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Hp91 with very dim display
08-02-2020, 11:05 AM
Post: #1
Hp91 with very dim display
I recently acquired a very nice 91 TopCat with a display so dim that I have to be in a dark room to see what’s on the display. I found no loose connections inside, no corrosion issues, and there were no obvious burnt components on the PCB’s. What are the likely causes for this problem? I’m using a known good, fully-charged battery pack and hooking up a known good charger changes nothing. Just guessing I thought maybe replacing one or both pairs of identical chips on the display board with the 24 wire ribbon cable going to the display might’ve my next move. Any help would be appreciated. I’m capable of de-soldering and soldering and have spare 97 display and all other 97 parts to use for repairing the 91. Thanks for your help!
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08-02-2020, 01:23 PM
Post: #2
RE: Hp91 with very dim display
I've yet to see a dim display on the three HP-97's that I have acquired. If you have a new battery and the battery contact tabs are nice and shiny, that should eliminate the power issue. I would tend to doubt that the integrated circuits are bad that drive the display. I'll have to look at the display circuit since I'd bet that the HP-91 uses the same display drive circuit as the HP-97.
Does the printer work? If the printer works, then this shouldn't be a main power supply issue.
~ Jim J. ~
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08-02-2020, 02:19 PM
Post: #3
RE: Hp91 with very dim display
I have 2 97’s and 1 92 that work fine too, so unfamiliar with the display issue. Printer makes noise and advances paper but no characters appear on paper. I will tackle that issue after fixing the display issue first. Based on what you have advised I believe that I should check power supply to see if output Voltage is correct. Can you tell me what components make up the power supply? A picture would be best of course. Thanks
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08-02-2020, 03:23 PM (This post was last modified: 08-02-2020 03:24 PM by teenix.)
Post: #4
RE: Hp91 with very dim display
(08-02-2020 11:05 AM)James Linder Wrote:  I recently acquired a very nice 91 TopCat with a display so dim that I have to be in a dark room to see what’s on the display. I found no loose connections inside, no corrosion issues, and there were no obvious burnt components on the PCB’s. What are the likely causes for this problem? I’m using a known good, fully-charged battery pack and hooking up a known good charger changes nothing. Just guessing I thought maybe replacing one or both pairs of identical chips on the display board with the 24 wire ribbon cable going to the display might’ve my next move. Any help would be appreciated. I’m capable of de-soldering and soldering and have spare 97 display and all other 97 parts to use for repairing the 91. Thanks for your help!

It looks like the only common part of the display driver circuit is what looks like a constant current source built from a transistor, 2 resistors and 2 diodes. Either of these could be faulty or have a poor connection and is limiting the current to the display segments. Both anode drivers on their pin 14's which are common, connect to the collector of PNP transistor Q1.

cheers

Tony
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08-02-2020, 04:30 PM
Post: #5
RE: Hp91 with very dim display
Thanks for your help! I’m going to take voltage readings with my multimeter working from the battery to the power supply and beyond to see where the voltage first drops and replace or re-solder the component where the problem begins. I expect it will be within the power supply but will also focus on the press/pull connectors between boards since they are obvious potential problem sources. Since it appears this 91 had never been opened up before I got it last week, it’s not likely a connector unless it was dropped and this doesn’t appear to be the case.
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08-02-2020, 06:26 PM
Post: #6
RE: Hp91 with very dim display
Lately I had an HP-97 with a dim display. Maybe not as dim as that one of the TO.
I disassembled the display PCB and found Q1 "exploded" -broken into pieces. After replacing that one and measuring around with a DMM I found that Q3 and Q4 had disintegrated, too. I also replaced those with no better result.

It showed that the resistance was not correct of R5. Instead of 4.0 ohms it showed up with nearly exactly 1000 ohms. After replacing that one (I only had several 10 ohms handy I solder 2 of those in parallel) the display showed up in normal brightness.

If you have the service manual I found out during that repair that in figure 4-23
a) connection from Vb going to R5 does not exist going to R7 that goes to ground
b) bat trim R1* should be R2* - but R2* is replaced by a cap. My guess: 2.2µF
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08-03-2020, 02:34 PM (This post was last modified: 08-03-2020 02:39 PM by teenix.)
Post: #7
RE: Hp91 with very dim display
(08-02-2020 04:30 PM)James Linder Wrote:  Thanks for your help! I’m going to take voltage readings with my multimeter working from the battery to the power supply and beyond to see where the voltage first drops and replace or re-solder the component where the problem begins. I expect it will be within the power supply but will also focus on the press/pull connectors between boards since they are obvious potential problem sources. Since it appears this 91 had never been opened up before I got it last week, it’s not likely a connector unless it was dropped and this doesn’t appear to be the case.

It could be the power supply, but I would suspect something else, otherwise the calculator overall would probably not work. The drive for the LED display is not part of the power supply, as a clever design kept it separate. My starting point would be the 4 ohm the resistor feeding Q1. Only one of the 106 LED segments is on at a time, and the logic circuits appear to work ok, so a higher resistance, or other faulty component around there, would easily dim the entire display.

Be careful with the display ribbon cable, it could be damaged removing and re-inserting it.

The printer working, but not printing, could be a failed battery connection in the ribbon cable or connectors. This power is also cleverly isolated from the logic supply and most of the analog smarts are on the circuit board covered in the maroon coloured coating. As you mention, it may also be just a dodgy circuit board pin connection. Also, please be careful if you feel the need to bend these pins, they break easily.

All just ideas of course :-)

cheers

Tony
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08-03-2020, 04:03 PM
Post: #8
RE: Hp91 with very dim display
Thank you for your advice sir! I sincerely appreciate you sharing this information. I plan to put it on the bench and start testing either today or tomorrow.
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08-03-2020, 04:09 PM
Post: #9
RE: Hp91 with very dim display
To remove the display DON'T remove the ribbon cable but just unhook it from inside the frame.
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08-03-2020, 04:21 PM
Post: #10
RE: Hp91 with very dim display
Will do. I've had a 97 apart a few times and both ends of the display ribbon cable appeared to be soldered onto the the two PCB's. I see that the printer ribbon can be removed, however I did not think the display ribbon was removable. Being new to Topcats, I'm just telling you what I tought I saw, not for I know for certain of course. Thanks!
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08-04-2020, 12:26 AM
Post: #11
RE: Hp91 with very dim display
(08-03-2020 04:21 PM)James Linder Wrote:  Will do. I've had a 97 apart a few times and both ends of the display ribbon cable appeared to be soldered onto the the two PCB's. I see that the printer ribbon can be removed, however I did not think the display ribbon was removable. Being new to Topcats, I'm just telling you what I tought I saw, not for I know for certain of course. Thanks!

Just in case you are not aware, you cannot just pull out the printer ribbon cable without damage. You must use something thin and wide enough which can be inserted between the cable and connector to move the connector fingers away from the cable. If you just try to pull it out, the fingers will dig harder into the cable. I used a piece of aluminium soft drink can with smoothed edges, but there are other ideas in various Forum posts.

The display ribbons I've seen can be carefully removed from the circuit board end, but the pressure required for removal/installation may damage it because of its age. There are no connector pins as such, just tinned wire ends and it would be a horrible repair job if any of those wire ends broke off. I think unless necessary, it is best to leave it as is.

cheers

Tony
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