Spice Calculator - dim display - HP38C

[James Linder]

I repaired the battery terminals of an HP-38C recently and installed
some replacement chips because the calculator would not boot up.

Now everything works fine except the display is about half as bright
as it should be. I'd like to know if this is likely due to the internal power
supply not putting out the correct voltage or current to the LED display
strip, or if this is likely due to another issue.

I know that the battery is good and that the LED light strip is good by
testing both in other Spice calculators.

Your help would be greatly appreciated.

[teenix]

The LED display current comes from the power supply unit.

2 Transistors and 2 resistors form a constant current supply for the cathode driver inside the main CPU IC.

The 16R resistor sets the current so if this has gone higher resistance then the current will reduce.

It may be worth checking these components, but my "stuff" is in boxes at the moment and I'm not sure where on the board they are located.

cheers

Tony

[James Linder]

As usual you know how to repair these. Thank you very much for your help!

If you can identify position of R16 and let me know proper resistance I can replace it.
Would it help if I send you a close up picture of power supply to look at and mark up?
I have several power supplies for parts so I can either swap parts or install new resistor for R16.

When a continuous memory Spice power supply goes completely dead, what usually goes bad and what do they look like and approximate position on board? I’m guessing this is probably a capacitor since they are usually the weakest link over time.

What is that brown component on the continuous memory power supply circuit that doesn’t exist on the power supplies without continuous memory? I’m guessing this provides power to CMOS memory when calculator switch is turned off.

Did HP put a dot on capacitors to identify the negative side connector?

James Linder-USA

[teenix]

I found a supply buried in a box and I marked the 4 components on the attached image.

Hard to say what might die in the supply, caps and transistors are probably the main culprits.

I think my supply is from an E model, so not sure about the brown "thingy"

If you have a spare supply, that would be the easiest to replace and try out.

Also, check pin #8 of the large CPU chip, it is the constant current input for the displays. If it is an unsoldered version, it may not be contacting the flexible circuit properly. You might notice some changes in brightness if you apply some pressure around this area while turned on. The current also passes through the power supply ribbon connector (pin nearest the center of the board) so this might not be connecting properly as well.

cheers

Tony

[James Linder]

Thank you Tony for your help! Spice calculators can be quite the challenge!
At first I wasn’t that keen on them but eventually came to appreciate them more because of the larger displays. I still find the symbols on the front of the keys difficult to see but I like the size of the unit. Charger connection isn’t the best and it’s obvious that HP made these cheaply. Even the plastic case is a more brittle plastic. Big difference from the military grade Classic series!