Woodstock Display Problem

[cjmcc]

I have an HP-29C that is working great except for the legibility of the display. The difference between lit and unlit segments is so small that it can be difficult to discern what is a 8 versus a 9, or where exactly the decimal point appears in the result. (OK, the last one is a trick question since I normally use the FIX 2 mode, but you get the idea.)

Any suggestions on how I can improve the display?

-Chris-

[teenix]

(02-12-2020 08:07 PM)cjmcc Wrote:  I have an HP-29C that is working great except for the legibility of the display. The difference between lit and unlit segments is so small that it can be difficult to discern what is a 8 versus a 9, or where exactly the decimal point appears in the result. (OK, the last one is a trick question since I normally use the FIX 2 mode, but you get the idea.)

Any suggestions on how I can improve the display?

-Chris-

I haven't had a chance to play with these, but I see on the circuit there is an adjustable resistor attached to the cathode driver, so I assume it sets the display current and thus the brightness.

cheers

Tony

[BobVA]

Maybe a power supply problem? I'd check the batteries / connection (supply 2.5v directly to the battery terminals with a power supply if you can), then have a look at the 4.4v "Vdisp" voltage (you'll need to google up the schematic) and see if that's in the right ballpark. The next suspects would be one of the driver chips.

(The adjustable resistor indeed looks suspiciously like a drive current adjustment, but it actually sets the "low battery" warning level.)

[teenix]

(02-14-2020 05:15 AM)BobVA Wrote:  Maybe a power supply problem? I'd check the batteries / connection (supply 2.5v directly to the battery terminals with a power supply if you can), then have a look at the 4.4v "Vdisp" voltage (you'll need to google up the schematic) and see if that's in the right ballpark. The next suspects would be one of the driver chips.

(The adjustable resistor indeed looks suspiciously like a drive current adjustment, but it actually sets the "low battery" warning level.)

Thanks Bob, I didn't know that :-)

The HP29C schematic is here if needed...

http://www.hpcc.org/cdroms/schematics5.0.../hp29c.pdf

Looking deeper, there is another resistor connected to pin #6 of the cathode driver that according to the Woodstock block circuit diagram, controls the display brightness. It may have gone high (or low) resistance or has a poor solder joint. The schematic lists it as 3K ohm.

cheers

Tony

[cjmcc]

I've attached a photo of the display showing 908172635.4

Going left to right, you can see the decimal point is ambiguous - several look possibly lit.
The 9, 0, 8, and 1 are reasonable, but the 7 has an extra segment 'almost' lit, as do the 2, (6 looks OK because that segment is supposed to be lit), 3 and 5 (which look like a 9 and a 6).
   

For reference, the on-line emulator HP29u gives a much cleaner look.
   

[BobVA]

Doesn't seem to be a consistent malfunction (e.g. one segment stuck on or off). That and the ghosting of some decimals would make me suspect some kind of a larger system problem, like a bad Vdisp supply (either low or perhaps with a lot of ripple).

I'd take at look at that first (ideally with a scope), both connected and disconnected and see where that might lead.

[teenix]

(02-14-2020 11:58 AM)cjmcc Wrote:  I've attached a photo of the display showing 908172635.4

Going left to right, you can see the decimal point is ambiguous - several look possibly lit.
The 9, 0, 8, and 1 are reasonable, but the 7 has an extra segment 'almost' lit, as do the 2, (6 looks OK because that segment is supposed to be lit), 3 and 5 (which look like a 9 and a 6).


For reference, the on-line emulator HP29u gives a much cleaner look.

Only 1 LED segment is lit at any one time and are excited in order a b c d e f g h etc

It looks like segment b is lighting along with segment f, ie the 7 and 2 digits.

As Bob mentioned the only real way to check the problem is with a scope. You could then see the signals that are exciting the display with and without the display connected.

Check the [strobe] pins on the anode and cathode drivers. If that signal is "fuzzy" it might explain the ghosting.

cheers

Tony