Hp 25 LED brightness

[Graan]

Hello
I bougt a 25C and a 25 online couple of years ago. 25C was working but not the 25. This weekend i opened it and found some corrosion on the main pcb. Cleaned pcb and contacts and it works. If I compare the LED brightness of the calcs the 25C is a bit brighter.
Is it possible to increase the brightness on the 25?
Read there is a resistor controlling the brightness.
Also a pot at the cathode driver, but could not find what its doing.
/Andreas

[BobVA]

The pot is for adjusting the "low battery" warning level.

If you're really curious about what's causing the different brightness levels you could swap the displays - that would tell you if it was the LEDs or the calculator main board.

If it's the main board you can google up the schematics and check the discrete "Vdisp" voltage to see if it's in spec and go from there if you think it's a problem.

[teenix]

Not really, the display power it tapped from a coil in the switch mode power supply.

I measured about 4V volt on a voltmeter on pin 5 of the ROM0 chip which is Vdisp.

If you think it is too low and the battery low LED indication is not active then you may consider replacing the capacitor and/or diode coming from the coil connection that supplies Vdisp.

The potentiometer is there to adjust the battery low level sense for the Cathode Driver. Mine is set to 1.18V into pin 4 of the Cathode Driver. If you have been twiddling the adjustment, this may not be set properly now.

The HP-25 schematic can be found here...

http://www.hpcc.org/cdroms/schematics5.0...k/hp25.pdf


cheers

Tony

[Graan]

Thanks for replies.

I assembled the calc already. Might take a look later. I measuerd the voltages when I had it apart and remember Vdisp was about 4.1 V
The 25 display is just a bit less brighter than the 25C so not really a big problem.
Since the pcb had some battery corrosion it might be something related to it. Might not have cleaned off all of it.

/Andreas

[GreyUser]

You’ll typically find the difference in the lcd display itself. HP sorted them by intensity and used the lower intensity units primarily in the low cost 21. It would not surprise me to learn they went into 25’s when volumes went up.

The 5 digit bubble displays used on the classics had a brightness code letter printed on the back of the package. This wasn’t needed with the Woodstocks since it was only one unit but they absolutely had brightness variations in the individual led chips, wafer to wafer.