|Re: HP 27|
Message #3 Posted by katie on 16 Apr 2002, 12:44 a.m.,
in response to message #2 by Frank
Thanks Frank, I think, sort of!
Steve, Actually, the warm-up problem would always show something on the display, like a zero, so that's probably not your problem. If you've got nothing on the display, the first check that you should make is to see how much current is being drawn by the batteries. You should hook up an external 2.5 volt power supply with an ammeter in series. If the power supply is working, you should see around a 100 ma (blank display) to 200 ma (most segments on) current draw. Assuming that you're getting that, you should next check the output voltages from the inverting supply. If you meassure them relative to the negative battery terminal you should find three of them: -3.3, +6.5 and +12.6. The easiest place to find these is on the non-component side of the main circuit board on 16-pin IC pattern that doesn't have a chip installed in it (labeled U5). Pin 9 = 6.5v,
Pin 12 = -3.3v, pin 15 = +12.6v.
If you're getting all that, the most likely problem is a bad chip and it's probably U1 (HP 5061 0430). They seem to have the highest failure rate of the chips used in this calculator. And if it has failed, the display will likely be blank. The really bad news is that this is the only HP calculator that uses this chip (that I know of) so you'll need to find another 27 to use as an organ donor.
I hope that helps.