HP48s won't turn on


Hello all, my HP48s won't turn on anymore. I tried the paper-clip reset a few times and dug around the archives for all the tested listed. It did come on once in several days of trying. It wouldn't turn off so I reset it again- Big mistake. Is it a keyboard button problem? I have the 48S opened up and I found the schematics- I'm not that great of an EE. Any suggestions/test point to look at would be greatly appreciated. Thanks for your help in advanced.


Doesn't it turn on if the ON switch is pushed while gently pressing the panel between the LCD and the keypad?

I have ever had such troubles on some 42s, 32sII and 48G.

Though the cause of a hp42s was the leakage current of the keypad,

the others were insufficient contact pressure by the deformed foam rubber under the keypad connector.

I hope this can help you.

Reparing the hp32sII with faulty keypad



I've tried this and there was no luck. I've applied pressure to the area just above the row A-F - the bezel and no luck. I've made sure the capacitors were fully drained and tried to start things up again.
I found the schematic and will try to see where the power goes.
Anyone know how to send the "on" signal to the chips?
Thanks for your help in advanced


I've done this with a HP48G (that is, bypassed the 'ON' key.

I have photos and will post them in a couple of days.

Of course this requires that you open the machine. I'm not certain how similar this part of the PC board layout is to the HP48S



Thanks for the reply TC I have the 48s in three pieces- back cover/keyboard/main board. I'm looking forwards towards your posting.
Thanks again.


I have posted an image here:
(Thank you to Marcel Flipse' original pictures and schematics.)

Near the center of the PCB (just left of the 'NI9714' paint and below the 'I12' paint) you will see two red ellipses with detail. The 'ON' pad connects to pin86 of the Yorke IC. The 'VBAT' pad is the main Battery line in the calculator. The 'ON' key on the keyboard connects these two points together. I've found a couple of failed HP48s that must have corrosion onder the 'ON' key contacts, as connecting these two points together on the PCB turns the calculator on. I have brought these two points outside of an experimental HP48. This is NOT recommended practice; I've connected a small capacitor between these two points, but there likely should be more protection as bringing this (Yorke) pin out of the machine to the outside world exposes the Yorke to damage from ESD (Electo Static Discharge). An optocoupler would be the safest, but my application will not allow that. I'm investigating adding current limiting resistors and/or diodes to further protect this line. I'll post pics of my mod'd machine. Again, this is NOT recommended.

This is an HP48G board, and I have not yet verified how close this may be to your HP48S. I have a failed HP48SX here (SN3229S......) that I can open up and document if you think it may help.

I have noticed that a 48G I've opened is a slightly different PCB than the HP48G PCB that Mr. Flipse had documented long ago.

If anyone has any details on PCB versions versus datecodes in both the HP48G and HP48S series, this information could be helpful.



Hello TomCee, Thanks for the photo and the locations. My 48S board is similar - I'll check it out with a meter before going ahead. There is no corrosion on any part of the board. I have access to a zoom microscope so when I have the chance I can look at the board under high mag. I'm guessing it could be a crack board/ bad "on" key or one of the chips went bad. Your posting should help me zero in on the problem. Thanks again for your assistance


Try spraying around the keys with contact cleaner. It worked for me on a 42s six months or more ago—it's still working now.


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Never spray "contact cleaners" into Pioneers and 48 keyboards. The keyboards are plastic, not metal and contact cleaners are formulated for metal contacts. Isopropyl is relatively safe but allow the unit to dry for several hours without batteries installed. The worst that could happen is you'll get white hazing in some areas.


I second that NONO!!! and I third it!!!!

By the way, aren't the actual contacts below a shield such that they are protected from liquids entering from the keyboard?

At least in the case of HP48 machines, I've noticed that corrosion seems to travel from the battery compartment up to the keyboard (from the back side). ...and unfortunately, the 'ON' key is in this area.



This was raised in an earlier topic last year, and comments was made that it may not be a permanent solution, and that possibly the effects of the spray would have a shorter life with each spray cycle. Nobody posted that it shouldn't be done.

As the spray goes almost directly to the contacts, and all around is metal, apart from the keys, I can't see why this should be a no go solution. Some plastics are prone to damage, but the 42s has a metal face plate separate from the plastic keys, with a metal plate underneath on top of the circuit board.

No whitening of the keys has occurred and the machine still functions nicely. The little amount of spray residue on them was wiped off after spraying, and obviously doesn't effect the plastic the keys are made from.

I used the plastic straw provided with the can of cleaner to spray between the keys and the face plate. The face plate on the 42s is made from metal, as are the face plates on my 50g and 17B11.

It resurrected my 42s with no damage to the keys, so I don't have an issue with using it again.

After the above posts, I now feel I should have posted with more detail, so If I am still on the wrong track, please show me where damage can be done apart from the keys. :)

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