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HP21 problem
12-19-2014, 04:19 PM (This post was last modified: 12-19-2014 04:25 PM by PANAMATIK.)
Post: #9
RE: HP21 problem
(12-18-2014 11:56 AM)MarkPalmer Wrote:  Before putting it in the non-working drawer I would take it apart and re-seat the LED display to the main board and the main board to the keyboard. There is a very good chance this will get it working again. The ROM chip in the 21 is tied in with the LED anode drivers, so if it still doesn't work it's probably a bad ACT chip.


I would recommend that too. Its worth a try.

However, the LED contacts can't produce this effect. They are just connected passively to the display driver chip. The display driver ROM chip has no capability of counting and it has no internal RAM memory, it shows the digits on the fly, that were sendt on a serial line from the ACT, which is permanently sending the actual digits. It needs about 4 ms for the whole display, each digit 310us. Thus it is rather sure that the ACT has problems. But it could be also the power switching unit, that has noisy voltages.

As I understood, You always tried with batteries, without AC Adapter, then there is no mains frequency present. I do not think that mains frequency does the counting.

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Messages In This Thread
HP21 problem - vk6ti - 12-17-2014, 05:26 AM
RE: HP21 problem - Katie Wasserman - 12-17-2014, 02:57 PM
RE: HP21 problem - PANAMATIK - 12-18-2014, 08:28 AM
RE: HP21 problem - MarkPalmer - 12-18-2014, 11:56 AM
RE: HP21 problem - PANAMATIK - 12-19-2014 04:19 PM
RE: HP21 problem - MarkPalmer - 12-18-2014, 04:09 PM
RE: HP21 problem - aj04062 - 12-18-2014, 04:24 PM
RE: HP21 problem - PANAMATIK - 12-18-2014, 05:55 PM
RE: HP21 problem - vk6ti - 12-19-2014, 12:07 AM

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