|Re: HP80 repair|
Message #4 Posted by Tony Duell on 26 Sept 2007, 1:45 p.m.,
in response to message #3 by Mark W Paris
My next move, I suppose, is to pop out the 3 individual 5-digit LED banks and test the good one against the others -- I was thinking to check the resistances across various connections with a multimeter to see if I can definitively say that they behave differently. This assumes that the 5-digit segments are the same part, which by their look, they are.
Yes, the 3 LED modules are all indentical
If your multimeter has a 'diode test' function (most digital meters do), use that. At least on my Fluke, the meter displays the voltage drop across the diode-under-test, which is much more meaningful than a 'resistance'.
IIRC, the individual segments of the displays on an HP classic-series machine are made of several LEDs (but I can't remember if they're in series or parallel). You mention 'spotty' segments, this could mean some of the LEDs in that segment have failed.
In general, a completely blank digit is a problem with the cathode driver chip (or maybe something on the logic board). The same segment missing on all the digits is a problem with anode driver chip or the inductor. Random defective segments are nearly always defective display devices themselves
Note that just about all the parts are custom. The only way you'll get spares is from another HP calculator. I think parts can be taken from any classic-series machine (note the for this purpose, the HP67 is a Woodstock, not a classic, the circuitry is very different)
I think if this fails to isolate the problem, I'm going to have a more difficult time figuring out if the problem is with the drivers or inductors since I don't really have much of an idea what those are!
The drivers are the 2 chips on the keyboard/display PCB. With the PCB in the normal operating position, the anode driver is the one on the left.
The inductors are 8 coils (look like fat resistors) standing vertically on the bottom (solder) side of that PCB, 4 each side of the battery connector space. Note that the inductor for the decimal point segment is a lower value than the other 7.
Thanks for the FixThatCalc reference. The last time I posted questions to this Forum asking for help with a (now fully functioning) 41c, Randy pretty much single-handedly helped me through the process. I was hoping that I might hear from him again but maybe he's had enough of holding my hand.
If anyone knows of a resource out there that diagrams the 80's PCB's and names the components, that would be a big help.
The HPCC schematics CD-ROM has a schematic for the HP80 (both versions of the processor board). But no PCB layouts, it's assumed you can find the components from the schematic. There are not that many of them, so it's not hard.
If you contact the HPCC secretary (address on the HPCC web site), he might be able to help you.