I use to have 25+ HP calculator over time in my life before and after college, well some of these calculators were sold because I was an student and no money other I gave to my sister an so, some months ago decided to start over again now after 10 years bought a bunch of them over ebay, but the more difficult and expensive one was the HP28S, well bought it works but drains the batteries very fast, no matter on off or idle consumes ~300mA (WOW), I noticed that one are of the calculator was very hot the upper left corner close by the IR LED! well I have opened and fixed some HP28S before and gave it a try again,

Well I found 5 components in the area where seems to be located the power regulator circuit(all surface mounted components) these are the only discrete electronic components in the calculator besides the big 1000uF Capacitor and the IR LED, 3 looks like are small capacitors, 1 (NPN?) transistor with the emitter and collector soldered together (don't know why) and one Zener diode.

Well I discovered that the transistor gets horrible hot !!! so there is something wrong looks like the zener diode is short and the transistor is taking the hit,

My questions is after the long story, if some body knows the schematic of this circuit will be perfect but more important than that is the specs of the transistor and the zener diode, if I find the model numbers I will be able to buy and replace them fixing the power drain!

The calculator works ! but in 10 seconds with new fresh batteries will show the battery symbol and of course will die in minutes

Thanks for all for helping this die hard HP calculator lover!

Edited: 13 Dec 2008, 11:59 p.m.

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I use to have 25+ HP calculator over time in my life before and after college, well some of these calculators were sold because I was an student and no money other I gave to my sister an so, some months ago decided to start over again now after 10 years bought a bunch of them over ebay, but the more difficult and expensive one was the HP28S

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The 28S was difficult and expensive to get??

On the contrary, the 28S would have to be the easiest to get and one of the cheapest vintage HP calculators on the market.

Dave.

Hi,

the component you titled as a transistor is actually a Z-diode. The marking Z3 on it says that it is a BZX84C5V6 or similar (The code is used for some oher components too, but all I could find in the short time are Z-diodes). You can find a data sheet at http://www.fairchildsemi.com/pf/BZ/BZX84C5V6.html. One of the connected pins is the anode of the diode, the other is not connected in the package.

You should try to get the schematic of this parts of the circuit from the layout, to deduce its function.

I wish you success,

what happens if you just take the diode off (carefully) ?

Yes, it is a zener and it is there for reverse battery insertion protection. The only problem was it didn't protect the calculator and they simply shorted, leaving you with the problem you currently have.

Remove the zener and forget about it as it was removed from later designs.

More importantly, how do you intend to reclose the case?

The voltage will not be regulated and probably damage the calculator!

Hi DaveJ looks like works in waves all depending who is interested out there if by any change there is a lot of people looking they will fight between them selves and raising the price always above 100 bucks, but look at my luck bought both HP48SX and HP48GX (even having my old ones) new almost from the box for 40 and 45 bucks!

Lately the HP28S is more difficult I guess just luck!

Hi Randy Thanks for your help!!! I will remove the transistor look alike (Zener Diode) and let you know what happens when I get back home next week!

I have opened before 3 HP28S and was successfully closed no problems!
to open just peel off the keyboard and top part plastic labels use a 7/64 drill bit with a fast drill or dremel(preferable) push fast and clean the bit for each thermal seal, to close (your question)take a clean soldering iron, put all together and press very hard per sector then with the soldering tip melt the top leftover plastic and most likely will be good enough, put the labels back and that's it!

One question I have if the black one is a Zener what is the cristal red Diode? third component left to right!

Thanks!
LEO

Okay, a one time chance to reclose!

The diode (a Schottky type) and the 100uf tantalum cap to its left form a voltage doubler circuit for the LCD voltage at 2x Vbat. The inductor on the far right side (102J) is also part of the doubler. This same circuit was used originally in the Pioneer units.

The Zener diode you can remove was later found in the original 48 series units but was removed when it was found it shorted and drained batteries more often than it protected anything...

Edited: 14 Dec 2008, 6:32 p.m.

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Hi DaveJ looks like works in waves all depending who is interested out there if by any change there is a lot of people looking they will fight between them selves and raising the price always above 100 bucks, but look at my luck bought both HP48SX and HP48GX (even having my old ones) new almost from the box for 40 and 45 bucks!

Lately the HP28S is more difficult I guess just luck!

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True, they do go through cycles a bit.
28S's were practically being given away at one point a couple of years back!

Dave.

Thanks Randy looks like you are very familiar with these circuits!

Yes it's one chance to reclose the HP28S!!

I posted one picture so you can see the condition of the pins after drilling

Thanks for your help

LEO