HP15C On Button won't respond

[jeffjensen04]

My older HP15C, circa 1986, has been super reliable. Today it shut off and would not turn on. I replaced the batteries and still no response. I removed the batteries and shorted out the terminals. When I replaced the batteries I received a PR ERROR code. Nothing would remove it except the F button.

Now the ON button won't turn it on, I have to press ON and then F back and forth several times and it turns on. Once it is ON, only the F and G buttons work. I do get a response from the SQ RT and Y^x buttons, but they give a series a strange numbers. It turns off when I press ON but will not turn bback on uness I repeat the ON -> F pattern.

I tried all the reset options I could find. Hold the minus key and then ON and release, hold the decimal key and ON and release, hold the multiplication key and ON and release. None of those options has an affect.

I removed the cover and made sure it was not corroded or dirty, it looks very lean.

Is it dead? Did I lose my best friend?

[damaltor]

Remove the batteries, short the terminals and leave it like that for a few days... and hope the best. but it somewhat sounds like a corrupted rom.
can you make the self test sequences work?

[Steve Simpkin]

One other thing to try is step 3 from the battery replacement troubleshooting section of the HP-15C manual (see below).

[GreyUser]

Run the self test which checks that the program in rom is valid. The test reads all of the rom and calculates a checksum and verifies it matches the original checksum stored in the units rom. If the checksum matches, all the segments in the lcd display will illuminate.

The keystrokes required to test are outlined here: https://www.rskey.org/~mwsebastian/selft...tm#voyager

If it does not pass self test, it is time to remove the case back and have a look. You peel the four feet off and remove the four #1 Phillips screws underneath. Lift a corner of each pad with a round wooden toothpick to avoid scratching the case back.

Once the screws are loose, carefully lift the back off the front half, looking for two very fine gauge wire coil springs that electrically connect the front metal aluminum keyboard surround plate to the case back aluminum plate for ESD protection. They are approximately 1/16” (1.5mm) in diameter, and different lengths, approximately 3/8” and 1/2” (~8 and 12mm) long. The long one lives in the hole by the battery box in the upper left quadrant, the shorter one in the lower right quadrant near the edge of the case.

Once open, count the black IC packages. There is still hope if it is a three chip unit as it’s most likely a fractured solder joint. You’ll need at least 5x magnification to see the failed joint if your eyes are the age of a typical RPN user. Reflow the failed joint(s) and it will be good for another thirty years.

There’s less hope for a two chip unit but have a look anyway, corrosion or a failed solder joint will cause the behavior you describe but a failed rom is more common on two chip units.

If you don’t want to open it, I should be able to tell you if it’s a two chip or three if you post the first five digits of the serial number. I don’t have super-easy access to that database but can get to it if it helps.

[jeffjensen04]

Thank you for the replies. I truly appreciate the help.

I have tried the keystrokes to run the internal tests, but the tests don't run. I tried all of the battery removal and "shorting out" the battery terminal techniques as well. I thought those were working because the PR Error message would come up. The only key that seems to work is the F key. That clears the message but nothing else. The ON button will shut it off but won't turn it on.

The broken connection intrigued me. I re-opened the back and maybe this is the issue. I attached two pictures of one of the connections. I don't know if this is a broken connection or if it was never supposed to be connected. There are a lot of unused tabs (not sure the correct name). If someone can let me know if this is supposed to be soldered to a specific path I can try and fix it.

THANKS!!!!

[teenix]

It looks purposely disconnected as there is no solder residue on the PCB pad beneath the pin.

cheers

Tony

[damaltor]

Those pins with the shortened pins with no solder on the trace are purposefully disconnected.
you could use a wooden toothpick and gently (!) push on the pins of the chips which are soldered to the pcb below. if any pin moves (in respect to the pcb) the soldered connection has failed.
if you can use a soldering iron, you might as well just resolder all of the pins anyways. Use leaded solder if possible.