HP-67 repair display



#6

I have an HP67 which has a common segmment out on each digit of the display. Does anyone know where I can get a schematic or pinout of the 5x7digit LEDs and display driver chips? How about replacement parts? Thank you much if you respond.


#7

Numbering the pins on the displays as on a normal DIL IC package, with pin 1 on the top right, the pinout seems to be :
1: Ag, 2: k2, 3: Af, 4: N/C, 5: Ab, 6: k4, 7: Aa, 8: k5, 9: Ae, 10: Ac, 11: k3, 12: Ah, 13: Ad, 14: k1.
These are common cathode displays (one cathode line for each digit, and a segment anode line for each segment (which I've named conventionally, h being the decimal point). Since you are missing a segment (rather than a digit), it would appear that you have an anode driver problem.
7 of the 8 anodes (all but 'g' -- the middle horizontal bar) are driven from the 16 pin chip on the keyboard/display PCB. It's normally house-coded 1858-0050, and is actually an RCA CA3082 transsitor arrayu (not custom HP :-)). The g segment is driven by discrete transistors on the PCB, so check those if that's the missing segment. The inputs to the anode drivers come over 8 of the pins from the logic PCB (start at thr fifth pin counting from the R/S key side of the machine). Check for bad contacts here. On the logic PCB they're driven by the 18 pin ROM0 chip (1818-0268), which is, of course, HP custom.
That should get you started....


#8

Thank you Tony!
Your description of the display driver was precise.
With a couple of quick scope measurements on the driver IC
I determined it was at fault. Replacement of the CA3082
fixed the broken segments.
It is almost back to healthy... When I depress any key in the third row (from bottom), i.e. 4, 5, 6 I get strange results. The display will flash abnormally and the digit will often not register. If I hold the key down I also see some abnormally bright segments. Looks like interference between the display driver and the keyboard registering function. Other key rows work normally. Any further suggestions or methods to get a schematic?
Thanks again for the guidance.


#9

The keyboard is a 5*8 (IIRC) matrix. The column lines (5 of them) go to pins on the ACT CPU. The row lines come from the same lines that drive the display
cathodes (in other words the display and keyboard are scanned together).
Maybe you have a short somewhere in the keyboard PCB, such that when you press one of the affected keys, 2 of the cathode lines are shorted together. You might want to check for solder bridges, etc on the PCB.
As for a schematic, the HP97 service manual is on the MoHPC CD ROMs I think. It's not the same machine at all, but many of the chips are similar in pinout and function. Using that manual you can work out most of the details of the HP67 in an afternoon at most.


#10

Thank you for the guidance Tony. I found a B-E short in the transistor closest to the power connector; it's now working well. Even the battery pack (labelled 1984 ) is holding a good charge. I find myself spending hours reading the book and programming it (just a geek at heart ).
Thanks again.


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