HP Forum Archive 02

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HP55 display problem Message #1 Posted by Erik Wahlin on 12 Feb 2000, 4:55 p.m. I am working on a HP55 that has a problem I was wondering if anyone has ever encountered. When you turn the calculator on, one particular LED segment lights up across the entire display.If you looked at the segments that make up the number "8", it would be the lower right segment that is lighting up for all digits. I switched out the calculator component board with one from another HP55 that I have, and the problem dissapeared, so there must be a fault in this board. Any insight anyone might have would be greatly appreciated.

Re: HP55 display problem Message #2 Posted by Viktor Toth on 12 Feb 2000, 11:26 p.m., in response to message #1 by Erik Wahlin Erik, I have had an HP-55 with a display problem that I eventually traced back to a faulty diode and a dead transistor on the main board. Two questions; first, does this dead segment light up with normal, full intensity, or is it lit just faintly? Second, does the calculator show any signs of life? When you press buttons, for instance, does the display flicker or change in intensity? Viktor

Re: HP55 display problem Message #3 Posted by Erik Wahlin on 13 Feb 2000, 12:56 a.m., in response to message #2 by Viktor Toth Thanks for the response. The calculator works (including the timer) except that the led segment at the lower right of each digit lights up (normal brightness).The segment never flickers, and is of normal brightness. I have a couple other HP 55's that work, so I have compared the signals at the CPU board input with an oscilloscope. All signal were the same except for the far right most connection. I have found the the right most connection on the CPU (calculator pointing up) has a 3500HZ, 8V square wave signal on the good HP 55, and a 4V dc signal with the bad HP55. This connection goes to the MOSTEK 1820-1169 chip ( see http://www.hpmuseum.org/55intern.jpg ) This connection also goes to pin #1 of the anode driver IC #1820-1029 next to the LED. I am not sure if this is similar to the problem you encountered. Thanks

Re: HP55 display problem Message #4 Posted by Viktor Toth on 13 Feb 2000, 5:31 a.m., in response to message #3 by Erik Wahlin Erik, I think I am a little bit confused: are you saying that the display shows only one segment, which is constantly lit across all digits, or are you saying that the display operates normally, except for that one segment which is lit all the time? Viktor

Re: HP55 display problem Message #5 Posted by Erik Wahlin on 13 Feb 2000, 8:43 a.m., in response to message #4 by Viktor Toth Yes, the display is operates normally but one segment is lit all the time (except when replaced by a decimal point). thanks

Re: HP55 display problem Message #6 Posted by Viktor Toth on 14 Feb 2000, 7:08 a.m., in response to message #5 by Erik Wahlin Erik, I'm afraid you were right, my problem was different from yours. Since you already traced the problem back to a chip on the main CPU board, I'm afraid it is possible that the problem lies with that chip itself. Replacing these chips is a real pain even if you have a replacement part (presumably from a cannibalized unit.) If by any chance you identify the exact cause of the problem, please let us know! Viktor

Re: HP55 display problem Message #7 Posted by Erik Wahlin on 15 Feb 2000, 1:07 p.m., in response to message #1 by Erik Wahlin I have finally isolated the problem I am having with the HP55. It is a bad IC chip (MOSTEK 6020P, 1820-1169) on the CPU board. I found this out by desoldering and exchanging the chip from a good HP55 CPU board. If anybody out there has a bad HP55 CPU board with this chip and could sell it to me, please let me know! I am not sure if there are any replacement chips that would work, but I am going to look into it.

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