hp-91 transistor substitution

[Paul Berger (Canada)]

(09-20-2014 05:10 PM)joseaem Wrote:  Hi all.
I'm owner of an old HP-91 calculator (year '79 more less). It makes all functions so well, but it have a 'little' problem at display. All results and all digits have the minus sign activated or bright.
Open the chassis, I discover a transistor phisically break (see picture attached).
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.

The numbers are 'M712 3-393', 'M712 3-374', '4-071 M741'

Thanks in advance

From 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. In any case there are no transistors in the drive circuit for the display. The segments are turned on by ROM 0 through the anode driver chips. 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. 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. 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 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.

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. 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 HP part numbers for the transistors are 3-393 = 1853-0393, 3-374 = 1853-0374, and 4-071 = 1854-0071 which in at least one cross reference says the industry number is CP4071 which does not seem to be helpful. I have had difficulty in the past finding substitutes for transistors used in topcats.