fighting with a HP41c

[aurelio]

the match is always lost

happy new year to everybody....

I plugged into a not working tall key a block, restored with the Diego's kit, but the calculator still does not show signs of life.
Cosmetically very nice, the calculator does not seems apparently to have defects, I tested by changing the parts, the CPU, the "golden" connector, and they work, the problem seems to be located in the part which contains the display and the keyboard.
Apparently nothing to be remarked ..no corrosions, I mean, on the PC board interruptions or not soldered points, nothing that I can see.

I'm scared to disassembly the keyboard and the display to investigate the "dark side of the moon", for the reasons I wrote in older posts: I don't want damage the display with the soldering iron.

Is there a method to check the display without disassemblying it, and to check the healt of the two toogle switches?

[Raymond Del Tondo]

(01-01-2017 06:46 PM)aurelio Wrote:  the match is always lost

happy new year to everybody....

I plugged into a not working tall key a block, restored with the Diego's kit, but the calculator still does not show signs of life.
Cosmetically very nice, the calculator does not seems apparently to have defects, I tested by changing the parts, the CPU, the "golden" connector, and they work, the problem seems to be located in the part which contains the display and the keyboard.
Apparently nothing to be remarked ..no corrosions, I mean, on the PC board interruptions or not soldered points, nothing that I can see.

I'm scared to disassembly the keyboard and the display to investigate the "dark side of the moon", for the reasons I wrote in older posts: I don't want damage the display with the soldering iron.

Is there a method to check the display without disassemblying it, and to check the healt of the two toogle switches?

Happy New Year, too:-)

Normally there is no need to actually disassemble/dismantle the LCD, however the solder joints between the LCD and the keyboard pcb should be checked first.

I had several 1979 and 1980 HP-41C units where the solder connection of one or more joints between the LCD and the keyboard pcb was broken. On some units you won't see the broken connection on first view, since the foot of the metal feather hovers only a few um above the solder tin landing point, but with a magnifying glass and a small screwdriver, which you can use to gently press on the metal strip, you might be able to find 90 percent of the defective solder joints, and then resolder the ones in question.

If it still doesn't work, you could resolder the remaining solder joints.

And if it still doen't work, check and maybe replace the zebra strip between the CPU and keyboard pcbs, which is another common source of malicious contacts.

If it doesn't work after all this, maybe the LCD is defective;-)

As usual, soldering in this area should only be done using a suitable low power soldering iron for electronic gear.

[Brad Barton]

(01-02-2017 06:24 PM)Raymond Del Tondo Wrote:  As usual, soldering in this area should only be done using a suitable low power soldering iron for electronic gear.

As a bit of a soldering rookie, I'd like to get some more detail on this. I've got a pretty decent (to me anyway) Hakko soldering station. The temperature is adjustable, and I usually have it set around 700-720 degF for most general soldering jobs with a medium sized tip.

From what I read a smaller tip (which I have available, but haven't used) would concentrate the heat in a smaller area, and not need to run so hot to accomplish the same job.

I assume in the case of the LCD leads to my 41C, I'd be using the smaller tip. Can you give me any idea what temperature I should be running at, and any hints on technique (time of contact, use of flux, etc) that could help keep me from damaging the unit?

Thanks,
Brad

[aurelio]

(01-02-2017 06:24 PM)Raymond Del Tondo Wrote:  Normally there is no need to actually disassemble/dismantle the LCD, however the solder joints between the LCD and the keyboard pcb should be checked first.

Thank-you Raymond, I've checked it with a multimeter and does not seem to be an interruption between the PCB and the contact strips

(01-02-2017 06:24 PM)Raymond Del Tondo Wrote:  And if it still doen't work, check and maybe replace the zebra strip between the CPU and keyboard pcbs, which is another common source of malicious contacts.

checked as I wrote (the first test actually I did) and it isn't

(01-02-2017 06:24 PM)Raymond Del Tondo Wrote:  If it doesn't work after all this, maybe the LCD is defective;-)

...aaaaarghhhhhhhhhhhhhhh

[d b]

One more addition to Raymond's suggestions:
I have had one of the contact/prongs disconnect at the solder joint but look fine.
Depending on how you were holding the probes to them, you might have been improving the contact.
Gently using a pin, you could pull or push one side away to see if they separate.

Good luck on your beautiful Tall Key unit.

[aurelio]

(01-03-2017 10:02 PM)Den Belillo (Martinez Ca.) Wrote:  One more addition to Raymond's suggestions:
I have had one of the contact/prongs disconnect at the solder joint but look fine.
Depending on how you were holding the probes to them, you might have been improving the contact.
Gently using a pin, you could pull or push one side away to see if they separate.

Good luck on your beautiful Tall Key unit.

Thank-you Den, I'll try again tomorrow gently, like the flight of a bee

[AndiGer]

As it is a tall key 41c I assume made in USA.
So there should be the golden balls at the tunnel to the battery pack.
Did you check the connection from the IO block to these balls?
Outer left to upper ball 0 ohm, outer right + of DMM and lower ball - about 1000 ohm.
If not there is a contact problem between IO block and keyboard PCB.
Wish you success
Andi

[aurelio]

(01-04-2017 05:51 PM)AndiGer Wrote:  As it is a tall key 41c I assume made in USA.
So there should be the golden balls at the tunnel to the battery pack.
Did you check the connection from the IO block to these balls?
Outer left to upper ball 0 ohm, outer right + of DMM and lower ball - about 1000 ohm.
If not there is a contact problem between IO block and keyboard PCB.
Wish you success
Andi

not considering the accuracy of my multimeter

I found 0 oms for the first an 78 ohm for the second
and on another tall key got 0 and 39 ohm....

[aurelio]

(01-03-2017 10:46 PM)aurelio Wrote:  

(01-03-2017 10:02 PM)Den Belillo (Martinez Ca.) Wrote:  One more addition to Raymond's suggestions:
I have had one of the contact/prongs disconnect at the solder joint but look fine.
Depending on how you were holding the probes to them, you might have been improving the contact.
Gently using a pin, you could pull or push one side away to see if they separate.

Good luck on your beautiful Tall Key unit.

Thank-you Den, I'll try again tomorrow gently, like the flight of a bee

edit: tested again it seems there is no interruption I assume or bad contact
How may I test the display without removing it?

[AndiGer]

Re-measured. 0 ohm and 1335 ohm on the tall key