HP 48g - Screen won't turn on -HELP!!!


Right - I may well deserve what I get here. I opened my 48g and put it back together (I am/was planning to upgrade it...that aint been done yet). However, after putting it back together it no longer turns on when I push the 'on' button. When I put the batteries in, the screen does momentarily flash dark and then it stays off. Did I some how kill it? Have I lost all memory (if it does get lost, would this stop the calculatot turning on)? Can anyone help me find out if I have completely fried it? Thanks,

Edited: 4 Nov 2005, 9:04 p.m.


Well, before trying something like opening the calculator, I'd advise archiving anything that you want to save to your PC. But what's done is done.

Are you sure that you're inserting the batteries with the proper polarity? Are you sure that the batteries are good?

It does sound to me like some sort of hardware problem, but strange things could happen if RAM gets corrupted (such as by being without power for too long).

It could be that the calculator really is on, but isn't displaying anything. The following solutions assume that the calculator is on.

Try a warmstart, that is hold down the ON key, press and release C, release ON.

If that doesn't work, try forcing a warmstart with a paper-clip reset. Remove the upper right (as viewed from the back) rubber foot. You'll see a small hole with the letter R next to it. Insert a straightened paper-clip (or something similar) straight in as far as it will go, and release it.

If that doesn't work, and you're willing to lose everything you had stored on the calculator, try clearing all user memory. Hold down ON, hold down A, press and release F, release A, release ON.



I tried all that you said and still nothing shows up on the screen. I don't mind losing memory - I just want to get it working again. The batteries are good and they are the right polarity. When I reinsert the batteries, the screen still momentarily flashes on. I am wondering if I damaged the 'on' button when opening it. If there is a possibility that the 'on' button is damaged, anyone have any info on how to fix them? Any other ideas? I think that I may have pushed the 'on' button without any batteries (thus causing memory loss apparently.) Would this cause the calculator not to turn on? I have tried connecting it to the PC via serial and then tried to grab an image of the screen but nothing happens (which is leading me to thing that it isn't turning on).Thanks


Edited: 4 Nov 2005, 10:07 p.m.


I am so rapt at the moment cause I got my wonderful 48g going again! After trying a number of things I just thought that I might as well put the batteries in the opposite way (reverse polarity) and then put them back the correct way (though that I might as well try) - then I heard it beep back to life and it asked if I would like to try and recover the memory. Mate that nearly knocked me off my chair as I was sure it was dead! I don't know what this did electronically to it but what ever it did, it fixed it! Someone out there might be able to explain what happened there. Anyway, maybe this will help someone else some day, who after opening it and putting it back together, seems to have a dead 48g. Thanks James for your help!


Well, I'm glad (and surprised) that that worked.

In general though, inserting the batteries backwards is a very bad idea. One reason is that there's a Zener diode across the batteries, for protection against electro-static discharges. Putting the batteries in reversed is likely to destroy this diode. If it fails "open", the calculator will probably work just fine, but if it fails as a "partial short", you may go through batteries very quickly.

For more on this, see these posts from a hardware designer of the 48 series.


Edited: 5 Nov 2005, 5:33 a.m.


James' advice should be followed rather than what I did. I guess I will find out in a few days or so if the zener has failed as a 'partial short'. The calc seems to be working fine at the moment so I am hoping that the battery reversal didn't cause any damage. What would have been another way to fix the problem that I had? Can the zener diode be replaced if it is no good? (Do you think that the battery reversal would have definitely cause some internal damage or could I have got away with it?).



Simply shorting across the battery terminals might've done the trick. Or removing the batteries and just letting the calculator sit for a few days, to give the capacitor time to discharge.

I suppose that the Zener could be replaced, but I really don't know exactly where it's located on the circuit board. It may well be a "surface-mounted" component, which would require skill with a soldering iron.

It seems to me that a better design would've been two Zeners in series, "head-to-head", that is, always one forward biased and the other reversed biased. If I recall correctly, these are available as discrete components, with both Zeners in one package, that is. I suppose that the designers weren't planning on people putting in the batteries reversed.

If the Zener fails "open", then probably little harm results, although it might not meed the original ESD requirements.

No, it's not certain that the Zener was damaged at all. Particularly if the reversed batteries were connected for only a few seconds, you may have "gotten away with it". My advice is to wait and see if you still get normal battery life.


Edited: 5 Nov 2005, 6:36 a.m.



Also see http://www.engr.uvic.ca/~aschoorl/faq/48faq-4.html#ss4.4.



You know of some very handy links thats for sure. I don't seem to find them when I do searches. I did part 8 of point 4.4 in the above link (though I certainly would have tried the previous ones beforehand had I know how damaging it could be) - I did only put them in 'VERY BRIEFLY' so I count my self very lucky that my 48g works (even if it possibly is damaged). Thanks again James - you're worth your weight in gold mate!!!


If your 48G serial number begins with the letters "ID", it ** will not ** have a zener installed. They were eliminated in late 1995 to early 1996, depending upon the model (G/GX). All S/SX's have them.

The easiest way to wake up a dead 48 when it's out of it's case is to discharge the 1000uf memory back up cap. Putting the batteries in backwards achieves the same thing but it's a bad idea IMO. If it's not open, leaving the batteries out overnight should work as well.

Of course I'm assuming a good 'ole standard paper clip induced warm-start didn't help...

Edited: 5 Nov 2005, 7:58 a.m.


I don't have the letters "ID" in the serial number but it was made in the 45th week of 1995 (I must say that it is in great condition for such an old calculator - I had it given to me). So not sure if it would have a zener or not


I can't say either as I don't have enough data on date codes in that range :(

What I can tell you is if the unit worked after the reversed polarity batteries and but now eats a set of batteries in a week or less, it has a zener and it has failed as a result of the forward biasing.


Speaking of Zeners, I miss Norm :-)


No kaka Bill. Norm is one of the good ones. I sent him an email
a while back but got no answer. Some of the superheros like spiderman, superman, & batman have come back in the form of movies. Maybe we can hope for "Captain Zener Rides Again".


Gee, I hope he's all right.


Do you happen to know whether the 49G and 49g+ have this Zener?



Does any one know what amount of current flow out of the batteries you might get if the zener does have a partial short. (I am not an electrician, but shouldn't I be able to tell how quickly the batteries are draining by comparing the flow that is going through now to what it should be?)Anyone know what is the normal current flow when the 48g is off? Caribe


Does any one know what amount of current flow out of the batteries you might get if the zener does have a partial short.

More than normal.
(I am not an electrician, but shouldn't I be able to tell how quickly the batteries are draining by comparing the flow that is going through now to what it should be?)

Certainly, provided that your ammeter is capable of registering such low currents.
Anyone know what is the normal current flow when the 48g is off?

If I recall correctly, a fraction of a milliamp with the calculator off. Try some of the posts in this search. I'd trust information from Dave Arnett a bit farther, as in this other search.

I'd recommend checking the current with the calculator off, but also after leaving it off for at least a half hour. Even better, put it into coma mode by holding down ON, pressing and releasing SPC, then releasing ON. In coma mode, even the system clock is stopped, so it's only drawing enough current to maintain static RAM, that is, the CMOS leakage current.

To wake up from coma mode, simply press ON, and of course the clock will need resetting if you use it. Coma mode will also clear the warmstart log, which you can view in the 48G (and 49) series by executing WSLOG.


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