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HP Forum Archive 02

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HP67 display problem
Message #1 Posted by Nick Kronenberg on 27 Dec 1999, 3:04 a.m.

Hello, UPN fans,

it’s me again (see my question below).

Even owning additionally to this 28C a HP41CX with card reader a still love my third HP calculator, an old HP67 being bought in the 80’s at a horrible price! After some years of not being used the part now is showing a display failure. All „c" segments on all three modules are non functional. I have found the helpful information of A.Knight (Thanks!) and did some measurements during Christmas holidays.

Pin 3 on all modules (anode c) - even showing a signal - (measured by scope) is kept at an non activated level (in relation to other anodes). As far as I could identify the signal it is been generated by 16pin IC 1858-0050-1D (A.Knight thinks this being the cathode driver, but I assume he is not right).

Does anyone have an idea what could be the root cause and can give assistance (Probably a „dead" signal path from the main controller is causing trouble?). My e-mail address is

I would love to make the machine work again! Thank you very much in advance!

Nick Kronenberg (German UPN fan)

Re: HP67 display problem
Message #2 Posted by Joe on 28 Dec 1999, 1:43 p.m.,
in response to message #1 by Nick Kronenberg

I've worked on dozens of HP 35s, 45s, 55s, 65s and 67s with the same problem. The segment c select is an anode, it's pin 3 on each display IC. Pin 3 is connected together on all three display ICs. The cathodes select which digit to display. The anode driver IC (the one on the left below the display) is almost certainly bad in your 67. That's common. There's no replacement for it. It is NOT a standard IC. It's a special hybrid that was made by HP for the calculators. I think segment c select is on pin 13. The only place you can get one is from a similar model calculator.

The anode select also ties to a coil directly above the displays. The early machines used individual coils but your's probably has them in two molded blocks. It's very unlikely that the coil is bad but you can check it. The coils are used to cause a brief high current pulse in the display. That causes a very bright but brief display to be generated. That way they can turn each digit on for a very short period of time and then switch to the next digit. Each digit is only on for about 5% of the time. If they used a normal drive current (and therefore normal brightness) the display would be too dim.

> (Probably a „dead" signal path from the main controller is causing trouble?).

Nope, Everything is done serially in the HP calculators. A dead signal path would cause it to lose a lot more than one segment. It's common to have dead select lines in both the cathode and anode driver ICs in the LED models. A bad cathode select causes an entire digit to be lost. A bad anode causes the same segment in all of the digits to be lost.


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