Dead HP 35

[krazyk]

Oh no, Ebay got me.

I absolutely loved working on my first vintage HP 25 a few weeks ago. The build quality and that electric RED display had me hooked :-)

I then scored an amazing boxed HP 33C complete with everything in EC and even though I was told it had issues, after I installed charged Eneloop NiMH batteries and installed a thin metal taped conductor in its 2 x AA battery compartment, and used slightly oversize screws in its cracked chassis posts to secure the shell it passed its internal self test and works faultlessly so far.


All this success made me then purchase this beauty, an apparently wonderful looking HP35.


Errr wonderful only very briefly for when I opened its battery compartment I was greeted by corrosion EVERYWHERE. I CANNOT upload any attachments to this website for some reason otherwise I would show you.



I tried cleaning the compartment up as best as I could and then tried to apply 3.78V DC to the battery electrodes from a 3 x AAA battery box with negative above and positive below. NOTHING when the power switch was applied. I then increased voltage to 4.1V DC as the HP25 needed more power to be reliable. Nothing at all.

I have contact cleaned the power switch, No Change.

What would be the next step?

Its obvious by the creases in the rear sticker that somebody has been in this unit before as they have folded down the top left and right corners.

Anyway for the price I paid I knew this wouldnt be easy ($100AUD) but I am unfamiliar with the HP35.

Any particular things to test once I open the unit fully?

Worst case I have a lovely non functional example of the first pocket scientific calculator ever made.

Kris :-)

[teenix]

Try and clean up the CPU board if it has corrosion on it. Mild warm vinegar/water solution may help. Thoroughly rinse and dry after wards. Bear in mind that you should ground yourself before handling the circuit board, lest you damage it with static discharge from yourself, best by holding the kitchen sink tap, which is where you can wash the board, the sink will most likely be grounded.

Check the wiring and make sure any corrosion has not broken the wires where they are soldered to the board.

Check the connections sticking up from the lower keyboard which the CPU board plugs into. Make sure they are clean.

Check the battery terminals, the charger port brass connector, and 3 pins are clean.

Do not be tempted to increase the operating voltage. It may damage the old circuits, and that will be that.

If you remove the keyboard from the top case, 8 screws, then check the power switch. The original lubricant HP used will probably be hard and may be interfering with the switch. Clean with isopropyl alcohol. Some proper switch lube should be placed back there, or maybe just a hint of Vaseline for temp use.

If cleaning fails to get it working, I attached an overlay of the CPU board with voltage points that you can test if you have a multimeter. If the voltages are not there, try tracing the wiring back from the battery. If the voltages are there then you might have an issue with the electronics. The only real way to test that is with an oscilloscope to see what signals are on the board.

You mentioned Aus$ for the purchase, I am in Melbourne if I can help.

cheers

Tony

[Hiwi]

Hi Chris,
you have to open the calc. The corrosion as you described is to heavy. There may be corroded cables.

Without openeing there is one thing to check.

The little gold plate at the charger contatcs. The middle of the three contacts is ground. When no charger is attached, the left and right contact are tied together through the plate. Check this.

And measure the current consumption, it schould be approx. 180mA.

The classic series operates down to about 3.7 Volts. Apply max. 4.5V.
The HP25 operating voltage should not exceed 3V!


Ralf

[krazyk]

Thanks guys for your fast responses

And Tony, I live here in Montrose in the outer east of Melbourne :-)
What are the chances of THAT.

I have seperated the shells and inspected the main board and it looks physically PERFECT. The gold keyboard connector pins all look great and are making equal contact.

Tomorrow I will fully seperate the battery box and inspect underneath for corrosion.

I had heard people mention the importance of that GROUND PLATE on the external PSU connector re power. So to understand properly it connects the lower left pin to the lower right pin normally. There was some green corrosion on the earth pin at the top centre when i first started

Wow, I have 25 years of IT repair experience and I STILL cannot upload any attachments in Chrome or Firefox on this website. I would love to send some pictures.

Thanks for your help so far.

Kris.

[HPing]

I use Firefox too and there are a few quirks. Maybe this helps with images (which I would like to see!)

At the bottom of your newreply, there is a section titled "Attachments".
- Next to "New Attachments", click "BROWSE" button
- Select (only) ONE file from the "file upload" popup on your computer, click "open"
- Click "ADD ATTACHMENT" button

After adding all your text and images, just click the "POST REPLY" button as usual.

I think:
>you can add up to 5 images
>if you select "preview post" or delete an image, you can no longer add more images
>image size is limited to 500KB
>some image types don't work (jpg does)
>"INSERT INTO POST" button would put up big images in the text (rather than small thumbnail images like the diagram above)

you can practice uploading images with the test forum. also you will see a few posts highlighting quirks
https://www.hpmuseum.org/forum/forum-18.html

[rprosperi]

(10-08-2024 02:58 PM)HPing Wrote:  I use Firefox too and there are a few quirks. Maybe this helps with images (which I would like to see!)

At the bottom of your newreply, there is a section titled "Attachments".
- Next to "New Attachments", click "BROWSE" button
- Select (only) ONE file from the "file upload" popup on your computer, click "open"
- Click "ADD ATTACHMENT" button

After adding all your text and images, just click the "POST REPLY" button as usual.

I think:
>you can add up to 5 images
>if you select "preview post" or delete an image, you can no longer add more images
>image size is limited to 500KB
>some image types don't work (jpg does)
>"INSERT INTO POST" button would put up big images in the text (rather than small thumbnail images like the diagram above)

you can practice uploading images with the test forum. also you will see a few posts highlighting quirks
https://www.hpmuseum.org/forum/forum-18.html

This is the step that messes up most people just starting to do this. It still happens to some of us...

[John Garza (3665)]

Good old fashioned BAD UI.

The button to 'Add Attachment' should be up near the other editor controls, not stuck by itself at the bottom - the user never looks there.

-J

[krazyk]

Will try adding some pics keeping things simple.

   

Corrosion damage when opened

   

[krazyk]

HP35 Opened Up

   

[krazyk]

Previous image was after cleaning the battery box area too

This shows other side of keyboard PCB after cleaning.

   

[krazyk]

The biggest area of concern is the display area but I note its protected by plastic for good reason.

   

[teenix]

Looking good.

If you are able to test the wiring and the connection between the 2 outer charging posts, with an ohmmeter, that would seem the next logical step. I would remove the CPU board for this.

cheers

Tony

[krazyk]

What I have found so far.

There is continuity between the left and right external PSU pins provided by the gold connector shield when the back shell is installed.

This is a close up of the power switch





   

I noticed that my circuit board looks different to the board that featured in the test point picture.

What do you guys think?

[Geoff Quickfall]

Hello,
Without a set of photos I cannot diagnose the problem.

From the photo I educe that you have opened the case.

1) the grease has hardened on the gold plated switch contacts.

2) I can only assume the grease on the PCA contacts is the same.

3) is the contact clip that is responsible for shorting the right and left pins in the charger receptacle corroded. Is it making a secure contact between the pins.

4) are all the PCA pins clean as well as the vias the pins contact?


To test the board outside the case:

1) the logic PCA must be attached to the keyboard PCA via the contact pins.

2) the right and left pins must be shorted to each other.

3) power (no more than 4v should be applied to the battery contacts.

4) the on switch contacts on the keyboard PCA must be shorted (tweezers) to allow for the LED display to illuminate to the default display of 0.00

# 2 above must be done or no B+ makes it to the logic and keyboard PCA.

Please refer to the following posting:

HP 45 restoration (g.quickfall)

Let us know, and supply pictures!!!

Cheers and good luck, Geoff

[krazyk]

Teenix

Just tested 0 ohm resistance from the following with the PCB CPU board still disconnected

Positive lower battery post to left external charging pin
Right most charging pin to right side red wire positive input on PCB
Upper centre external PSU pin to left side black wire negative input on PCB
Negative battery post to upper centre negative External PSU pin.

So it looks like the basic wiring in the main corrosion area is fine.

[HPing]

I suppose you have seen these electronics and testing pages for the HP 35 from Jacques Laporte (there are 9 bubbles to click on)
https://archived.hpcalc.org/laporte/HW%2...ration.htm

https://www.hpmuseum.org/forum/thread-72...l#pid64562

[teenix]

(10-09-2024 01:19 AM)krazyk Wrote:  Teenix

Just tested 0 ohm resistance from the following with the PCB CPU board still disconnected

Positive lower battery post to left external charging pin
Right most charging pin to right side red wire positive input on PCB
Upper centre external PSU pin to left side black wire negative input on PCB
Negative battery post to upper centre negative External PSU pin.

So it looks like the basic wiring in the main corrosion area is fine.

Sounds good. Next, I would reassemble and connect a charged battery, or 3.75V to the battery connectors, -ve is topmost. The gold bar on the charger port needs to be in place. If it is still a no-go, then I would check the voltages mentioned in post #2.

If the voltages are correct, and still not working, remove the power and check if any ICs are warm to the touch. It might indicate a failed IC.

[AnnoyedOne]

(10-08-2024 10:07 AM)krazyk Wrote:  I CANNOT upload any attachments to this website for some reason otherwise I would show you.

Yeah. Attachments don't show if you "preview" your post. You have to post to see them.

I "preview" the text first but often have to go back and edit the post anyway.

BTW, as you've learnt "the hard way" some eBay sellers are scam artists, others are just plain ignorant, and so on. I check listings very carefully these days. No photos of the actual item (vs stolen ad photos from the web) and its "next...".

A1

[krazyk]

I knew the HP35 was sold as not working but I didnt expect that it was going to be this hard to get going. I dont blame the Ebayers as I have successfully got a Ti 58 and that HP 25 and the HP33C all working and they were all sold as NON FUNCTIONAL.

I have reassembled the unit, jumpered the top psu left and right pins and applied 3.78 volts DC to the battery prongs, neg top, + bottom and have taken voltage measurements from the test points posted in post 2 diagram.

EVERYTHING is down by 20%. So Vcc of 8.2V expected is about 7V, 6V expected is 5V and the -12V expected is -10V.

I am guessing that this implies an issue with the power supply from the main keyboard pcb. The smaller logic PCB appears immaculate but is simply recieving an inadequate amount of power at 7 volts to START.

One interesting phenomena is that the Vcc voltage seems to CLIMB and starts out at as low as 5volts when I first apply power and then rises to a maximum of about 6.9 to 7 volts.

What components could be causing this?
I was concerned about the vias looking very dark and possibly corroded near the display ICs Anode and Cathode. Is there an easy way of testing their logic?

I have a desoldering station and a Siglent DSO although I am still learning how to use the DSO. :-)

I have printed out the circuit diagrams for the 35 but wish I had truth tables for the logic ICs so I could check the display chips.

I am enjoying this mystery, like a detective novel.

Kris

[teenix]

The logic diagrams for the various ICs used in the HP-35 would boggle the average mind :-)

You can check out some basic operation with scope traces in my Classic Notes.

Have you measured the circuit current when operating, with just 0. on the display. (Post #3 180mA or so).

It may be that a component or two are "on the way out", after all the years since manufacture. You might like to replace the capacitors in the power supply section. If the original solder appears black at the joints, they might be a bit difficult to get new solder to "bite".

I attached a diagram locating the caps, these should be available from Altronics or Jaycar(maybe). Through hole tantalums of 16-25V rating should do.

Also, the transistors in the top right corner might be failing. Jaques Laporte mentioned that 2N3904 types should be a suitable replacement. Altronics Part No: Z1200, although you will have to check the pinouts to see if they match.

If the circuit board is non useable, I have a replacement design here that you could get made up. Lot of work though.

cheers

Tony