Classic Display Problem - Suggestions?


I have a classic calculator that has some extra segments lit. The segments are not hard on. They come and go, to some extent, based on which keys are pressed.

The calculator, otherwise, works fine.

My problem is 'where to begin'. If you have worked on these before, is this a problem that is more likely in the display, the driver chips or the processor. I have extra drivers and LED modules but both are a pain to replace.

Which is more likely the cause?


I'd start with cleaning the circuit boards in case there's any residue or oxidization (under the chips, too!) This problem is more prevalent with VFD displays due to their higher operating voltage, but I've seen that even a small amount of residue between traces is enough to cause visible crosstalk between digits. Since the keyboard and the display share some scan lines, it is also normal for the crosstalk to change its appearance in response to keystrokes.



The affected digits are the 1st and the 11th. I removed the main processor board and powered up the calc. The same 1st and 11th digits had segments lit. The 1st and 11th are in the same digit position within each module. Wonder if that is a coincidence?

So, it appears that these digits are being driven independent of the processor. Looks like it might be one or both of the LED driver chips (I assume that is what they are just below the LEDs) or the LEDs themsleves. Possibly the traces but they look good.

Anyone know where a display circuit schematic is for an HP-65?



I don't think the fact that it's the 1st and 11th digit has any special significance. A question: when you say 'lit', exactly what are we talking about? Lit to full brightness? Which segments are lit, and how do they respond to keystrokes?



On digit 1, segments a, f and g are lit. These are lit slightly dimmer than what would normally be on.

On digit 11, segment e and g are lit. normal brightness.

If I press keys, some segments go off some do not.

If I press '.', '1', '4', or '7' as the first digit, all these segments go off. But pressing other keys turns one or more back on.

The board is very clean; no apparent oxidation or shorts.

I think I'm going to remove the driver chips tonight and 1) look under the chips and 2) look for shorts between leads.

But I don't think I will find anything.

I do also find that if I disconnect the CPU board, that these two digits still have segments on but at a much brighter level.

I think some basic troubleshooting will lead to the solution. I just wish it were easier to remove the parts.



The fact that your segments are on even when the CPU board is removed suggests a short somewhere. At this point, I'd begin by poking around with my 'scope and a continuity tester. At least you should be able to determine what is shorted to what, and whether or not the short is likely within a driver chip. Those are rather difficult to desolder, so I'd not attempt it until I am reasonably sure that they're the cause.


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