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HP Forum Archive 13

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LED failure
Message #1 Posted by Matthias on 29 July 2003, 1:26 p.m.

I`ve got a HP67, which is in pretty nice shape, even the card reader still works, just the minus sign for the exponent doesn't show up.

Did anyone have a similar problem, is it somehow likely to be a bad soldering joint, or am I really "lucky" enough to have a damaged LED?

What's your best guess, I'm still hesitant to open it.

      
Re: LED failure
Message #2 Posted by Vieira, Luiz C. (Brazil) on 29 July 2003, 4:27 p.m.,
in response to message #1 by Matthias

Hi, Matthias;

If I am not wrong, all classics have a complete position in the display dedicated to the minus sign for the exponent of ten. From the Woodstocks on, you'll see less digits and a "shared" portion (two or three digits) that act sometimes as least significant digits for the mantissa and others as the exponent of ten.

I'm mentioning these facts because if it is possible to activate all segments in this position then we could verify if it is a matter of bad contact or not. Anyway, based on what you described, all of it may be out of operation. This is not bad, because each digit has a dedicated connection common to all cathodes (in some cases may be anodes) and each anode in all segments taht share the same position (a, b, c, d, etc.) are connected together. This is a common arrangement and is driven by multiplexed signals. You probably know about that, but maybe others don't.

Well, it is possible that a single bad connection is the cause. IN all Classics (HP55, HP65, etc.) and in the HP67, the display is arranged in four blocks with four digits each. I would do the following: I'd open the calculator and identify the block that contains the exponent signal (the rightmost) and I'd carafully and re-solder all of its terminals. If you decide you should test before or inspect to find with pin is not connecting, no problem. To be honest, I'd do the same if I have the time to search for the problem.

Hope this helps.

Success! (I did not spell check the text, so forgive my mistakes, please...)

Luiz (Brazil)

Edited: 29 July 2003, 11:50 p.m. after one or more responses were posted

            
Re: LED failure
Message #3 Posted by Ernie Malaga on 29 July 2003, 11:39 p.m.,
in response to message #2 by Vieira, Luiz C. (Brazil)

Luiz:

Quote:
Success!

Qapla'!

-Ernie 8^)

                  
My first word in...
Message #4 Posted by Vieira, Luiz C. (Brazil) on 29 July 2003, 11:46 p.m.,
in response to message #3 by Ernie Malaga

Hi, Ernie!

Thank you! As you have probably noticed, I love learning. In Portuguese it's almost the same as in English, with one single "c" and an extra "o" in the end: Sucesso.

Please, in which language is Qapla' written? Forgive my ignorance 8)

Best regards, my friend.

Luiz (Brazil)

                        
Re: My first word in...
Message #5 Posted by Ernie Malaga on 30 July 2003, 2:04 a.m.,
in response to message #4 by Vieira, Luiz C. (Brazil)

Quote:
Please, in which language is Qapla' written?

Klingon! According to the Star Trek universe, that's the word for "Success" in that language, and is used as a "farewell" word.

Quote:
In Portuguese it's almost the same as in English, with one single "c" and an extra "o" in the end: Sucesso.

But it's "éxito" in Spanish and "sukceso" in Esperanto. In Spanish, "suceso" is a big event.

Once I read that "escova" is a brush in Portuguese (but it sounds like "escoba" in Spanish, which is a broom. And "vassoura" was the broom, but that sounds like "basura" in Spanish, which is the trash. If any of this is true (and I have no way of knowing, since my knowledge of Português is extremely limitado), it's got to be quite a source of confusion. Incredible, since Portuguese and Spanish _should_ be more closely related!

-Ernie

                              
Universal knowledge! :^)
Message #6 Posted by Vieira, Luiz C. (Brazil) on 30 July 2003, 2:52 p.m.,
in response to message #5 by Ernie Malaga

Hi, Ernie;

yes, all Portuguese terms you used are correctly "translated". And you are also right: Portugus e Espanhol _deveriam_ ser mais prximos (Portuguese and Spanish _should_ be more close {to each other}). 'Get the picture?

Live long and Prosper. :^|

Luiz (Brazil)

                                    
Re: Universal knowledge! :^)
Message #7 Posted by Ernie Malaga on 30 July 2003, 3:31 p.m.,
in response to message #6 by Vieira, Luiz C. (Brazil)

Obrigado!

-Ernie

      
Re: LED failure
Message #8 Posted by David Smith on 29 July 2003, 4:34 p.m.,
in response to message #1 by Matthias

It could be a bad LED, but the HP67 has some really weird circuitry involved in generating the minus signs. There are several transistors on the keyboard used to make them. If I remember right, both minus signs are generated from the same digit driver. Tony Duell could tell you a lot more on the subject.

            
Re: LED failure
Message #9 Posted by Matthias on 1 Aug 2003, 9:53 a.m.,
in response to message #8 by David Smith

Thanks David,

would I leave a very naive impression if I asked you who Tony Duell is and how to reach him (just in case he isn't a Borg or thelike and still alive)

Matthias

                  
Re: LED failure
Message #10 Posted by Vieira, Luiz C. (Brazil) on 1 Aug 2003, 10:11 a.m.,
in response to message #9 by Matthias

Hi, Matthias;

You're not asking me, but as many of us "walking around this neighborhood" for a while, let me tell you Tony Duell, from England (London?), is not a Borg. And it is not futille!

He knows internals (electronics) of many HP portable devices as he had post many concise and precious information at this very Forum. Be sure he's a valuable "voice" in here. As David himself, Ellis, Vassilis, J.F. Garnier, Glynn, etc. etc. etc...

Best regards.

Luiz (Brazil)

Edited: 1 Aug 2003, 10:15 a.m.

                        
Re: LED failure
Message #11 Posted by Michael F. Coyle on 1 Aug 2003, 10:57 a.m.,
in response to message #10 by Vieira, Luiz C. (Brazil)

Hi Luiz!

Quote:
[Tony Duell] knows internals (electronics) of many HP portable devices as he had post many concise and precious information at this very Forum. Be sure he's a valuable "voice" in here. As David himself, Ellis, Vassilis, J.F. Garnier, Glynn, etc. etc. etc...

Since modesty prevented you from adding your own name to the list, please let me do so now.

- Michael

                              
Re: LED failure
Message #12 Posted by Vieira, Luiz C. (Brazil) on 1 Aug 2003, 11:37 a.m.,
in response to message #11 by Michael F. Coyle

Hi, Michael;

I'm flattered; I hope I deserve this honored mentioning of yours. But there are many, many others that should also be mentioned. To be honest, I can not even think of names I would not add...

Thanks.

Luiz (Brazil)

                  
Re: LED failure
Message #13 Posted by David Smith on 1 Aug 2003, 1:13 p.m.,
in response to message #9 by Matthias

Tony is the Lord High UberPope of all things HP9100 and most things HP classic. He is at Cambridge in England. I don't have his email handy, but he does post in the HPFORUM from time to time. You can search the FORMUM archives for any of his postings or Google him. Be aware that he uses a funky email system and his emails arrive to me as file attachments.

      
Re: LED failure
Message #14 Posted by Jon on 30 July 2003, 1:52 p.m.,
in response to message #1 by Matthias

In the Victor Toths page you have some information about similar problems. I will try to copy that information and post it. And in the service manual of the HP-97 you have also some descriptions of the leds working (HP-67 and 97 are very similar)

Cheers

Jon

            
Re: LED failure
Message #15 Posted by Jon on 30 July 2003, 4:55 p.m.,
in response to message #14 by Jon

Here you have the promised information. I hope it can help:

Usually, if the same segment is dead across all digits in a calculator, the segment driver is suspect. In the case of the HP-67, the segment driver chip is the smaller of two chips located underneath the LED display modules. It is easy to map this chip's inputs and corresponding outputs. This chip contains seven independent driver circuits. When a single segment is dark across the entire display, chances are that one of the driver circuits is dead. Replacing the entire chip is often not an option, since substitute chips are hard to find. However, a single driver element can be replaced using a 2N4401 transistor. The base of the transistor should be connected to the corresponding input, the emitter to the corresponding output, and the collector to pin 15 of the segment driver chip. Using fine wire-wrap wire, you can place the transistor in such a way so that the keyboard/display PCB still fits snugly against the calculator's plastic backbone, and the case can be closed securely. If the driver circuit in the segment driver chip is shorted, it may be necessary to disconnect (cut) one or both corresponding pins in order for the substitute transistor to work correctly.

                  
Re: LED failure
Message #16 Posted by David Smith on 30 July 2003, 6:27 p.m.,
in response to message #15 by Jon

The anode driver is a standard off the shelf chip that is still made. NTE even has a replacement in their product line. I cant remember the non-HP part number, but it is something like CA3080 or CA3083. Again the HP67 has weird circuitry driving the sign segments that is independent of the anode driover chip.

      
Re: LED failure - Conclusion?
Message #17 Posted by Matthias on 2 Aug 2003, 4:01 a.m.,
in response to message #1 by Matthias

Thanks to all, a good deal of helpful infos you guys can provide indeed. I'm impressed.

That much for the positive side. After a rough evaluation of what I learned from you and the manuals and considering the fact that ALL elements of the display (even the minus sign for the mantissa) but the one for the exponent show up, I have to assume that the LED is likely to be the problem.

So, now some further questions (always assume I'm not an expert like you, I know basics about electronics but I never opened one of those so far): What's the cure? Does every digit have a single 7seg LED, are there groups or is it a complete bank? Can it (to whichever extent I need) still be purchased or can anyone provide it as a spare part?

I waited almost 20a to get one of them, it's supposed to be perfect, even if the only calculations I do nowadays are how much month will be left after the end of the money, hi.

Thanks again.

Matthias (Germany)

            
Re: LED failure - Conclusion?
Message #18 Posted by Vieira, Luiz C. (Brazil) on 2 Aug 2003, 8:12 a.m.,
in response to message #17 by Matthias

Hi, Matthias;

What I can tell you is that HP offered LED digits pack as standard product, so I guess you'll be able to find the same (or equivalent) LED assembly in other HP products or even HP LED assemblies in other brand equipment as well. remember finding a four-digit HP assembly in a portable, battery operated RPM metter, a small, rugged handy tool used in our electric lab when I was in the university. One digit was missing and I and two other guys were assigned to repair it. I was surprised for finding an HP logo stamped in the LED assy.

I think that the only way out is finding a replacement, from a donator unit. Old, trashed equipment may be a source as well. Why not to try? They will be "rated" 1975 to 1982, probably. I hav a Test and Measurement catalog where these LED's are announced. I'll scan the image and post it here later.

Best regards and success.

Luiz (Brazil)

            
Re: LED failure - Conclusion?
Message #19 Posted by David Smith on 2 Aug 2003, 4:03 p.m.,
in response to message #17 by Matthias

The problem could still be a bad transistor, etc on the HP67 keyboard. The HP67 middle segments are really weird circuitwise. They are all totally independet of the rest of the segments.

The LED's are packaged like a DIP circuit. Five digits to a package. Your only source is another calculator. All the classic series machines use the same displays. They come with either gold or silver leads. They are not quite the same! The digits are slighty different sizes. There is also a letter on the back of each package. It indicates the relative brightness level. The letter is usually in the range of K to M. One or two letters different is usually a pretty good match.

            
Re: LED failure - uneducated suggestion
Message #20 Posted by Renato on 3 Aug 2003, 6:04 p.m.,
in response to message #17 by Matthias

Matthias,

Ive never examined a classic display module, but Ive seen 97, woodstock and spice displays. I have a woodstock display module with two bad segments in the third digit, and I dont think there is anything I can do, except find a parts donator machine.

In your case, I would try to use another segment to indicate exponent sign, by rerouting connections on the display assembly. If this did not work, then i would look for a single , third party led, to indicate exponent sign. I have no idea if these are feasible, but I think it would be fun to try to make it work.

Good luck,

Renato


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