![]() ![]() Since this would appear to be similar to the 97s display circuit and comparing part numbers that are visible these transistors control voltage supplied to the anode drivers and if one was failed I would not expect the symptom you are observing. The 1976 vintage 91 I just finished rebuilding, (for sale) does not even have any transistors on the the display board so it would appear that by the time the one you have was built the display board was very similar to the 97. If it is a segment driver you may be able to get a replacement as I believe this chip 1858-0044 was used in all of the topcats, however the ROM would be unique to the 91. You could isolate the anode driver by swapping them if the anode driver has failed it should cause a different segment to be lit up all the time. If the anode driver had failed I would expect the segment to come on as soon as you turned on the calculator and remain on, however this symptom could also be caused by a failed ROM 0. The fact that the this segment is on for all digit positions would suggest that the driver is on all the time either due to a fault in the driver chip or in ROM 0. The segments are turned on by ROM 0 through the anode driver chips. In any case there are no transistors in the drive circuit for the display. They have lots of manuals including Service Manuals as well (the HP-97 is included and it is close enough for this task).įrom your picture that does not look like a break in the transistor, that looks more like there is a coating of some kind on the back. You may like to acquire the Manuals on CD/DVD from this MoHPC. There are thousands of possible replacements that will work on that position. I would use fast switching high gain transistor for both NPN and PNP positions, capable of about 0,5 to 1 Amp of current, working with at least 50 Volt Vcb, and a minimum of 0,5 Watt of dissipation power. If the reading is negative, the transistor should be a PNP type. If the reading is positive, the transistor should be a NPN type. If you are lucky, a voltage will be there during the normal operation of the calculator. Take a digital multimeter set to measure Volts, put the negative probe to the Emiter pin and the positive probe to the Collector of each transistor. You are lucky because the transistors cases have the Emiter (E), Collector (C) and Base (B) pins well indicated, so that is a good starting point: it means they are bipolar silicon NPN and/or PNP semiconductors. (just for reference, Motorola identification code is "185") My cross reference comes from this information here. "4-071" -> this probably is a NPN transistor "3-374" -> this probably is a PNP transistor "3-393" -> this probably is a PNP transistor The 3 transistors are from Motorola (the "M" indicates the manufacturer, and the 3 digits are the manufacturing date). I need a valid commercial reference of this transistor, and perhaps the oher 2 additional transistors near, because for me it's impossible find them in tables and internet. Open the chassis, I discover a transistor phisically break (see picture attached). All results and all digits have the minus sign activated or bright. It makes all functions so well, but it have a 'little' problem at display. I'm owner of an old HP-91 calculator (year '79 more less). ![]()
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