HP80 repair



#7

I'm seeking some technical advice on repairing the HP80.

The model I'm working on appears to function correctly in computations. The display, however, is faulty. The LED's in places 5,7,9,10 and the exponent are partially functioning - not all the segments light, some are too bright and/or "spotty."

I've managed to disassemble the unit (without too much damage to the back plate). I've checked for any obvious loose soldered joints or damage. There don't appear to be any.

Any advice on how to proceed? I'm thinking that it's not all that likely that the LED's have been fried. Is this right? Any suggestions on how to further check the connections?

The unit is in nearly perfect condition and I'd like to get the display working again.

Much thanks.

Mark


#8

I was hoping that one of the real experts might respond to your query, but since that hasn't happened, I'll offer up the tiny bit that I know.

First, did you look over this description of the HP-35 internals? I'm pretty sure that the display circuitry is identical to the HP-80. The display system consists of three chips that each have 5 8-segment LED digits, the anode driver, the cathode driver and eight inductors. Perhaps a study of this information will allow you to isolate the problem. If I recall correctly, if the same segment is out in every digit across the display, that would indicate a bad anode driver chip. Problems with individual digits might be in the LED chips themselves or the cathode driver chip. (Maybe. I used to know more about this, but forgot most of it and now can't find any information. Sorry.) In any case, if you identify the problem, the only way to fix it will likely be to replace the bad part with one "harvested" from another classic (35, 45, etc.) calculator. I believe that the actual repair work is possible for most people to do, if they are handy with a soldering iron and know the precautions to take when working on electronic components. Finding the parts might be the tricky part. You could watch eBay for cheap and/or non-working units, or post a wanted ad at the MoHPC classified section.

If repairs are beyond your abilities, you might try contacting Randy at Fix That Calc to get his prognosis. I have never used his services, but many others have and speak highly of his abilities.

Hope this helps, sorry I couldn't offer more. Good Luck!


#9

Dear Jeff,

Thanks for your reply - esp. the reference to the 35 internal layout. That's good reading on its own right.

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.

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!

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.

Thanks again.

Mark


#10

Quote:
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)

Quote:

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.

Quote:

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.


#11

One thing I forgot to mention. In some classic-series machines, the inductors are little modules on the solder side of the PCB, not individual components. Each module contains 4 inductors, and has 5 connections. \The 2 modules in a given machine are different, one contains 4 identical inductors, the other contains 3 of the same value and a lower-valued one (for the decimal point).


#12

Dear Tony,

Thanks for this - it's going to be very useful when I get a chance to sit down and work on it - hopefully this weekend.

Mark


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