Faulty LED's on HP 55


I have an HP 55 with some faulty LEDs that I'd like to replace, however, first I'd like to be sure that the problem is confined to the LEDs themselves. One of the LEDs has segments that fail to light up when powered, which I assume means that they are burned out. However, another LED has segments which are dimly lit when NOT powered, and become progressively brighter as other LEDs are powered. So, I am wondering if this is indicative solely of an LED problem or some other electrical or IC problem as well.

Also, is there an alternate source of LEDs for this model, other than another HP Classic series calculator?

The calculator is very clean, and there is absolutely no indication of any battery corrosion or outgassing problems.




I have a similar problem on a 45

Specifically a decimal point does not light up. No other problems so I am looking at replacing that block of LEDS.

I also have an hp 65 that exhibits the ghost led effect. That is, one of the LEDs lights up with the digit you have input in a previous block, producing a very faint ghost image.

ex.  input 8.80, fix 2 display

digit display reference 1 2 3 4 5 6 7 8 9 0
should see 8 .8 0
do see 8 .8 0 8

the 8 in position 6 is a very faint image.

Cheers, Geoff

P.S. the circuit board has been cleaned of corrosion and the traces or check for continuity. I don't have an ocilliscope or the expertise to run 'yet'!


It's been a while but I'm willing to take a stab at what I think is wrong. I would first suspect the display driver chips before the display LEDs themselves. If the LEDs are "burned out" due to excessive current then something in the display electronics had to fail to allow that.

As I remember, to reduce the parts count, the display driver strobes through each LED display with "ripple blanking" one side of the LED to turn on and off each element very quickly with the other side being fed with the proper character for it. That's why if you look at a classic calculator display that has been video taped, it will have a flickering pattern as it beats with the 30 Hz of the video scan.

If you are getting dimly lit LEDs or 'ghost' elements, that implies that the display switching is allowing leakage current to partially turn on elements that are suppose to be off. Again, I would suspect the drivers before the display elements though I could be wrong.

I don't know how the driver chips are wired with relation to display so I can't say how easy it is to repair. I do remember that the driver chips are custom HP parts and I doubt that there is a third-party source for replacements. Where the display chips sealed in epoxy? I also seem to remember how the display worked in the classic calculators being discussed in an old HP or PPC article.

Finally, I have an HP-55 that I bought new back in 1974 and I remember being very curious about how the display worked and why it was so bright. I was surprised to learn that there are lenses in front of each LED 7-segment display to enlarge them. In reality they are much smaller and even brighter than they appear in the calculator.




I was afraid the display drivers would be at fault.

In the case of the dissappearing decimal I am thinking it is an actual connection fault as this calculator was severely corroded. I bought it for parts and found out that the PCB is fully functional.

The ghost images would appear to be chip drivers. Fortunately they are IC's and soldered into place. Now to find good spares!!!



No problem, Geoff. I'll see you at HHC 2009. Vacation is set and reservations made.


Edited: 5 May 2009, 6:37 p.m.


Darn tootin,

Hotel booked and I will be presenting an HP 41 restoration paper and slide show; replete with the screw post problem, back case screw head washer problem, LCD replacement (maybe) and of course a general spa.

Included in the presentation (slide show) will be an HP 41 card reader restoration also.

All in aid of the conference theme this year. I was going to present aviation specific programs on the HP 41 but I will leave the programming to the consumate pros.

Cheers, Geoff


Also, is there an alternate source of LEDs for this model, other than another HP Classic series calculator?

In a word, No.

Within the classics, the LED arrays can and will be different. On the back of the package is a letter that is the brightness code. Mixing arrays that are more than one letter apart will be noticeable. Original sets will all be matched to the same letter.

Some arrays have gold leads, some have tin. Mixing them will result in different vertical character positions.

The problem with ghosting segments in the order of occurrence:

1) Bad LED array
2) Leaky keyboard contacts or corrosion
3) Bad driver IC's

You really, really need to get Tony Duells schematic set from HPCC to do any real troubleshooting. Anything less is just guessing. Further, attempting LED or driver IC replacement with anything less than a professional desoldering station will result in board damage.

Just my opinions, your mileage may vary.


Your absolutely right on the "Leaky keyboard contacts or corrosion". I had this problem on an HP 35 with ghosts prior to a restoration.

Lots of 'blue' oxidation and crystalline accumulation from battery outgassing. After a thorough cleaning (vinegar, soap and water, and alcohol) the ghosts had all disappeared.

In fact, nothing displayed!!! Just kidding, it now functions perfectly.



This is a finding of Mr. Timmerman, full credits to him :)


decided that maybe the driver for the left digit needed to be pulled up a
little. This was because it just seemed to sink current during the time that
the other digits were driven, causing the cross-talk from the other digits.

Indeed the problem vanished with a resistor of 150 Ohm between Vcc and k15.
To avoid possible damage and to stay on the conservative side I chose to use
200 Ohm. In the pictures you can see that the display is now much better
looking, though a little less bright than the other HP45 that I have.

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