|HELP!!! Fixing HP28S (Battery Drain)|
Message #1 Posted by lsegovic on 13 Dec 2008, 11:57 p.m.
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.