re: 27S display - a show stopper?
Message #11 Posted by Thomas Radtke on 12 Oct 2011, 1:36 a.m.,
in response to message #9 by Lyuka

Thanks for the links! So, replacing diode and capacitor might help?

                        
Re: 27S display - a show stopper?
Message #12 Posted by Thomas Radtke on 12 Oct 2011, 1:27 a.m.,
in response to message #4 by Neil Hamilton (Ottawa)

Quote:
What is the date and source of the 27S?
I can tell not before tomorrow, but it's an early Singapore model sold in Germany.

Same here with the anunciators, they are perfectly readable.

                              
Re: 27S display - a show stopper?
Message #13 Posted by Thomas Radtke on 12 Oct 2011, 11:15 a.m.,
in response to message #12 by Thomas Radtke

S/N 2917Sxxxx

      
Re: 27S display - a show stopper?
Message #14 Posted by M. Joury on 11 Oct 2011, 7:15 p.m.,
in response to message #1 by Thomas Radtke

My 27S (3224S04249) is perfectly legible. No issues whatsoever.

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