Broken HP-41C (won't turn on)

[dpauw]

Dear forum members,

this is coming from a new member and absolute HP-41C beginner, so please forgive me for any stupid questions / wrong jargon, etc.

I have a HP-41C (square display edges, so "full nut"?).

The calculator was in a drawer for 15+ years and would not turn on. Upon opening it, it turned out that the battery contacts were corroded. Since then, I have taken the following steps to repair the unit:

• I got a new flex PCB from TheCalculatorStore and installed it.
• Cleaned all contacts and battery springs.
• The screw posts seem fine; nothing broken (yet).
• Replaced the capacitors C1 and C3 (they looked pretty blown up).
• Zebra strip does not look corroded and has (according to my multimeter) a clear electrical passage on all sections.
• I tried to install additional spacers around the lower screw posts to increase pressure on the zebra strip.
• When the unit is assembled, I measure around 5.5 V through the expansion ports (when battery is connected). The battery pack - fresh batteries - directly measures in at 6.1 V.
  

Still, the calculator seems dead.

The only reaction: When the batteries have been out for a while and I bring the top and bottom half of the calculator together, I get a short flicker of "hieroglyphs" in the top part of the display. So there seems to be some life left in it.

After that, it goes dead again and will not react to anything.

Does anybody have any idea what more I could try? It's been weeks now and I feel like I have tried any idea I could find on any related forum...

Thank you very much!

Yours,

Dominik                    

[TallKey]

Spacers applying more zebra strip pressure is probably not a good plan.

Calculator was stored rubber feet down (15yrs)?

Mulitmeter indicates depressed ON closing the circuit?

I use the service manual as my map of the traces, if all other steps fail.

What I've done with success, not in this order:
-Clean and dust everything as it is assembled. Always assume dusty circuit pads and zebra strip.
-Alcohol swab the circuit pads
-Use a 10x loupe to verify all is good with the traces, switches and the zebra looks 100%

41 face down, all assembled but for battery holder and no screws:
-do you feel a bit of resistance / sponginess from the upper flex and the zebra flex connectors as you press the calculator halves together? Guessing it is about 1mm of to close the halves. Did the flex strip foam/rubber look OK?

Check switches:
Expect a shiny disk at each switch circuit board through-hole. Actuating a switch with good light and a loupe, you can verify the switch passes a visual. If the switch disk is not shiny, it is probably corrosion in that switch.

I have revived problem 41Cs to 100%. Assumed nothing, took my time.

Have experienced the reassembly of a previously working calculator and it not work. Hurried or I was lazy.

Today I always assume contacts ares dirty / dusty and verify all looks good as I reassemble (using the 10x loupe).

Good luck

[Johnboy]

I have fixed a few 41c/cv/cx calcs, mostly broken screw posts or corrosion due to battery leakage, but maybe not as many as other folks here.

I agree with TallKey about cleaning all your contacts with alcohol, I use isopropanol (rubbing alcohol) or a specific purpose circuit board cleaner spray.

I'm not sure about the virtue of replacing the capacitors, I have never done that. If it's the black electrolytic caps you replaced, make sure you have the polarity correct.

I'm guessing your 41c is not the model which has nuts on the lower two screw posts, so pretty hard to do any testing without the back case half installed.

I'd clean everything thoroughly, using fresh batteries, reassemble and try the various reset methods.

Good Luck

Cheers,
John