HP65 display issue

[SteveO]

Hi, Have just purchased a HP65 and found the second digit on led display is faulty, (only displays the top bar of the 8) see attached pic. Would someone know if this is repairable or if I can replace the whole display? If anyone could help it would be greatly appreciated.
Thanks in advance.

SteveO

[ThomasF]

Hi SteveO!

The display in the classics consists of three modules with 5 digits each.
Most probable the first (leftmost) module is broken. I had a similar problem with a HP-55 calculator, where some segments in the middle module was lost.
In some cases it could be a bad connection, but then either the same segment in all digit is lost, or a complete digit, is this case it is neither.
And if you look with a magnifying glass you could also see that only parts if a complete segment (which to me looks like it consists of three smaller segments) light up - making a "dot" instead of a segment ("line"). (Probably more noticeably if you fill the display with "5" - it seems as if the top segment is only a "dot" in your case.)

Fortunately (depending on how you see it), the display module is the same in all classic models, so if you have the possibility to get some parts from another (non-functional) classic model, you can replace the single module that is failing.

This I did with my HP-55, I got a replacement module from a HP-45 that was broken, and I'm very happy with the result! Check the before and after pictures below!

   

Check out Teenix document (http://teenix.org/ClassicNotes.pdf) if you want some more information about the display in the classics - and if you have the possibility to replace it - remember the tip I got from Teenix that if the part gets too hot you risk damaging the internal LED segments as they don't like a lot of heat - but if you are familiar with a soldering tool - it should not be any problem!

Best regards,
Thomas

[SteveO]

Hi Thomas
Thankyou for the information that really helps, don' t have a spare classic to get parts from yet but saw that vintage HP led modules are also available on ebay so just got to make sure correct model for the classic. Going to purchase the HPQDSP 6061 led module (see pic) as looks the same as in the calculator.
Thanks again for the information.

SteveO

[ThomasF]

Hi SteveO,

The modules that I removed from the 45 and 55 are both labeled "HP 1990-0335".
If these are compatible with the ones you have found I don't know.

EDIT: Added link with more information:
https://www.keesvandersanden.nl/calculat...isplay.php

Best wishes,
Thomas

[GreyUser]

Please be aware that if you are attempting to repair the display in a classic, replacing one 5 digit module with a random part will in all likelihood not match the other two with respect to intensity.

Why? Because HP graded the part by intensity and indicated that with a code letter on the back of the device. I've been told that HP allowed for one letter of difference in the three LED packages but personally, I've never seen this, all units I've repaired always had the same intensity code parts.

IIRC this is mentioned in the linked keesvandersanden article.

Your best bet may be to look for a cheap 45 or 80 that can serve as a parts donor and replace all three. Only in that way will you be assured the displays will match in size, intensity and alignment, with the first two being out of your control and the latter being a genuine challenge for the installer.

You will absolutely need professional vacuum through hole desoldering equipment. The boards are too fragile to be attacked with hand pump desoldering tools and solder wick will not get enough of the solder out to free all the pins of a device simultaneously. While you can certainly cut a defective unit out and then remove the pins and clean up with solder wick, that doesn't make sense for otherwise good units that have to come out due to intensity differences.

[ThomasF]

That can be seen in my "after" picture, the middle display is slightly brighter then the other two. In my case, the first module I changed apparently already was broken (as the 45 calculator was DOA I couldn't have known, but at my second attempt with another module I got it working. The brightness issue is seen in the picture but not really noticable for the naked eye.

I'm used to solder my own boards for hobby, and I had no problem desoldering the modules with a non-professional solder iron, and with a cheep de-soldering "gun", just being careful when doing it. It might be I was lucky, both with the soldering and with the modules.

Anyhow, that was my short story on how I managed to get the 55 working again and I'm happy to share that story.

Best regards,
Thomas

[teenix]

Hi all,

I'd be using a soldering heatsink like this for removing the good displays, especially if using de-soldering braid. They are cheap to buy.

cheers

Tony

[ThomasF]

And I use a de-soldering tool like this, it combines the soldering iron and the solder suction pump, so I can heat and remove the solder very quickly without heating the components too much. I have found it very efficient.

Cheers,
Thomas

[GreyUser]

The use of heatsinks is a fools errand. They don’t work for the application being discussed. In fact, they’re a general waste of time.

A integrated vacuum pump desoldering tool is the only effective way to extract the displays. There is more to component removal than just buying the appropriate tool, technique is just as important. While it is very dated, this video provides an excellent overview of the process as it applies to older HP through-hole boards.

https://m.youtube.com/watch?v=e8KRPFOD1RE

[teenix]

(08-08-2021 05:03 PM)GreyUser Wrote:  The use of heatsinks is a fools errand. They don’t work for the application being discussed. In fact, they’re a general waste of time.

Maybe, maybe not, but even so it can't hurt to try for a bit more protection on these rare parts

cheers

Tony

[GreyUser]

Soldering requires raising the temperature of the connection to point the solder liquifies and can be vacuumed away. Given the thermal mass of the tip being used and the required final temperature, it becomes a question of time. How long does it take to reach the required temperature? The absolute minimum time required is the goal.

Heatsinks create the opposite of the desired action as they require the application of a higher heat, for longer times than would normally be required to overcome the added mass of the heatsink. Anything that is removing heat from your connection requires you either use a higher tip temperature, applied for a longer time or a combination of both.

In this specific case, the thinner than normal FR-4 board, which heats faster than a thicker board, sets up the likely possibility of lifted pads. It’s the board that is at risk, not the component.

[teenix]

I whole heartedly agree with those points, especially the time element.

I've been using a soldering iron for 50 odd years now and still learning things especially with surface mount components - mostly that I need new eyes. I did learn in my early teen years, don't solder in bare feet. The iron slid off the table and the tip landed between my toes. Sizzling your toes like sausages is not much fun. I remember my father thought he knew how to solder better than me and got me some acid to dab on the PCB connection to help the solder wet - obviously watched a plumber once. It worked well of course except for the stinging nasal passages and a ruined project after a short time. I was too young to know the dangers myself, but that little bottle of acid soon disappeared into the garden.

I've used a Letter Clip on some DIP IC's including the HP displays which contacts all the pins. I have a temperature controlled de-soldering gun and is usually on the PCB for 2 seconds or so which is enough time to melt and remove the solder and move the pin away from the side of the plated hole (if there is one), so it comes out easy. Longer time can be required depending on the oxidization of old solder and how much of it has leached onto the pad on the other side of the PCB and up the side of the pin. Re-soldering before de-soldering can help here.

To my mind, the IC pin is so thin in the PCB hole that most of the applied heat will be on the solder. I haven't noticed any problems having the clip on. A worse condition is any large copper surfaces on the PCB drawing the heat away. This heat does travel up the pin of course, and the letter clip is warm after all pins are de-soldered. This suggests that a lot of the heat transferring up the pin (not all) has been trapped by the clip.

Its hard to say how much heat will hurt these old ICs, but a lot will be working on their last legs (oops - bad joke) and any excess heat won't help. I've got a partially dead HP display, maybe I should see how long it takes for the iron to ruin any of the remaining LEDs.

cheers

Tony

[aurelio]

(08-08-2021 05:03 PM)GreyUser Wrote:  The use of heatsinks is a fools errand. They don’t work for the application being discussed. In fact, they’re a general waste of time.

A integrated vacuum pump desoldering tool is the only effective way to extract the displays. There is more to component removal than just buying the appropriate tool, technique is just as important. While it is very dated, this video provides an excellent overview of the process as it applies to older HP through-hole boards.

https://m.youtube.com/watch?v=e8KRPFOD1RE

Dated maybe, as you wrote, but very impressive, thank-you for sharing.
Aurelio

[[kby]]

My preferred desoldering tool has become the Hakko FR-301 (there are different suffixes depending on your country’s electrical setup; the 301P is the US model). Not particularly cheap (about $250–350 IIRC) but it’s quite good. Has temperature controls and different tips. The default one should be ok for this job.

https://hakkousa.com/fr-301-portable-des...-tool.html