HP-45 display issue - segments stuck on

[Dave92F1]

Hi, all! I have an HP-45 with a display issue - in all digit positions the decimal point (.) and the lower-right digit segment (the bottom segment of "1") are stuck on. (The LEDs themselves seem fine.)

Otherwise it appears to work OK. Is there a known fix/place to look for the source of this problem?

[teenix]

(02-18-2023 05:39 PM)Dave92F1 Wrote:  Hi, all! I have an HP-45 with a display issue - in all digit positions the decimal point (.) and the lower-right digit segment (the bottom segment of "1") are stuck on. (The LEDs themselves seem fine.)

Otherwise it appears to work OK. Is there a known fix/place to look for the source of this problem?

Normally, when all decimal points are on, is an indication that the battery is low and should be charged. The Anode Driver IC 1820-1029 is responsible for detecting this condition by monitoring the battery voltage and comparing it to a reference voltage. This reference is controlled by an external resistor connected to pin 10 of the IC (330K for the HP-45).

Possible causes are flat battery, non standard battery, poor battery connections including through the wiring and key to CPU board connectors or dirty power switch, change in resistance of the 330K resistor, corrosion around the IC or resistor, faulty Anode Driver IC.

It is difficult to determine why just the LED segment lights across the entire display. It could be a short in a display module, or the keys circuit board, (possibly with the decimal point), or the Anode Driver has failed, or some other component.

If the there are no shorts and if you can test the signals going to the Anode Driver with an oscilloscope and they appear ok, then I would suspect the driver IC.

As the Anode Driver IC is responsible for controlling the (somewhat high voltage) display inductor coils, it is possible that it may have failed and caused an internal short.

If you are interested, more info on the display can be found in my Classic Notes download.

cheers

Tony

[Dave92F1]

I don't have a battery pack or a power supply so I was giving it 3.5 volts from a bench power supply. (Maybe that wasn't enough.)

(02-19-2023 12:48 AM)teenix Wrote:  It is difficult to determine why just the LED segment lights across the entire display. It could be a short in a display module, or the keys circuit board, (possibly with the decimal point), or the Anode Driver has failed, or some other component.

If the there are no shorts and if you can test the signals going to the Anode Driver with an oscilloscope and they appear ok, then I would suspect the driver IC.

I'll check that out tomorrow. On the chance that it's an Anode Driver, are working replacement parts available somewhere?

[teenix]

(02-19-2023 06:59 AM)Dave92F1 Wrote:  I don't have a battery pack or a power supply so I was giving it 3.5 volts from a bench power supply. (Maybe that wasn't enough.)

I'll check that out tomorrow. On the chance that it's an Anode Driver, are working replacement parts available somewhere?

The batteries typically are 3.75V, (written on back label somewhere) so maybe 3.5V is fooling the Anode Driver.

You could ask Forum members if one is available.

cheers

Tony

[Dave92F1]

Thanks again. The decimal points was definitely the voltage - at about 3.6+ volts the dots go out.

But now I have different problem - the stuck-on segments are OK now, but the upper-left segments on the 5th thru 9th digit positions (from the left) are NOT illuminating.

They were before!?!?

[teenix]

I assume you are using a regulated 3.75VDC supply for the calculator power.

There are 3 x 5 digit LED modules for the display, maybe the middle one is faulty.

Does it revert to the previous way of working when back at 3.5V?

Have you measured the power supply rails inside the calculator?

cheers

Tony