Topcat HP-97 with very dim LEDs


I just bought the HP-97 in very nice physical condition but with the usual card reader gummy wheel and printer gear problems. I can fix those issues, but am stumped by something I've never seen before. The entire LED array is extremely dim. I tried boosting the voltage at the battery contacts with my DC power supply but it doesn't brighten the way it does on other calcs. All the segments are there and all work properly. All are exactly the same low level of brightness. I have two other Topcats, and the displays are much brighter. Is there some part of the circuitry that is at fault, and how can I fix this? Thanks.




I do not remember quite well the LED circuit, but it surely is multiplexed. Is there a way to compare (oscilloscope) the sweep waveform, mainly the cyclic relation? I cannot see another possibility, given that the voltage levels are OK.

Have you checked the HP97 Service Manual (MoHPC CD#11)?


Luiz (Brazil)

Edited: 27 Nov 2009, 9:49 p.m.


The display multiplexer (aka cathode driver)is essentially a one-of-15 decoder. It wouldn't' matter what rate you ran this at, each display would still be on for 1/15 of the time -- if it were too slow you'd see flicker. A dim display means that you have:

1) a bad cathode driver chip (likely)
2) a bad ground connection going to the cathode driver chip (very unlikely)
3) a bad resistor (R2) on the display board that limits the current draw through the display (this is epced to be either 200K or 330K). I'd try changing this first because it's easy.



I don't see a 200K or 330K resistor, the largest I see is an 18K in a cluster of 3 near the top of the PCB that is part of the keyboard. Which is the cathode driver chip? The resistors checked out to their values, so I don't see any point in changing them.


Here's the layout from the 97 service manual, it sounds like you might have a somewhat different layout. I know that they changed the main PCB layout in the 97 over time, but I have no idea if the display PCB layout changed too.

Edited: 28 Nov 2009, 9:42 a.m.


My layout matches the layout in the service manual, except that there are only 3 resistors on the top right side, instead of 4 and there is a small cap (?) just below the right side of U4. The S/N is 1909Axxxxx, 4th year of production, so HP may very well have made some changes.

Update: I just got the Museum DVD set V7 and looked at the HP-97 Service Manual. It is now clear that my PCB version has all the resistors except R2, however, the P/N on the cathode driver U1 is the same. Also, the manual troubleshooting guide makes no mention of my condition, namely, all LED segments/digits working correctly but just very dim. So maybe there is something else at work here? I do know that on my other Topcats if I increase the voltage at the battery terminals, the display will get brighter, but on this one it seems to have no effect. Anyway, I'm waiting for a replacement printer idler gear replacement to arrive, so I can run the diagnostic program to see if it reveals anything.

Edited: 28 Nov 2009, 7:44 p.m.


It wouldn't' matter what rate you ran this at,

But it could be the pulse width that's the problem, again only visible with an oscilloscope.

Edit: perhaps R2 is missing since production but the original owner may not have realised the display was dimmer than it was supposed to be.

Edited: 29 Nov 2009, 1:27 p.m.


I have since acquired another HP-97, and the keyboard PCB layout matches the one with the dim display LEDs, namely, no R2 and other configuration differences from the design shown in the HP service manual. However, the LED display on this unit operates with normal brightness, so I can only assume some component is faulty on the first unit. I swapped keyboards and now have one complete fully functional unit. Upon further examination, I've determined that there are many other differences between the service manual and the units I've been working on. The logic boards are arranged very differently; in many cases the ICs are rotated 180 degrees such that pin 1 is on the opposite side, resistor/transistor differences etc. I wonder if anyone else has noticed these differences, and if there are other circuit diagrams and component lists for these revisions.


There was a substantial redesign on the main PCB when the 97S was released. This was done to accommodate the 16-pin DIP header that connects to the pod, but the same board was used in all subsequent basic 97 calculators as well. There's more about this here..


Yes, both of my units are like the 97S main PCB. For now I'm done messing with these units. The unit that inherited the dim display, is missing the battery door and printer tear-off bar and won't write cards, so it's a spare parts unit for now. Thanks for the info.

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