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HP41CX - dead or asleep?



#9

I may have killed my HP41CX. I think I had
the clock display running when I removed
the card reader. But, since the power ON
switch was very intermittent prior to this
(it was very difficult to turn on or off),
there might be a slim chance the ON switch
finally failed completely and the chips
are OK. Anyone do this and did your
HP41CX survive?

I have not opened the case yet to check
it, but thought, could it be asleep and
not responding to the power switch?

I read the HP41CX repair article by Stefan
Vorkoetter. I would really like to get a
schematic of the HP41CX. Anyone know if
schematics are available?


#10

Me again.

Removing the card reader with the clock being displayed is a major unsafe procedure, indeed. It may cause the system to fail or, even worse, a hardware failure.

The differences between the fullnut and the halfnut are better explained here at the museum under

http://www.hpmuseum.org/hp41.htm

The halfnut does not allow hardware repair over the mother board cause it has a one-piece board with one IC only, the display assy with the CX ROM and the Time assy. It is almost impossible to repair any. If your CX is a fullnut, the classic assy allows a mboard maintenance.

In both cases, getting to the [ON]-key contact for cleaning needs the same procedure, fairly discussed here under

http://www.hpmuseum.org/cgi-sys/cgiwrap/hpmuseum/forum.cgi?read=10579

If you still need any help, post new messages.

#11

I have done this same thing more times than I care to admit and have not permanently killed anything yet. My bet is you just glitched the CPU (or your switch has finally died).

You can try to fix the switch contacts by finding the small feedthough hole on the keyboard that is directly under the power switch. Place a drop of 91% isopropyl alcohol on the hole, and then gently scritch around inside the hole with a small brush made up of three wire brush bristles chucked in a pin vise or hobby knife holder. Be careful not to break off any bristles in the hole.

It can be VERY difficult to make a HP41CX forget its memory when you want it to. I have some units that will retain their memory for well over three weeks without any batteries installed. The only sure fire quick way I know of it to open the case and short out the large power supply capacitor.


#12

I opened my CX ( I was relieved to see it is a fullnut )
and discharged the big cap and tried cleaning the
power switch. Reassembled carefully. Still won't turn on.

I'll try cleaning the power switch again.


#13

Arrrrgh! Two of the 4 NiCADs were dead. I checked
them the other day and they were fine. How did
they die with the calculator off?

Atleast now, I know how to clean the power switch.
Works like new now.

Now I need to replace the melted rubber wheel
in my card reader. Just need to fine size 005
o-rings.

Thanks Luiz, thanks David and thanks Tony!


#14

I believe it's possible for the Phineas time chip to get into a state where it takes excessive current from the batteries. That's why it's a good idea to turn a 41CX on and off after installing the batteries, or to turn a 41C/CV on and off after plugging in a time module. Actually, I guess most of us
do that anyway to make sure the machine (or device) works properly. Turning the machine on/off will initialise the chip.
Anyway, paerhaps unplugging the card reader got the chip into that state and you didn't realise it before the batteries went flat.
Size 005 O-rings (15/64" OD, 7/64" ID) are not hard to find -- I know of at least one supplier in the UK (which is where I get the rings to rebuild my card readers) but I guess a UK supplier is no use to you.


#15

That would explain why my NiCAD batteries
measured good right after the problems started
then turned up drained two days later

I'll be more careful now and use your tip when
replacing the batteries.

Thanks Tony.

#16

It is a bad idea to remove or insert any module when the machine is turned on. However, I've done it (by accident) a few times to one of my 41CVs and not done any lasting damage. However, it does normally 'crash' the machine.
I assume you've tried the standard methods of getting a crashed machine back to life. Start by holding down ENTER and prssing the power switch. That will reset most CXs. And try alternately pressing the CLX key and the power switch. Finally try holding down CLX while pressing the power switch. This will (should) cause a MEMORY LOST, but it might get the machine to work again.
If that doesn't help, take out the battery pack, peel off the feet and take out the screws under them. Take the case apart (the procedure has been given here many times in the past) and discharge the electrolytic capacitors by touching a 100 ohm (or so) resistor between their terminals. Put the machine back together and try again.
There are 2 types of HP41 construction. The older machines (fullnut) have a separate logic board with pin-through-hole chips (the large one is the CPU and keyboard scanner, there's a number of 8 pin memory devices (ROM and RAM), a 14 or 16 pin chip (I forget) that's the PSU circuitry and a 20 pin chip (in the CX), the real time clock). The display drivers are mounted direct-on-board at the back of the display. With these machines, HP used to replace individual components, and if you can get the parts (e.g. from other old machines), it's possible to do the same.
The later verson (halfnut) has everything on the keyboard PCB. There's a surface-mount chip at the front of the keyboard -- the CPU and keyboard scanner. Just about everything else is in the display module (ROM, RAM, display drivers, PSU). Halfnut CX machines have another PCB mounted on top of the CPU chip on solder posts. This PCB contains the extended RAM, extended functions and time module ROMs and the time chip. [Note, though, that the ROMs in the display driver are different between the halfnut CX and CV, so you can't upgrade a CV into a CX by adding this board. If you do, it'll find the time module but nothing else]
Anyway, the only official repairs to this type of machine are to replace the 'extra' PCB or to replace the top case (including the CPU, display driver, etc). It is possible to take the the machine apart, but it involves undoing the heat stakes holding the PCB to the top case, and it's non-trivial to get it back together reliably.


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