HP-65 sensor chip failure

What are the symptoms of a failed chip (from over voltage via charger)?

Does the drive cease to operate?

I am in the middle of restoring a 65 and I just corrected all the damage from a severe battery leak and before I get into the gummy wheel repair, I'd like to know if the chip is good.

If I start a card into the slot, the drive does come on, but because the wheel is toast, I can't tell if a program will load.


I believe that the motor would not run if the card sensor were bad, but I can't say for sure, since I've never had this situation. Perhaps an expert like Randy can say for sure. I'd do the gummy wheel repair, regardless, and just see if it reads/writes cards correctly. You may also need to repair the motor/worm gear coupling (aka clutch).

Also, it's not certainty that the sensor chip will fail if operated from the AC adapter w/o a battery, just more likely.

Edited: 1 Aug 2010, 12:19 p.m.


Well I think it is OK. I have replaced the wheel and now have a smooth running card reader.

Smooth running EXCEPT it doesn't appear to read or write a card. There was a fair amount of crud on the head. Anyone have a suggested method to clean? Could this be the source for the read/write problems?

Also, any suggestions on whether I should replace, touch up or glue back on the label? What would retain the best value?

to my dismay the small piece of black flaked off due to the battery leak (yes it was pretty bad)


I use GooGone to disolve any buildup and then isopropyl alcohol to clean the heads. This may solve the read/write problem or it might not. There are many other possible reasons for read/write problems, including bad reed switch adjustment, incorrect card reader speed and bad circuits. If there is an error during card reading, you will get a blinking display, but also if you attempt to read a blank card. During normal card reading, the display does not blink and does not change.

As far as the back label is concerned, I have always simply replaced it with a replica that you can buy on TAS. As to value, I don't think about such things, since I buy calcs to repair/collect rather than resell, but IME when I've decided to sell excess inventory of a given model, I've mostly at least recovered my "investment". I noticed that the bottom feet are missing; did you remove them? Also, you have a Version 2 of the HP65, based on the serial number label being located outside of the battery compartment. Were the card reader wire connections sockets or soldered?


Have you checked the resistance among the two couple of wires of the read/write head? You must disconnect the wires from the sense amplifier PCB and read the value between the couples red/yellow and blue/orange to know if all the coils are ok.
Let us know about

Don't worry and try again with your baby!


BTW, if you need a couple of green feet, I can help you.



Good on the green feet, they are in the "box of parts"

wires are soldered into the board. I will try the goo gone/alchohol first, then move to the resistance checks. Presume that some resistance is ok and I am primarily checking for open circuits.

I too am in this for the challenge of fixing these vintage rigs and don't plan to sell this one unless I come across a better one.

How can you tell if the card WROTE to the card? The display does blink indicating nothing was read, but I don't know if it's telling me it can't read or if I'm reading a blank card.

I have no period cards with this machine, only blanks ones from my 41C.


Try writing a program to a card first, and then reading it back in.


Tried that, it blinks.


OK, then at least you have a problem with the card reader writing cards. This doesn't necessarily mean that it won't read cards, but you would need a card that had been written to test it. You can read a card written for the HP41C into a HP65 w/o getting a blinking display, although the program won't execute properly to test the read function of your card reader.


I could send a tested card on my '65, so you can test the reading capability of your calculator. It's possible that it be able to write but due a bad capacitor, it does not read properly the cards.




I tinkered some more. Cleaned and inspected the wires. None appear to be broken @ the board.

It will read a 41C program on some of my cards.

It will not read a card that I tried to write. Does it give any feedback when it writes a card correctly?

Ignazio, thanks for the offer. I think I'll pass as it does seem to be reading. Now I need to fix the writing.

I suppose the next step is to detach the wires and check the resistance across the coils.


The '65 use a 6 bit code instructions, so it's not able to read cards from '67 or 41C. Unfortunatelly it don't leave any feedback after a writing operation. And if you are not SURE that your calculator is reading correctly the cards, you must detach the five wires and check them with an ohm-meter to verify the coils.

Lookin forward for good news.



Measured 50ohms across the following:
Remaining wire is Black. It measures about 15 ohms to the outer shell.

Sounds about right, agreed?

Must be a problem in the mechanical drive. I will try to adjust the eccentric to get better force.


yes, the value of resistance is ok. The black wire is the ground, and must be connected to the body of the magnetic head.
And now? If you want I can send you a card with a trivial program to verify the read capability of your reader.
Let me know your address.



Well through continued help from Ignazio, I am slowly trying to find the fault.

I have connections between the motor control board and the Sense Amplifier Chip. It seemed I had found a fault when I discovered high resistance from RA connector to the RA pin on the IC.

Well, I jumped the trace (the motor control board has seen severe battery acid) and it STILL doesn't work.

I'm Stumped. I don't have a background in electronics so this question may be dumb, but I can't check the caps & resistors while soldered in, correct?

I also can't seem to be able to correct the break in the RA trace without desoldering the IC from the board. Is this advisible?

One thing I noticed playing around with the head is that it is slightly loose. If I push it in a little, the card will not go through and jambs up. If I wiggle it out, it allows the card to pass.

More suggestions are welcome, let me know if anyone has spare parts to buy!

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