HP 41CX back from the dead

[Jeff_Birt]

I have had this HP 41CX in the bottom drawer of my desk at work for about 7 years. It was inherited from the previous occupant of my desk. I pulled it out today and realized the LCD was not cracked as I recalled, just the plastic covering was. Hmmm....check EBay....Holy Calculators Batman!....they are going for crazy prices.

The batteries were long gone bad had corroded at some point. The spring contacts cleaned up easily but the flexy circuit board battery contacts were toast and what a perfectly goofy idea for battery contacts. In general the miniaturization techniques used are quit a treat to see, a combination of clever and insane.

When I got the cover off I saw that the impact that broke the plastic screen cover also knocked loose most of the wire ribbons connecting the screen and one of them was missing entirely.

[Jeff_Birt]

I cleaned up the small amount of battery crud that had made its way inside and soldered the ribbons in place. Then I wondered how I was going to power it up seeing as how the flexy board battery contacts were gone. Luckily the flexy board seemed to be made of Kaptan type material so I was able to scrap and peel enough of the clear over layer away to solder on a couple of small wired which were routed through the battery charging port.

[Jeff_Birt]

I set the bench power supply to 6V and about 100ma on the current limit and powered it up. The word 'LOST' appeared briefly, perhaps an omen? I pressed the on key and nothing happened, darn. I poked around a bit more and when I hit the PRGM key a few characters on the right hand side of the screen came on! I played around a bit more and it was all very confusing and uncalculator like. Hitting the ALPHA hit let me type characters and only about 6 shoed up on the right side of the screen. After another few minutes it dawned on me that no all of the LCD leads were making contact.

All the solder joints looked OK so I started ohming out each ribbon to the trace if connected to, just past the solder joint. Three of them on the left side of the screen were open. The troublesome ribbons were desoldered and then it was clear that when the ribbons had gotten knocked loose it pulled up about 1mm of trace. Since the traces were not covered in solder mask I thought I could solder to them, but trying to go from the tiny traces on the LCD to the main PCB would not work as the traces would break again in short order. After puzzling on it for a while I pulled some strands out of a stranded wire and formed them to match the traces, tinned the traces and wire strands and soldered them from the trace to the original ribbon pads on the LCD. The wire strand was then folder back and soldered to the main PCB.

It works! Two bodges and it is working again. I can't sell it in this state as who knows how long my wire strand repair will last, not to mention the batter contacts are shot, oh and the screen cover is still broken...

[Le_Bassiste]

congrats on bringing this gem back to life! 
i've restored many of these through the years, and i found the flex blocks by Diego Diaz extremely helpful:
http://www.clonix41.org/Maintenance/IO_Block/IO_Block.htm

[VK5RC]

Great outcome,  sad someone did that in the first place.

[HighVoltage]

Great repair.
This was an amazing calculator in the days and still is today.
The good days of HP ...

[Jeff_Birt]

Thanks for the link to the replacement flex PCB. I sent him an email just now about obtaining one.

I just measured up the broken bits of the display cover plastic. The thickness was the biggest concern but it is close enough to a standard thickness (about 1/16"~1.5mm) that a new one should be able to be milled out. Now to scrounge through my scrap box to find a piece of clear plastic.

[Eduardosky]

I set the bench power supply to 6V and about 100ma on the current limit and powered it up. The word 'LOST' appeared briefly, perhaps an omen? I pressed the on key and nothing happened, darn. I poked around a bit more and when I hit the PRGM key a few characters on the right hand side of the screen came on! I played around a bit more and it was all very confusing and uncalculator like. Hitting the ALPHA hit let me type characters and only about 6 shoed up on the right side of the screen. After another few minutes it dawned on me that no all of the LCD leads were making contact.

All the solder joints looked OK so I started ohming out each ribbon to the trace if connected to, just past the solder joint. Three of them on the left side of the screen were open. The troublesome ribbons were desoldered and then it was clear that when the ribbons had gotten knocked loose it pulled up about 1mm of trace. Since the traces were not covered in solder mask I thought I could solder to them, but trying to go from the tiny traces on the LCD to the main PCB would not work as the traces would break again in short order. After puzzling on it for a while I pulled some strands out of a stranded wire and formed them to match the traces, tinned the traces and wire strands and soldered them from the trace to the original ribbon pads on the LCD. The wire strand was then folder back and soldered to the main PCB.

It works! Two bodges and it is working again. I can't sell it in this state as who knows how long my wire strand repair will last, not to mention the batter contacts are shot, oh and the screen cover is still broken...

Excellent work!   


You can add more photos of the calculator please?

[amyk]

Given how much they sell for, it's not surprising that someone produced a replacement PCB for them. Flex PCB prototyping services are available, and while not ultra-cheap, are probably still good value.

[Jeff_Birt]

The flex board was a reasonable 22EUR which included shipping. I've attached a few more photos. One is with the back off the unit and the other is a close up of the stacked ICs which I'm assuming is from the factory. I've done something similar with an HP-48 to increase the memory but this is the first factory board I have seen with stacked ICs.

The set of connector pads on the top of the board toward the LCD is where the flex board connects. The main PCB sets on a zebra strip and is held in place by the two screw posts and the back cover. I'll take more pictures when I take it back apart to install the LCD cover and flex board. I'll have to unsolder the main PCB side of the LCD to pill the LCD in order to replace the plastic. Hopefully the wire strand bodge holds up.