HP-41CV Fullnut disassembly

[grsbanks]

So, I have a fullnut 41CV that has an unimaginable amount of crud that is wedged between the LCD and the plastic window. Compressed air has not budged it one iota so I'm thinking of maybe using a more drastic cleaning procedure.

This photo isn't my 41CV, but it's the same hardware as mine:



How is the PCB with the LCD and the two driver chips held in place? OK, there's the 15 or so metallic pins soldered both to it and to the keyboard PCB but is there anything else holding it in place? Basically what I want to do is remove the LCD without having to break the heat stakes holding the keyboard PCB in place, so am I going to have any nasty surprises if I unsolder the connectors between the PCBs or will the LCD come out easily enough giving me access to the inside of the window so that I can clean everything out?

And yes, I have already disassembled this unit once to install a 41CL board and the posts, flexes and everything else are in good condition.

Thanks in advance.

[Sylvain Cote]

Hello Godwin,

I have replaced several HP-41 LCDs and I found it quite easy to do.

I unsolder the pins on the main PCB and keep the pins on LCD PCB soldered.

• use a wick to remove most of the solder from all the pins
• then unsolder pin by pin, if you are right handed, start with the rightmost pin and the reverse if you are left handed
• for each pin being unsoldered:
  • use a small fiber or wood pen to put a little pressure between the main PCB and the pin
    warning: be gentle with the pressure, if you put too much pressure you risk removing the pad from the main PCB
  • then unsolder the pin
  • validate that the pin is completely free from the motherboard before going to another pin
    
• once all pins are free from the main PCB you will see that you can remove the LCD unit quite easily
  

Soldering the LCD unit back is also easy,

• clean up all the pad from the main PCB
• insert the LCD unit into its place
• push all pins, nearest to the main PCB as possible
• apply flux then solder the pins
• clean up excess flux
  

Sylvain

[grsbanks]

Thanks for that, Sylvain. That's what I wanted to hear

I'm getting a 41CL Time Clone to mount on the CPU board in the next few days so I'll kill two birds with one stone and get both jobs done with just one disassembly.

How does that crud get in there in the first place? Is it maybe through the keyboard overlay latch?

[Sylvain Cote]

(08-11-2019 08:12 PM)grsbanks Wrote:  Thanks for that, Sylvain. That's what I wanted to hear

Great!

(08-11-2019 08:12 PM)grsbanks Wrote:  I'm getting a 41CL Time Clone to mount on the CPU board in the next few days so I'll kill two birds with one stone and get both jobs done with just one disassembly.

I would suggest to do one modification then test to see if the calculator is still working before doing the second modification.
If something goes bad it will be easier to find the problem.

(08-11-2019 08:12 PM)grsbanks Wrote:  How does that crud get in there in the first place? Is it maybe through the keyboard overlay latch?

The HP-41 is far from being a sealed system, crud can come from the back ports, battery space, etc.

[grsbanks]

(08-11-2019 08:26 PM)Sylvain Cote Wrote:  I would suggest to do one modification then test to see if the calculator is still working before doing the second modification.
If something goes bad it will be easier to find the problem.

That's a very valid point, and it's something that I've always stuck to in the 40 or so years I've been dabbling in electronics. This time, however, it's a slightly different game because of the fragility of the Coconut shell. I really don't want to take the thing apart any more than is necessary.

(08-11-2019 08:26 PM)Sylvain Cote Wrote:  The HP-41 is far from being a sealed system, crud can come from the back ports, battery space, etc.

That's a fair point. I was just surprised at just how much there is in this one. I also have a 41C and 41CX from roughly the same period, give or take a couple of years, and they are both absolutely pristine. Maybe I was just lucky with those two.

[Monte Dalrymple]

(08-11-2019 08:35 PM)grsbanks Wrote:  That's a very valid point, and it's something that I've always stuck to in the 40 or so years I've been dabbling in electronics. This time, however, it's a slightly different game because of the fragility of the Coconut shell. I really don't want to take the thing apart any more than is necessary.

Hint: use rubber bands to hold the case halves together when testing. Then you don't have to put undue stress on the screw posts, and you're not really putting it together. This is what I do when I am testing or updating boards before shipping them. One around the top of the case to get pressure on the battery contacts and one over the keyboard to get pressure on the CPU contacts. I use fat rubber bands from walmart that are big enough to wrap three times around the case.

[grsbanks]

Thanks both for the tips.

I got the LCD disassembled, cleaned and reassembled this evening and it's all working fine held together with elastic bands.

There will be more light tomorrow morning for me to see what I'm doing so I'll fit the Time Clone then.

I love the way you can save this calculator's state to flash memory and then restore it from there when you're finished if the batteries are going to be out for more than a few seconds

Edit: Sunday morning and all is working.