Hi
Lately I've bought 3 classic 2x 35 and 1x 80. Including the one in my junk box all exhibit some kind of ghosting.
The junk box one: Slight enlightening of the middle segment of all digits. Not severe but made difference between 0 and 8 not as clear as it could be. Corrected it by "shortening" one of the coils with a resistor. Found a value which caused no visible fade but also made sure it was dark when supposed to. Shortening with a resistor of the driver coil of the corresponding segment doesn't seem to change power draw any significant value.
The 80. Ghosting of one specific digit.
A 3rd edition 35. One segment dead this is also pretty worn and UV exposed so perfect for spares.
A 4th edition 35 (the one with molded key caps). Heavy ghosting on one digit.
I read somewhere some one else "corrected" such issue by loading the cathode of a bleeding digit. While that works for the 80 and with a low value (high current drain) also for the 35 4th edition that raise power consumption between 10 and 30-40 mA depending on needed value.
Since I already have swapped around the cathode driver of the 4th edition with another from one of the worn ones still no change i also tried swapping the anode driver. Surprise!! Still no change. Kept the original anode driver as I just relied on pressure to try the swap out. That leaves it up to the displays themselves. Took a module from the one with the dead segment. Presto! The 4th edition works perfectly - without any pulling resistors. Yeah the 4th had I marked (luminosity) and the modules in the one with dead segment was H marked. No visible difference though so I'm happy. Really like the 4th edition.
So far 2 working. The junk box one and the 4th edition. Guess I should try taking the last intact module from the worn 35 and put in the 80 and see it that solves the ghosting here. Can be pulled with a resistor to perfect visible result but raises power draw around 10-15 mA depending on how many digits visible.
So what's your experiences with the classics?
Is it really that hard to come by perfect samples?
Seems it's not the driver circuits to blame as I read on some page where one also corrected his 35 with a resistor pulling cathode on a digit. Seems like it's pretty much the displays themselves. Which makes me wonder - did the classics suffer from ghosting digits even back then?
Or is it the module themselves that can't cope with the reverse voltages they see as a result of the complex multiplexing or simply bad quality so some go bad with time?
Normally calculators with molded LED displays are very robust. I've _never_ seen a display module fault on neither TI nor Sharp or any other less known using molded or window chip like encapsulated LED modules. Sure the ones which are a PCB with plastic lenses heat seated or glued on top it happens from time to time they have faulty segments. Of course here the bonding wires are exposed to air and a bit of moist (incorrect storage) and the weak fastened ones let go on either the PCB or the LED chip - here I've both HP, TI and others.
-----
If only someone would display replacements for the classic series. Yeah the modules are long out of production but guess small discrete SMD diodes on a PCB and a mask could do the trick... If not small discrete ones aren't available to just put enough on a PCB.
Edited: 10 Mar 2012, 3:49 p.m.