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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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HP-25 calculator shows all zeros, or blank display
Recently aquired a HP-25 programmable calculator (1975) that had dead
batteries and wouldn't power up correctly. Sometimes the display indicated 12 zeros and other times the display was blank and a few keys would indicate a single "0" in various places. I took the thing apart. cleaned the board and keyboard with alcohol and toothbrush and contact cleaner and managed to get it working for a couple hours using a couple alkaline AA batteries. Put it all back together and everything seemed to work right but the next day it went back to the original erratic operation. I read an article indicating possible noise problems on the data line that was fixed with a 20k resistor termination on some particular line. I tried that idea on a few pins without sucess and cannot get the thing to power up with further cleaning. Haven't looked at the clock signals on a scope yet. Hard to find a schematic for old HP calculators. Any ideas? -Bill |
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