HP 35 display

[Thomas Puettmann]

Hello!

On my HP 35 one segment of the 7th digit does not light.

If I enter just 8s or 6s the display shows

8888889888.

or

6666665666.

Any hints?

[Dave Frederickson]

(09-23-2024 01:50 PM)Thomas Puettmann Wrote:  Hello!

On my HP 35 one segment of the 7th digit does not light.

If I enter just 8s or 6s the display shows

8888889888.

or

6666665666.

Any hints?

The obvious culprit is a bad display. Can you test the segment with a battery?
Schematics are available here.
https://hpcc.org/cdroms/schematics5.0.zip

[ThomasF]

Hi Thomas,

Unfortunately probably a broken display segment.
If it was a bad solder/connection, then either a whole digit, or the same segment in all digits would be lost.
I had a similar problem on a HP 55 and had to replace the faulty display segment (there are 3 of them, each with 5 digits) from another broken device.

But, maybe someone have another probable cause to this?

Cheers,
Thomas

[Thomas Puettmann]

Thanks a lot for the quick responses!

I think I will leave it as it is until I find some time to look for a not too expensive replacement unit.

[AnnoyedOne]

As I recall LED's and displays had* a lifetime of about 100,000 hours. That's about 11.4 years of continuous usage. Perhaps your HP 35 was left on for 8hrs/day.

* Modern ones probably much longer.

A1

PS: Many of these older calculators are over 40 years old. I wouldn't use them as "daily drivers".

[John Garza (3665)]

Although there are lots of relatively cheap donor machines on the auction site, replacing the LEDs can be tricky. You have to ensure you get ones with similar brightness (letter code stamped on the back), and ensure they are lined up properly, and that all pins are soldered. Usually, you have to replace all three LED modules from a donor machine unless you get lucky and your donor has the same brightness letter code. I recently repaired a 65 with a dim segment on a single digit by replacing all the modules with donors from a 67 keyboard that was a spare parts unit.

-J

[Thomas Puettmann]

I just noticed something in complete darkness, see picture.     You have to enlarge the picture to see it. Could this dimly shining dot be related to the not glowing segment?

The calculator is from my late father. In his company Graetz (Radio + TV, later acquired by ITT) they bought many HP 35 and attached them with chains to the desks. (Does this make sense for pocket calcs?). Some years later the employees were allowed to take them home. Must have been around 1976 when I was 8 years old. I was very happy to see a pocket calculator for the first time. The batteries were already not working properly at that point of time and so the machine went to the drawer. Only some months ago, I put some 3.8 V drone lithium polymer drone battery inside and recognized the segment error. Unfortunately, because, otherwise, the LEDs are a pure joy to look at.

[teenix]

The faint display might be due to the cathode driver for that digit not fully turning off causing a ghosting effect and is due to the multiplexed nature of driving the LEDs. You might notice that it changes when you key in numbers.

There are many instances of failing LEDs either fully or partially and can be simply put down to the age of the calculator. Nothing manufactured lasts forever so sadly that will be the fate of all these LED displays and/or the circuitry that drives them - but not for some time to come of course :-)

Chaining the calculator to a desk is probably desirable as they were new, expensive and probably tempting for someone to 'borrow'.

cheers

Tony

[Thomas Puettmann]

(09-24-2024 06:31 AM)teenix Wrote:  You might notice that it changes when you key in numbers.

Indeed!

If the display is empty and you enter 8s or other digits with the lower left segments on, the faint dot in the 7th digit gets brighter with every keystroke.

If the display is empty and you enter a digit where the lower left segment is off (e.g. 4) the dot disappears immediately.

[Harald]

(09-23-2024 08:27 PM)Thomas Puettmann Wrote:  The calculator is from my late father. In his company Graetz (Radio + TV, later acquired by ITT) they bought many HP 35 and attached them with chains to the desks. (Does this make sense for pocket calcs?). Some years later the employees were allowed to take them home. Must have been around 1976 when I was 8 years old. I was very happy to see a pocket calculator for the first time. The batteries were already not working properly at that point of time and so the machine went to the drawer. Only some months ago, I put some 3.8 V drone lithium polymer drone battery inside and recognized the segment error. Unfortunately, because, otherwise, the LEDs are a pure joy to look at.

Hi Thomas,

Interesting story. It's a shame all that industire is gone now.
The HP35 was so expensive, they got tied to the desks. A bit like using Kensington locks on laptop computers today.
Greatz was in Altena, do you still live in the area? If so you are close to me (I live in Wetter). I can see if I have a suitable donor part and maybe help with the repair.

Cheers,
Harald

[John Garza (3665)]

Poor HP-35s chained to a desk and forced to calculate for their meager ration of electrons...<sniff>

$400 USD in 1973 is roughly $2800 USD today.

And HP sold an accessory to help: a lock-on wraparound case attached to a steel cable to reduce pilfering.

-J

[Thomas Puettmann]

Hi Harald,

my father was the head of the quality control laboratory of Graetz (later SEL, ITT, Nokia) here at Bochum. They did all the bad things with TVs and Radios, like letting them fall from 2 m height and so on, and then inspected the consequences. When Nokia bought ITT in 1988 quality control was immediately shut down to zero. You can sell so many more units this way. Luckily my father was almost retired then.

Thanks for offering your help with the repair. That would be great! I'll send you a PN later.

Thomas

[Thomas Puettmann]

Hi John,

thanks a lot for sharing your valuable experiences with recent repairs. Although I am well equipped electronically (not at least by inheritance), I would not open the HP 35 and remove the instruction plate on the back without indication that a repair would be successful. I'm happy that Harald offers his help.

Thomas

[Thomas Puettmann]

I tried to see what is going on with some magnifier glasses. I should have done this at the very beginning, sorry. Could it be dirt?
   

[teenix]

That looks to be one of the partially failing LEDs that I mentioned, where only some of the segment lights. I guess over time the chemical structure of these LEDs begins to change.

cheers

Tony

[Thomas Puettmann]

Thanks, Tony. With the magnifier glass the dark area looked somehow 3D. I built a small 20 x microscope with a second lense. I see now that the dark area is totally 2D. You were right from the very beginning!

Somehow, this changed my mind. Having inspected the segment that closely, I can now distinguish a 8 from a 9 and a 6 from a 5. At least, if I have reading glasses on. Without no chance.

I will not repair this as long as this does not get worse.

I have taken the image as my Avatar. :-)

Thanks to everybody for the quick and generous advice!

[Edit:] I think we are all getting older, and some things don't work as we would like them to work. I consider the half-defect segment to add some kind of "personality" to the machine.

[AnnoyedOne]

(09-24-2024 10:07 AM)Thomas Puettmann Wrote:  I will not repair this as long as this does not get worse.

Frankly I'm surprised that a 50yr old calculator works at all. I'd "retire it" to a good home

A1

[John Garza (3665)]

(09-24-2024 07:53 AM)Thomas Puettmann Wrote:  Hi John,

thanks a lot for sharing your valuable experiences with recent repairs. Although I am well equipped electronically (not at least by inheritance), I would not open the HP 35 and remove the instruction plate on the back without indication that a repair would be successful. I'm happy that Harald offers his help.

Thomas

HP-35 back plates are available on the auction site, in case you damage the original during removal.
Usually they come off clean if you use a hair dryer to heat the plate. Then roll it flat.
And from what I've read here about your particular problem, I'd say a 90% probability it's an LED module fault. Regardless, you will need a donor calc.

-J