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shorting HP-41 cont. memory capacitor



#10

How risky is it to open 'er up and short this cap? The fuzzy service manual pics on the web make it look as though all I have to do is remove the bottom case and I should have immediate access to the cap.

How would this go? I should use a very tiny test jumper to connect both leads to the capacitor together? How long would it take to discharge using this method?

Thanks!


#11

I opened up my 41C, which already has the feet off, and compared it to the service manual on the CDs, it is the same. I guess it is a "fullnut". If yours has a black border with rounded corners around the LCD, you have a "halfnut" (isn't it a CV? some are fullnut, some are halfnut). Someone will be by shortly to give you a better answer!

Section 2-33 (on page 10 of the PDF) says C1 is the storage cap, figure 4-6 (page 35) is an outline drawing of the PCB showing C1 is the largest cylindrical cap near the upper left corner, almost under the edge of the beeper, and table 4-5 (page 37) says C1 is 470 microfarad, 6 volt.

To open the unit, peel up the rubber feet to expose the screws. The two screws in front are close to the inside, the rear feet are about in the middle, slightly closer to the outside, under the feet. Note that the feet are actually two layers, the rubber is stuck to a thin vinyl layer which is stuck to the case (I know this is the case with Voyagers). Remove the four screws and the bottom comes off with the I/O and Battery block, giving you access to the PCB.

With direct access to the cap (without the diode in the circuit), the cap will pretty much completely discharge as soon as you connect the two leads with a piece of wire. Spend a few moments making sure you are making good contact to both leads AT THE SAME TIME! If you use a test lead with little clips, you can be pretty sure you have made contact. If you have a voltmeter, measure the voltage across the cap before and after. Observe anti-static precautions!

#12

You should also short out the smaller power cap next to the larger one. Also while the unit is apart clean all the internal pressure contacts (battery assembly and the zebra connector beneath the fullnut CPU card). Also clean the mating contacts on the keyboard assembly. I use 91%isopropyl alcohol. I also treat them with a corrosion inhibitor such as the CAIG ProGold pen. Available at Frys for the princely sum of around 15 bucks.

If you have a half nut machine be VERY careful of the beeper wires. The wires have become very brittle at the solder joints. It is VERY easy to break them. Hopefully you have some experience with a soldering iron.


#13

Thanks for the tips!

I found similar cleaners in our electronic shop at work. 99% isopropyl and Gold Plus gold contact cleaner/inhibitor. Sounds like it'll work. I might try it tomorrow.

Can the black feet be reused or are they trashed by taking them off?

I have a 41CV, and it does not have the rounded black corners in the display.


#14

The feet can definitely be reused. They are held on by a double sided stick rubber tape. I remove any sticky items using a product called "UnDu". It is available at Home Depot, most hardware stores, office supply stores, etc.It is just the solvent heptane in a cute (and way overpriced) little bottle. It causes any adhesive to temporarily loose its stick. Some types of rubber cement thinner are also heptane (some are hexane). A quart can costs about what an ounce of UnDu sells for.

To remove feet I place a drop on the small blade of a Swiss Army knife and slide it under the foot. Hopefully you get under the tape and not between the tape and the foot. Place the removed feet on a piece of peel off label backing (i.e. post office labels).

If you loose your sticky stuff, you can use double sided scotch tape.


#15

I got it open, and tried shorting the caps. The first time it took a while to get to 0 V. I reassembled and still got MEMORY LOST as soon as I slip in the batteries. Maybe this time a little different in that I can barely see a flash about once a second or so.

I took it apart again, cleaned all contacts and have shorted both caps. I notice that if I take off my leads and measure, a tiny reading (0.003 V) starts to build after a while. I put my wires back on and am just going to leave it for a few days.

I don't have any conductive foam I've heard about, but maybe it would help to drain the residual charge off the calculator.

Would it be more effective to ground the cap rather than just connect it's leads together?


#16

The (-) end of the big cap is shown connected to ground on the schematic, and so is the (-) end of the battery. The way the voltage starts rising again after you remove the short suggests that another cap has retained some charge and it is leaking back through a diode. Here's a suggestion: put shorting jumpers on both caps at the same time. And leave them there for a little while. (I just re-read your post and I see you are doing that.)

In theory, a capacitor is never completely discharged because no wire has zero resistance (just as there is never a perfect vacuum). Also, I have read about a property of dielectrics (the non-conductive stuff between the plates of a capacitor) that cause them to be "twisted" or (something like that) by the presence of a voltage. This is probably just the fundamental property that causes capacitance (and which is taken advantage of in LCD's). But I seem to recall that dielectrics return to their "non-twisted" state at different rates - some kind of memory effect. Consider that the caps in HP41's have been holding a charge for at least 15 years!


#17

It didn't work. I shorted both caps and left it overnight. It was 0V when I came in this morning, and still gives MEMORY LOST as soon as the batteries hit the contacts.

Something is really screwed up. Can anyone fix my 41CV?


#18

I don't have enough experience but I know there are others here who do, also there is some information on the other Museum pages about people who do repairs.

I had a thought - is there any chance that the computer is OK and the keyboard is not? Is the "<-" key the only one that will clear a "memory lost"?


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