Classic Display Replacement

[Jeff O.]

I finally got around to ordering one of Harald's multi-emulator boards to install in a classic calculator. When I ordered, I had not yet determined in which of my various classic units I would install it. I recalled that I had a couple of HP-35 and a couple of HP-45 units packed away somewhere that had various issues. So I dug those out to find a candidate for the logic board transplant. I found that I had an HP-45 with a good keyboard, fully functioning logic board, but the middle segment of the 11th character in the display appears to be bad, i.e. an “8” is displayed as a “0”. If I recall correctly, this is indicative of a bad LED display module, not a problem with the anode or cathode driver chips. I had another HP-45 with a bad logic board, a bad keyboard (several collapsed keys), but all LED segments in all digits of the display are OK. The candidate for Harald’s board would have been an easy choice if the second unit had the good keyboard, but alas, that was not to be. Classic displays and their issues have of course been discussed many times here over the years, so I did a quick search of the archives. The search seemed to confirm my diagnosis that the missing LED segment means that the 5-digit LED module is bad. So I thought the best plan would be to transplant one of the LED display chips from the second unit to the first. But, my archive search also led me to the following ominous statement:

“Should you decide to have a display module replaced, I absolutely, positively, without a doubt recommend that you do not attempt it yourself, as this is a very, very difficult thing to do on a valuable Classic machine without messing something up.” (in the article found here: http://www.classiccmp.org/calcmuseum/clasdisp.htm )

I have soldered stuff over the years, but do not consider myself highly skilled at electronics repair. Should I heed the advice?

[teenix]

(01-31-2019 07:08 PM)Jeff O. Wrote:  I finally got around to ordering one of Harald's multi-emulator boards to install in a classic calculator. When I ordered, I had not yet determined in which of my various classic units I would install it. I recalled that I had a couple of HP-35 and a couple of HP-45 units packed away somewhere that had various issues. So I dug those out to find a candidate for the logic board transplant. I found that I had an HP-45 with a good keyboard, fully functioning logic board, but the middle segment of the 11th character in the display appears to be bad, i.e. an “8” is displayed as a “0”. If I recall correctly, this is indicative of a bad LED display module, not a problem with the anode or cathode driver chips. I had another HP-45 with a bad logic board, a bad keyboard (several collapsed keys), but all LED segments in all digits of the display are OK. The candidate for Harald’s board would have been an easy choice if the second unit had the good keyboard, but alas, that was not to be. Classic displays and their issues have of course been discussed many times here over the years, so I did a quick search of the archives. The search seemed to confirm my diagnosis that the missing LED segment means that the 5-digit LED module is bad. So I thought the best plan would be to transplant one of the LED display chips from the second unit to the first. But, my archive search also led me to the following ominous statement:

“Should you decide to have a display module replaced, I absolutely, positively, without a doubt recommend that you do not attempt it yourself, as this is a very, very difficult thing to do on a valuable Classic machine without messing something up.” (in the article found here: http://www.classiccmp.org/calcmuseum/clasdisp.htm )

I have soldered stuff over the years, but do not consider myself highly skilled at electronics repair. Should I heed the advice?

Probably a good move. De-soldering chips from old circuit boards can be tricky if you are unsure. LEDs can die if too much heat is applied and you risk melting the red plastic.

A proper vacuum de-soldering tool would be the best solution. Maybe a computer repair outlet would have this item in their workshop and could replace the faulty display module. It should only take half hour max.

cheers

Tony

[Harald]

Hi Jeff,

Your conclusion is correct, you do have a faulty LED module.
I wouldn't say replacing the modul is that difficult. But it does require some experience with a soldering iron. You will also need desolder braid or a desoldering pump. You have to desolder every pin and make sure it can freely move in its hole before attempting to remove the module.

Cheers,
Harald

[Jeff O.]

Harald and Tony,
Thanks for your replies. Except for the one missing segment, the good 45 is in very good condition. I think that for now I will not attempt the repair, lest I end up with two piles of junk. Maybe someday my skill level will improve, or I’ll find someone to do it for me.
Jeff