Time flies in my HP-41CX

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Time flies in my HP-41CX

Postby Drulli » Wed Apr 09, 2014 7:31 pm

Hello all,
after repair of broken screw posts my HP-41CX (coconut) runs very satisfying, almost without any problem. (I can’t say if the below mentioned problem existed before, I haven’t used the CX for years).

BUT - the time in the CX runs much faster than it should. One minute standard earth time means 2:56 minutes in the CX universum. With this big delta to the real time the Correct function does not help. Did anyone came across this problem? Does anybody has a hint what can be the reason?

All commands of the Timer Module U11 work correct, no problem at all (Date, CLKT, DDAYS, etc), thus I hope that the problem is outside of the U11 (HP Part Number 01LF6-0001)

The Timer U11 uses a crystal / quartz Y1 with a parallel switched resistor R4 (22 MOhm).
Has anybody seen a specification of the Quartz/Crytal (HP Part Number 0410-1381).
In the Service Manual nothing is mentioned.
(Pictures can be found herehttp://www.schwalvenberg.com/de/hp-41cx/108-time-flies-in-my-hp-41cx)

My assumption is that the crystal and R4 are part of an oscillator that provides time information, mainly build into U11, the Timer Module but the quartz could not be integrated into U11.
What can make this oscillator run three times faster than nominal, is itrunning on an overtone due to aging ???
It would be great to get some feedback or hints or ideas from the community.
Drulli
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Re: Time flies in my HP-41CX

Postby Garth » Wed Apr 09, 2014 10:20 pm

Aging of the crystal won't do that, and in fact aging decreases logarithmically with time, the largest changes occurring shortly after manufacture. A poorly designed oscillator may suddenly start oscillating on an overtone, but the 41cx's history and time modules for the 41c and cv indicate the design is sound. I do wonder why there aren't any loading capacitors from each terminal of the crystal to ground though. Perhaps those are built into the IC. My strongest suspicion is that the crystal is damaged, for example from mechanical shock. If it's a 32.768kHz crystal, you should be able to get a replacement that fits fine even if it's not exactly the same shape and size.
http://WilsonMinesCo.com (60 HP-41 links on the links page)
Garth
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Re: Time flies in my HP-41CX

Postby Drulli » Sat Apr 12, 2014 8:52 pm

Problem is solved!

Waiting for the ordered but not yet delivered components (Y2 and R4), I refurbished my card reader ("gummy wheel problem“) in the meantime.
I put the refurbished Card Reader in the CX, the reader was listed in Catalog 1, so far so good.
When I put a first card in the reader the power sucked by the engine led the CX lost its memory. The reader needed 2,3 attempts before the cards could be read without problems.

The clock in the CX, which ran 3 times faster than it should, is now running very precisely with the right "speed".
The power loss must have reset some internal states in the Timer IC (U11) which must have become somewhat scrubby in the initial power up phase of the reassembled CX.
At least that's my assumption and by far the best root cause! ;)

Next project to come - HP41 Rechargeable Battery Pack Refurbishment.... :roll:
Drulli
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Posts: 19
Joined: Wed Mar 19, 2014 7:13 pm
Location: Germany


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