HP-15C Won't Power Up - Mostly - Page 1
Forum section: electronics / repair

author: whirligig
author_rank: Contributor
timestamp: November 07, 2021, 10:58:25 pm
content: I'm trying to bring this HP-15C back to life. It's been asleep with the original batteries since new. I've followed every reset method described in the owner's manual to no avail. However, shorting the battery terminals will sometimes produce some strange symbols on display as shown here:I've opened up the case and found some corrosion on the traces around the main chip, 1LH1-0306-85011. I've cleaned it up but the pin connections to the board appears dull like cold solder joints as you can see in this picture:I'm tempted to spread some solder flux and reflow the joints with my solder iron. What do you think? Only thing that's stopping me is damaging these CMOS chips.

author: george.r
author_rank: Contributor
timestamp: November 08, 2021, 12:05:23 am
content: Greetings. Have been in the same spot before - with a 15C as well. You'll want to pop the board off the case by cutting the heads off the rivets, and entirely remove the display module, before conducting any further work. Sometimes it could be just crud under the zebra connectors. It's what I thought was wrong with the 15C I lifted from an ewaste bin. But, alas, no. I noticed the Nut CPU (smaller one) didn't have either phase of the clock (which is derived from an LC oscillator). So I swapped the CPU with one from a 12C. Wrong, that did nothing. So I caved in, and desoldered the R2D2 (larger chip, RAM, ROM, Display Driver), and found an eaten trace underneath that.Happily working now, Best of luck and shout if you need further help.P.S.: the display can be fiddly to get back on properly, but it certainly is doable. As for putting the board back on, there's some discussion online as to the best method of replacing the rivets. I've just kinda hotglued it in place.

author: WattsThat
author_rank: Frequent Contributor
timestamp: November 08, 2021, 12:35:07 am
content: QuoteNO NO NO, A thousand times NO. Do NOT do this as the first order of business, the calculator will never be the same, keyboard wise. While a battery leak can cause an open trace in the RC tank circuit, there is zero need to forever damage the keyboard action as the open trace can be repaired from the component size. Popping heat stakes is the absolute LAST resort. BTW, the symptoms you describe do sound like a dead clock which would be the result if the tank is open.Clean up the obvious damage and see if that fixes it. If not, check the pc board photos over on the HP Calculator Museum and you can see where the LC parts are under the battery box area and follow the traces. I can post the pinout of the nut processor which gives you the RC tank connection points if needed. They might posted over on Eric Smiths site, brouhahacom. I do have loose boards and can provide the connection points if needed.Last thought. Yes, solder joints do fracture but usually in the center area of the board where the most flexing movement occurs. Fractures can easily seen with 5X or more magnification. You don’t want to be spreading flux and reflowing the chips, you only want to repair any visibly broken connections.Edit: the nut processor is the far right square chip. Part number varies. The RC tank lands on pins 7 & 8, note that pin 6 position is blank/missing but counted. If you check with an ohmmeter between 7/8, you should see about 6-7 ohms if the traces are intact. Pins 7 & 8 are on the bottom of the chip, the side closest to the keyboard. If you need help this, let us know and we can markup a photo and post.

author: george.r
author_rank: Contributor
timestamp: November 08, 2021, 01:47:56 am
content: Yeah, but no. What's one to do? Apply soldering heat to the still-assembled display zebras? Soak that and the keyboard area with flux? Proper servicing requires the calculator to be fully disassembled, unless you're content with a crappy job. I'll accept 95% of the original rigidity and a properly-fixed calculator over some pearl-clutching about the keyboard heat stakes.

author: whirligig
author_rank: Contributor
timestamp: November 08, 2021, 03:07:57 am
content: Thanks george.r and WattsThat for the insight. A little history about this. My wife inherited this HP-15C from her Dad which was a present to him by her Uncle who worked for HP in Corvalis. It's likenew and never used but it's unfortunate that the batteries were installed and the calculator went back in its box and forgotten for nearly 40 years. I have my worn HP-15C from 1980-81 and was hoping to use it to compare pinouts but my early calculator is completely covered in sheet plastic that I don't want to peel it open quite yet.I will check the pinouts on the nut processor tonight. I registered last night on the HPmuseum.org site and had posted this very problem on their forum. I will definitely check their site for more info. I hope I don't have to cut the rivet heads off the stakes. We'll see. I have some experience soldering/desoldering SMD's so I may give that a try if it comes to that.Wish me luck, I'll report my findings.

author: whirligig
author_rank: Contributor
timestamp: November 08, 2021, 03:55:05 am
content: Quote from: george.r on November 08, 2021, 12:05:23 amI examined your photo and I see the missing trace!

author: whirligig
author_rank: Contributor
timestamp: November 09, 2021, 03:07:47 am
content: Quote from: WattsThat on November 08, 2021, 12:35:07 amIt's about 1/2 mega ohm. I think I'll scope it and see if there's any oscillation.Update: Just scoped it when I got the weird display to show up. On many of the pins I see a consistent 115hz square wave. When it's powered up with a blank screen, nothing. Well, the next thing I'll need to do is see if I can get the chips off without melting it.