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HP-41 CX dead overnight



#17

HP-41CX worked perfectly yesterday. Won't turn on today.

Tried:

  1. Repeatedly trying to turn it on

  2. Tried master clear

  3. Took out all modules

  4. Tried master clear

  5. Left batteries out for a few hours

  6. Still won't turn on

  7. Tried to reverse battery polarity

  8. Still no go

What now?


#18

Hi,

the usual indicators for causes of a malfunction are

- corroded battery contacts

- corroded battery springs

- bad contact between case halves

Does any of these indicators apply?

Actually the 'case halves indicator'

just indicates that the unit has fallen

off from somewhere, and possibly one or more of

the screw posts that hold the cases together has broken.

The actual malfunction cause is a bad contact between the pcbs

inside if the case halves aren't held together tightly.


BTW: I'd never reverse batt polarity.

Ok, the HP-41 isn't the HP-48, where this

would likely damage the zener diode,

but I don't recall to have ever heard

that an HP-41 could be revived by

reversing batt polarity;-)

Regards

Raymond


#19

No corrosion. Clean as a whistle. No faulty contacts. Two halves firm and tight together.

I've revived a 41 with reversing the polarity once before after getting that tip from someone.


#20

Quote:
I've revived a 41 with reversing the polarity once before after getting that tip from someone.

I've used that trick too with success. The 41C hardware experts will undoubtedly be able to tell us why it works in more detail, but I believe the basic explanation is that there are capacitances in the 41C that can maintain voltages in the CMOS circuitry. Reversing polarity allows these to discharge.

Another bit of 41C lore is that it is sometimes neccessary to leave the batteries out for an extended period - like 24 hours - to revive an unresponsive unit. Have you tried leaving the batteries out for that long?

Regards
Howard


#21

Please do not shoot me for this stupid suggestion, it's just that I have overlooked it once myself (why is this darn remot not working suddenly?!) - did you try new batteries?

Cheers

Peter

#22

I find that on full-nut models a quick way of making sure that the battery assembly is making good contact with the backplane is to use the two contacts on the side of calculator next to the hole for the batery pack (the ones that were supposed to be used with the A/C adaptor).

If you have a really old 41 (the one with the gold balls) you can simply measure the voltage across the two contacts. If not, you have to use thin wires to reach all the way to the backplane.

The whole point is to do this with the battery pack in place so that you can tell whether there is any problem with the battery pack itself, but most importatnly with the battery/module assembly and its connection to the backplane.

I do not believe that this method works with half-nut models.

**vp


#23

I get the same voltage from measuring the outer battery contacts as I do reaching down all the way to the backplane.


#24

OK, now the last check to verify end-to-end operation.

Check inside a module port to see if you get Vcc. (The I/O ports are one way of getting at the Vcc line without opening the 41)

These are the module contact assignments (View into I/O Port):

(you may want to check Vbat as well, but it should be OK).

If Vcc exists, then its not a supply problem because it means that power from the batteries cross the backplane, enter the CPU-daughterboard, go through the power supply and exit back into the backplane and then into the I/O port sub-assembly.

If Vcc is missing , then you should looking at a bad connection or a
bad PSU.

Assuming there is voltage present at Vcc, the value should tell you whether the CPU is running. Normally Vcc should be 6V (CPU running). If the CPU is sleeping or standby, Vcc should be Vbat-1V and it is not regulated.

While monitoring Vcc, press the ON key to see if anything is happening.

**vp

Edited: 26 Feb 2006, 9:36 a.m.


#25

oh-kay: Now here is a challenge; I have a multimeter in my hands. It and I are not very well aquainted. We do get along well, it's just that we have little experience being partners in matters like these.

So, Vcc? What is that? And how exectly do I measure the voltage there? I take the one thingy from the multimeter and insert it into the I/O port and the other one I put... where?

Excuse my ignorance in this area. I am a programmer of these beauties and are eager to learn more about the hardware.


#26

Geir Isene wrote:

> So, Vcc? What is that? And how exectly do I measure the voltage there? I

Look at the drawing. On the top row, Vcc is the second connector from the left.

You meassure it against ground which can be the battery (-) or the GND signal to the right of Vcc. They are the same.

> I take the one thingy from the multimeter ...
Hmmm! Given the above, are you sure you want to do this? Maybe you do not want to learn electronic equipment troubleshooting on your HP-41.

**vp


#27

Well, I am ready and willing to try a few non-dangerous recepies here before I send it to a competent doctor for revival.


#28

OK then.

The problem is that the contacts are very
difficult to get at because the leading edge of the module interface is insulated. So your voltmeter probes would need to make contact with the exposed surface further back inside the port cavity.

The way I'd do it is to use an old module (e.g. an 82106A) take out the PCB inside (or at least make sure the module contacts are isolated from any ICs on the PCB) and connect Vcc, Vbat and GND contacts to short wires coming out through the back of the module. Then you can plug your "troubleshooting" module in and be able to take measurements.

If you have more advanced instrumentation (e.g. scope or logic analyser) you can do the same thing with the clock signals and see whether there is activity apart from power.

**vp

for a discussion on how to take apart a module see http://www.tla.org/hp41/module.html

#29

In general, putting batteries in backwards or shorting the terminals in a 41 doesn't clear memory. Of course your mileage may vary.

It depends entirely upon the leakage current of the reverse polarity protection diode(s). HP used some good diodes there and as a result these methods usually meet with little or no success.

If you want to do a little experiment, remove the battery and measure the voltage present on the outer calculator terminals, the two center points are only a jumper. Leftmost is (-), right is (+) with the display at top. If the voltage is 4 to 6 volts, that's good because it means the unit is holding voltage as it should and holding memory . If it is zero-ish volts, that's bad and will require further diagnostics to find out is wrong.

For the purposes of the thread, the game is now to get that 4 to 6 volt memory maintaining level to zero to accomplish a master reset.

Short the terminals for a few seconds, now measure the voltage and see that it doesn't change.

Reverse the batteries, insert for a few seconds, remove and check. See that the level may have dropped a few hundred millivolts. Then watch it rise back up all on it's own. I'll leave it to reader to figure out why that is...

Reinsert the reversed batteries and leave it for awhile. Remove and measure. You'll find the level never goes below about 0.6 volts. Never (*see disclaimer below). It can't because you're dropping the voltage across a silicon diode, the very diode that prevents reverse current flow and the resultant damage. Because of that diode, chances are slim to none that you'll ever clear memory.

* Never disclaimer: Chances are the one unit you test will have a leaky diode and reset just fine when shorted :) Consider yourself unlucky and go buy a lottery ticket since no doubt your luck will change for the better.

** Never Never disclaimer. If that diode is leaky, reversing the polarity of the batteries MIGHT do some damage. Consider yourself sufficiently warned.

Now, back to the chase: Is the unit in question a Fullnut or Halfnut? BIG difference internally...


#30

First of all let me thank you guys for jumping to my (still potential) rescue. This is the HP Forum on its finest.

  1. It's one of my fullnuts
  2. I have tried new batteries
  3. Left it without batteries overnight
  4. Still not working.
Now to Randy's path:

Quote:
If the voltage is 4 to 6 volts, that's good because it means the unit is holding voltage as it should and holding memory . If it is zero-ish volts, that's bad and will require further diagnostics to find out is wrong.

It's 6.1 volts

Quote:
Short the terminals for a few seconds, now measure the voltage and see that it doesn't change.

It changed to 5.5 volts

Quote:
Reverse the batteries, insert for a few seconds, remove and check. See that the level may have dropped a few hundred millivolts.

Down to 0.01 volts(!)

Quote:
Then watch it rise back up all on it's own.

After around 5 minutes, it has climbed to 0.02 volts. After a few hours, its up to 0.12 volts.

Should I then reinsert the reversed batteries for awhile (considering the last two answers here)?

Edited: 26 Feb 2006, 6:38 a.m.


#31

Hi Geir, all,

I've been reading the whole thread, and so far the advices you've got are of a consistent troubleshooting practice.

Furthermore, the results you've encountered/measured are almost definitive to discard any Battery->I/O-Block->Backplane misconnection.

With this scenario there are still two different points where your 41 can suffer its illness:

- Backplane->Mainboard zebra strip (the more likely)

- Mainboard IC damage (the worst case, quite unprobable but not impossible)

You can still try to place your 41 onto a flat surface and push (hard) both halves together in the frame of the 1 2 3 row. Make sure you do not press any key while pushing!... then try ON again while holding the pressure...

Squeezing a bit (just a bit!!) before you try the above procedure won't hurt either...

If if turns on, then the failure is definitely in the above mentioned zebra strip. If it doesn't come up to life, this does not mean that your Mainboard is gone (not yet at least...) but you'll need to take it apart for further testing/repair... indeed you already need to take it apart whatever the case, but you can get a good feedback of what you'll find after doing this.

If you decided to face the task by yourself and need/want futher advice, do not hesitate in asking fot it... You're in the right place ;-))

Best regards.

Diego

#32

You should open up the machine (four screws under the rubber feet), take out the CPU card, clean all the contacts and pads in the machine and the back of the keyboard with 91%+ isopropyl alcohol. Keep the alcohol off the display window. It will leave a white stain. While the machine is apart, short across the large electrolytic capacitors. Reassemble. Cross fingers. Flame on.


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