HP65 Display Problem


When I turn my HP65 on three zeros (0.00) light up as expected, but I also get two other digits partially lit up further to the right side of the display. When I fill the display with digits ('8's say) those right side digits don't display properly. Can anyone tell me if this is likely to be a display, or a display driver problem or both, and also are the HP35 display/display drivers compatible ?. Thanks - John



LEDs are VERY reliable, the thing that usually kills them is over voltage, it would be very unusual for all 7 segments to fail (or part fail) at one time. Your problem probably is because the 'column' drivers have a fault. This could be due to either a dry solder joint between the display driver and the display or because the display driver has failed.

To 'cure' a dry joint you can use a soldering iron to reflow the joints (perhaps adding a little bit of new solder). If the tracking between the display driver and display has oxidised (and hence won't conduct well) you can either put in a jump wire (make sure you join up the correct points!) or flow a little solder all along the track. - I don't know how much soldering experience you have so be careful and remember: the new joint should look nice and shiny, use as little solder as possible and don't get the joint too hot, you can always let it cool and have another go later, best to practice on a scrap PCB where you can't do any damage.

If the display driver has failed you have little option but to replace it. I'm sorry but I don't know what is compatible. Your best bet is to look at the numbers printed on top of the display driver chip, if you can find one with the exactly the same number (ignoring the date code that goes YYWW, YY=year, WW=week Number) then go for it!




It's hard to tell from your description of the problem (e.g., are you seeing definite "crosstalk" between certain digits, or are the faulty digits always faintly lit, and not responding when they're supposed to display numbers?) but another possible cause (and much easier to fix than having to find vintage replacement chips) would be corrosion/contamination on the display/keyboard PCB. In my experience, enough corrosion can cause a fair bit of crosstalk between digits. This problem is usually more pronounced on calculators with VFD (tube) displays because of the higher voltages, but can affect LED machines as well. So if you seen signs of (greenish/bluish) corrosion on this PCB, I recommend that you thoroughly scrape it off (just don't damage the circuits themselves). Do hope that you don't need to remove corrosion from underneath the chips or the keyboard contacts...



Viktor, it's a case of faulty digits always faintly lit. I haven't actually opened the calc yet (it has a nice back label!), but there was some corrosion evident from the battery, not alot, but some nonetheless. Before I opened the calc I was hoping to have some idea of the chance of sucessful repair. Thanks - John



That doesn't sound good... chances are, it's a faulty driver chip, then.



The other thing I forgot to mention that may be relevant, is that I managed to drop the thing, from about three feet onto carpet, in it's leather case. I did this just after I got it, and before I had a chance to turn it on, so I don't know if this problem was due to the drop or not. I am trying to get in touch with the guy I bought it from, but he's away for a couple of weeks. He didn't mention anyhing like this when he described to to me though..

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