HP35 Startup


Calc has been unused for ? years & had no power supply.

Applied power supper Marked HP 67, obtaine thru your want ads.
All I get is random nos and alternate dots.

No response to keys.

Is there any way to bring it back to life?

I can still keep in my minimuseum and sell the power supply!


You need a working Nicad pack - this calculator won't operate without it. If you don't have a new one, it's not too hard to rework it or purchase a new on ebay.


This machine will work just fine without a battery pack installed. The power supply for the classic series machines delivers a filtered and regulated volatge to the electronics and a separate voltage to charge the battery pack. It sounds like the main electrolytic filter capacitor in the power plug is bad (a very common problem).


Or .........

The one of the wires from the netadaptor to the plug for the calculator is broken, I have have seen that once in one of my adators.
If you have a voltmeter, you can check easily. Connect the - to the central pin, the left and right pin should measure around 6 volts for the left and 15 volts for the right pin.




Since he is getting random numbers and dots at least the ground and main supply wires are most likely good. When the filter cap goes bad you get a pulsing power supply voltage that causes the display to flicker and generate random garbage.


Thanks for the prompt respnse.

I get the voltages Ronald mentioned.

I dont get flickering of display, just a difernt set of random nos when I turn it on.

How else can I trouble shoot the capacitor, and the calculator?


The power adaptor should be OK. I say that because I sold that one to George. I have been using it with my classics (HP-65 and HP-67) for quite a while and I checked the voltage before I sold it. It is probably something with the calc itself. Just to be 100% sure I would suggest that you get yourself a battery holder for 3 AA(A) alkaline cells and connect them with the calculator without adaptor and see what you get. You can buy those in Radio Shack for about $5. As a matter of fact I use those as battery packs in some of my classics and I put NiMH AAA inside. This way I can charge spare cells outside of the calculator itself.

George, if it turns out that the adaptor is faulty I will gladly refund your money.



Yes, it sounds like the charger is good. Could be something in the power on reset circuit on the CPU board. Tony Duell has some excellent notes on the innards of most of the classic machines.


Is there anything else I can do checkout/start up the calc itself, since charger is OK.

Im not too interested in major repairs, as I can still include it in my personal museum of souvenirs, which fits in a shoebox

I thot it was worth a try before deciding its fate!


This is probably too simple to be your problem, but have you cleaned the on/off switch? Dirty contacts here often cause a flickering/random digit phenomenon. Cleaning is quite easy to do. If you have not done so and would like to try and need any guidance, please ask.

Edited: 3 Sept 2004, 10:56 p.m.


Would like to hear Jeff Os method for cleaning on-off switch, or any other ideas for vitalizing my calc.


General disassembly instructions are here. More details from memory:

  1. remove the 6 screws that hold the back of the calculator. Two screws are under the lower two rubber feet. Don't remove the feet, just lift the outside edge of each one to access the screws. Two screws are in the battery compartment. The other two are under the instruction label at the upper right and upper left. Either peel back the label or poke holes in it. When you remove the back, the shorting contact in the battery connector well will be free, make sure you don't lose it.
  2. remove the 8 screws that hold the main board (called the
    spine in Dave's instructions) to the front of the
    calculator. No need to remove the small board that has
    all the electronics on it. It is connected through a 25
    pin connector at the bottom, which is usually just a
    friction fit with springy contact fingers, but I have also
    seen it soldered (I think).
  3. with the calculator keyboard-down, lift out the main
    board. The keys will be free at this point, so if you
    turn over the front half of the case, they will fall out.
  4. turn the main board over, on the front will be the key
    contact strips and the LED display at the top. At the top left right below the display are the ON/OFF switch contact pads. Clean these with the contact cleaner of your choice. The plastic switch slider itself will probably still be sitting in the front case. On the back of the switch slider you will find a springy metal contact. Clean this piece.
  5. lubricate the above contact that goes under the switch
    slider and the contact pads on the main board with a
    lubricant of some kind. I believe Radio Shack sells a
    silicone contact lubricant that works.
  6. while you have it open, clean all of the switch contacts
    as described here.
Also lots of good information here.

Good luck, ask if you need more assistance.


While the machine is apart it is a good idea to clean the shorting bar in the calculator charger port and the charger port connector pins. Also slightly retension the contacts by bending them slightly outward so they put more pressure on the charger pins whn the charge plug is removed. The same sort of retensioning should be done to the power switch contacts.

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