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HP Forum Archive 13

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HP42S: scary moments
Message #1 Posted by Vieira, Luiz C. (Brazil) on 30 July 2003, 7:57 p.m.

Hi, all;

again, one new episode of the not-so-unknown Scary Moments.

This time with an HP42S.

Yesterday I was reviving some healthy memories form the time I was a newcomer in the world of the HP42S and I took the previously mentioned Load Flow program in order to verify some storage techniques when dealing with indexed matrices. I remember that when I bought my NIB HP42S (still deceased) and I began to deal with it, I already had an HP28S, an HP15C and an HP41CX, amongst a few others. I also remembered that I took the HP42S as the easiest and most powerful "numeric" calculator. In time: I do not compare a numeric to a symbolic calculator as a whole because there is no way to balance their resources in a fair way.

Well, this is not the subject and, after being corrected once (thanks, Valentin), I'm actually "loosing focus".

I got the HP42S I recently took with the others after cordial and friendly arrangements (hey, R.L.A.) and switch it ON. I created a [100 1 Matrix] , pressed [ENTER], [SHIFT][COMPLEX] and while the busy indicator flashes, the low battery indicator turn itself ON, also. "New batteries, please!" I thought. At last, there it was: [100 1 Cpx Matrix] and the low battery indicator still ON. I decided to remove the MATRIX menu by pressing [EXIT] ([ON]) and then I store the complex matrix in DT ([STO][ALPHA] DT [ENTER]). Then the nightmare begins: Freddy Kruger and Jason in my back looking at the HP42S vanishing the display and... resting in peace?

New batteries, no batteries for a night, new batteries, shorting batteries connector... Cutter, hands, fingers... Back case removed and let's inspect. As I had an extra 32KRAM chip waiting for the chance to become something more than a number, I removed the original RAM in this HP42S and transplanted it. I did not think that it would put it back to work, but if it returned from the dead, then it would be back as an HP42S+.

I decided to inspect components and I saw a suspicious SMD that seemed to be a capacitor: no polarity marks and a somehow revealing inscription: 102K. I took 102 as 10E2 pF, or 1000 pF or 1KpF or 1nF. What called my attention was the fact that the soldering terminals were too "recessed", meaning it was easy to see a considerable space between the component body and its terminals. I re-soldered it and nothing. I re-soldered the terminals of all remaining components and nothing. I removed the mainboard and added a strip of tape between the foam and the keyboard "sheet" to increase pressure and enhance contacts, but nothing. I was about to give up when I remember that I'm the first one to insist with others not to give up. Then I thought with myself: "Are you a man or a rat?" It took me a couple of minutes till I realize there would be no answer, so I decide to move on. I removed the suspicious capacitor and measure it: nothing! Great! Let me take another capacitor and solder it in place. I searched for another SMD capacitor and the one I found that was closed to 1nF (1n2) was too small and tinny. Well, the HP42S is a 4.5V designed calculator, why using capacitor with a lot bigger isolation voltage? Something was not right. Then I decided to take extreme measures: I used the soldering iron and pressed both terminals till they could touch the body's surface, as I thought they should be. Then I decided to measure it again: I did not understand the reading, one thousand times the one I thought it should be: 1F. So, I should read 102K as 10E2 K = 1000Kpf = 1f. Interesting!

Best regards.

Luiz (Brazil)

Oh, yes, the HP42S? Now it is a working HP42S+, thanks! And please, don't ask me to explain what happened... I'm still collecting my brain, spread around the room.

Edited: 30 July 2003, 8:03 p.m.

Re: HP42S: scary moments
Message #2 Posted by Randy Sloyer on 1 Aug 2003, 12:02 a.m.,
in response to message #1 by Vieira, Luiz C. (Brazil)

The typically blue part marked 102J, above and to the right of the crystal is an inductor. It is part of the step up voltage converter for the LCD display. I believe it is 1000 uh, it measures about 27 ohms.

I've never seen a bad one but it is likely to have fractured solder connections as it is high enough to come in contact with the back esd shield. This means any mechanical shock of the case in that area will land on the solder connections. Check the esd shield for signs of transfer imprinting due to impact. I've seen many a 100uf tantalum memory backup capacitor broken off and rattling around inside the case from the same cause.

An inductor!!!
Message #3 Posted by Vieira, Luiz C. (Brazil) on 1 Aug 2003, 9:23 a.m.,
in response to message #2 by Randy Sloyer

Hi, Randy;

thank you very much.

After what you post, I'm somehow worried. This means that, in fact, the component was still opened when I measured it, what explains the "remaining" capacitance. When I soldered it in place maybe the extra heating finaly closed the internal broken solder, otherwise the calculator would not be working as it is (passed 24 hours).

I'll check if there is any mark in the ESD protection. But I'll first try to find an inductor with this value. 1KH or 1KnH?

Thank you very much.

Luiz (Brazil)

Re: An inductor!!!
Message #4 Posted by Randy Sloyer on 1 Aug 2003, 11:01 a.m.,
in response to message #3 by Vieira, Luiz C. (Brazil)

I don't know the value from the marking. I could just mail you one, send me an email, I lost yours.

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