The Museum of HP Calculators

HP Forum Archive 05

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Message #1 Posted by Hugh Wong on 19 Apr 2001, 11:18 p.m.

I recently won (bad luck?) an auction on two HP41cv(s), the seller claims one works 100% and the other one dead. However, when I got them, one works partially and one doesn't at all due to battery terminal corrosion. I repaired the terminals and hard wire the PC board to the keyboard and both of them partially working, however, after a minute or two the display became auto flickers and it display Chinese characters even I cannot understand. So I searched the Forum for answers and resetting, shorting the battery terminals, shorting the whole pc board with tin foils, strangely, after shorting it with tin foils, both calculator will work longer, like two to ten minutes, I even can do a TVM with an ADVANTAGE module plugged in but only for a few minutes. I notice the Museum has a CD set which has a HP41c “service manual” is that similar to the 41cv ? Can anybody help ?play became auto

Re: HP41cv
Message #2 Posted by Ion Abraham (New Mexico USA) on 20 Apr 2001, 9:27 a.m.,
in response to message #1 by Hugh Wong

Dear Hugh,

I've been down this road, not very successfully, so here is what I know so far.

I have the service manual, and yes, the service manual does also apply to the CVs, although more so if they are Fullnuts (rectangular screens). However, I don't think the manual will be very helpful in diagnosing the sort of problem you are describing, which I also have seen before. On the other hand, the manual does provide schematics, if you want to get that far into it, which I haven't yet.

It's really interesting that the units work longer after you short the terminals. It's my personal suspicion that this sort of problem is caused by a bad/old component, like a capacitor. I don't have any evidence for this, except that this is the only way I can think of to explain the sort of behavior you are describing.

I don't know how much this helps, but please post if you are able to make any progress.

Best regards,

Ion Abraham Albuquerque, New Mexico

Re: HP41cv
Message #3 Posted by db (martinez,california) on 20 Apr 2001, 2:11 p.m.,
in response to message #2 by Ion Abraham (New Mexico USA)

hugh; ditto on what steve says. plus - do the tin foil thing and let it sit for a week unpowered, then try it. the next step is to do it and wait about 6 months. also, thoes dozen or so tabs that connect to the bottom rear of the screen can come loose but still touch and look connected. they will give you conji if they are not soldered. gently pull on each to see if it is still tight. this dosn't sound like the answer but since you've got it wide open and it's easy, it's worth a try. good luck.

Re: HP41cv
Message #4 Posted by Hugh Wong on 20 Apr 2001, 3:39 p.m.,
in response to message #3 by db (martinez,california)

Thank Ion and Mr. Martinez,

I guess if the tin foil trick does not work next week - I'll have to wait 6 month by that time I'll brush up my chinese and talk to HP also.

I faintly remember when I first purchase a HP41cv 15 years or so ago, they always have reset problems. It might be the static memory they first designed has a discharge problem. Ion, if you can share the service manual, I can send you some money for deposit and later return your manual. Or if you can scan me a copy of the schematic and email me or put it on the website. There are hardly any components on the CPU circuit board. Are the components generally replacible ? Its such a shame that we are not able to fix it. By the way, I already replaced the two electrolytic capacitors. I also want to tell you that both calculators act similarly and thats so unusaual.

Re: HP41cv
Message #5 Posted by stefan (sweden) on 21 Apr 2001, 2:43 p.m.,
in response to message #4 by Hugh Wong

Hi Hugh As you can se at the forum 4 april j have similar problems with my Cx. Mine was modified for higer extra capacitor and a reed relay. I removed the extra capacitor and the relay. The result was that the chinese sign changed and became stabile and faded out after a while. Now to my question. You had TWO capacitors in the CPU.Is your cv modified too? or where is the second capacitor situated. And finally what is the walue of the resistor? stefan

Re: HP41cv
Message #6 Posted by Hugh Wong on 21 Apr 2001, 7:44 p.m.,
in response to message #5 by stefan (sweden)

I don't think my cpu board was modified. The two capacitors are 470 uf, 100 uf. They are mounted hanging out the circuit board where the pc board was notched out to accommadate the components. Mine 41cv(s) are identical and displayed the same problem. Mr. Ion Abraham furnished me with a HP41c service manual and now I may be able to identify the problem.

Re: HP41cv - Schematic
Message #7 Posted by Dan M (New England, USA) on 21 Apr 2001, 7:49 p.m.,
in response to message #6 by Hugh Wong

You may find a version of the 41C Schematic at Warren Furlow's excellent web site:

Look for "Reference" and then "Hardware," towards the top of the page.

Or, more specifically, try this link (but don't forget to check out the rest of the site)...

Many thanks to Warren Furlow for creating this site, and to Reinhard Breuer for re-drawing the Schematic

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