Transplanting 15C chips to 12C PCB

[Marcovecchio]

Last week I saw an HP 15C for sale, for a low price, because it had failing segments on the LCD. An easy fix, I thought, so I bought it.

Yesterday it arrived in the mail. Indeed, some segments didn't work, so I opened it. I got shocked when I saw the PCB situation: LOTS of blue corrosion, PCB traces and pads lifted. I tried to clean and made things worse.

   

But I noticed it's a newer model, made in 1988. It uses a PCB very common to HP 12C models made in the same time period. I immediately opened a 12C from my stash (which had been opened in the past, so I didn't have to pop all heat stakes) made in early 90s, and to my happiness, the PCB was identical, and perfect, free of any dirt or corrosion (12C above, and 15C below):

   

Then I started the surgery: to remove the ICs from both boards, and swap them (I have more pictures, but I can't attach more than 5):

   

   

I had to take EXTREME CARE with the IC leads, since they bend easily, very different from other SMD ICs I worked with. I also turned my soldering station to a very low temperature to avoid heat damage to the ICs. After two hours of delicate and tedious work, here goes the result:

   

Success! I was not sure, but now I know there are no other differences between 12C and 15C calculators of the this time period, all differences are inside these ICs. I just wanted to share this fix with you all. When I have some more time, I'll try to rebuild the corroded traces of the other PCB and fix it as a 12C.

Best regards!

Eduardo

[BobVA]

Very nice work! Thanks for keeping an HP-15 in service!

Did you use a hot air station? Or one of the low temperature alloy removal products?

[Marcovecchio]

Thanks!

I used a regular soldering iron (temperature controlled, set to about 260 Celsius), and a low melting temperature alloy. I think it's the best method, to keep temperature to the chip as low as possible.

Eduardo