Hello! I recently acquired an HP-55. After some cleanup and TLC, reassembly, and a new battery pack it's working pretty well now, except for one issue - no decimal points will illuminate. All other display aspects are fine; the segments & digits are all well behaved. I did some reading up (ClassicNotes.PDF) and suspect the root cause is an open coil in the 1810-0146 quad coil pack. All of the coils are low ohms to common except for the decimal point coil.
Other than taking a 1810-0146 from a 'donor' calculator, two solutions come to mind:
1) solder a small fixed 68uH inductor across the open coil in the pack, or
2) make a replacement "pack" from four small fixed inductors (1x 68uH, 3x 130uH)
Has anyone else faced this issue? I'd love to hear how you solved it.
-tom
I'd try soldering the inductor and see if it works. If so you might decide to leave it and try to find a replacement part. I'm sure there will be plenty of these on Classic display boards lying around as parts recovery - hopefully one not too far away and cheap to post.
I updated the
ClassicNotes file to include coil details and resistances.
Decimal point coil should be around 3 ohm, the others 5 ohm. (out of circuit)
cheers
Tony
Thank you, Tony!
I have some small-ish axial inductors on order. As soon as they arrive I’ll tack one across the suspect coil unit and see if the decimal points illuminate. Hopefully that solves the problem!
-tom
My axial inductors arrived, but I couldn't really find a nice way to tack in the 68uH inductor that didn't physically interfere with the innards of the HP-55. I decided to construct a replacement array from three 120uH inductors and one 68uH inductor, all "1/8W" size. I used a solderless breadboard as a jig to hold them all in place while I trimmed leads and soldered a common "rail" across the top of the four inductors. I carefully de-soldered and removed the faulty inductor array, installed my replacement, and then re-assembled everything.
At power on, instead of seeing "0 00" I was greeted with "0.00". Problem solved.
-tom
You could be inductanced into the HP calculator repair hall of fame :-)
cheers
Tony
Some of the original ones were not that different.
The DP coil is second from left.
cheers
Tony
Oh, funny, “what’s old is new again!”
I’m pleased that the 120uH I used vs 130uH you measured doesn’t seem to make any difference in terms of segment intensity. At least, not to my eyes.
Cheers!
Tom
(03-25-2022 02:53 AM)tomcircuit Wrote: [ -> ]Oh, funny, “what’s old is new again!”
I’m pleased that the 120uH I used vs 130uH you measured doesn’t seem to make any difference in terms of segment intensity. At least, not to my eyes.
Cheers!
Tom
I'd be more concerned with higher inductances as there may be segment bleeding from one digit to the next. That might also put a bit more stress on the transistors inside the Cathode and Anode driver IC's.
cheers
Tony
I had the same concern re:bleed, Tony. I studied your excellent writeup on how the display circuitry works and decided that using 120uH was the better choice over 150uH. The HP display circuitry is genius, by the way. I work for a semiconductor company, and I marvel at this ingenious low-power solution using 1970s technology.
Thanks again for the whitepaper on how it all works - some great reverse-engineering and patent spelunking you did there!
-tom
I had the same concern re:bleed, Tony. I studied your excellent writeup on how the display circuitry works and decided that using 120uH was the better choice over 150uH. The HP display circuitry is genius, by the way. I work for a semiconductor company, and I marvel at this ingenious low-power solution using 1970s technology.
Thanks again for the whitepaper on how it all works - some great reverse-engineering and patent spelunking you did there!
-tom
Nice job using the new inductors!