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+--- Thread: cathode for HP91 (/thread-4452.html)
cathode for HP91 - isanchez - 07-30-2015 06:29 AM
Hi everybody,
I am repairing an HP91 and to finalise it I need a working display cathode. The reference from the HP97 manual is:
- 1820-1629, integrated circuit , cathode driver
I have replaced it from a working 97 and works fine.
Could you please tell me if there exists an equivalent reference avaliable to buy?
If not, do you have one for donate/sale? Please give me hand with this. It is present in 97/91/92 models. If you have one to sell, please send me a PM to the google address:
sanreig "at" gmail "dot" com
I would appreciatte very much your help. It is really a pity to have this out of order just for the cathode.
Thanks in advance
Kind regards
Ignacio
RE: cathode for HP91 - Katie Wasserman - 07-30-2015 04:11 PM
(07-30-2015 06:29 AM)isanchez Wrote: - 1820-1629, integrated circuit , cathode driver
I have replaced it from a working 97 and works fine.
Could you please tell me if there exists an equivalent reference available to buy?
Unfortunately there is not an equivalent chip. The cathode driver is also the keyboard scanner and the low battery detection chip, it's relatively complicated and would be difficult to replace with anything other than a programmed microcontroller. (The project that I described here implicitly describes how the scanning works using the RCD and STR control lines.)
RE: cathode for HP91 - isanchez - 07-31-2015 04:59 AM
Dear Katie,
Thank you for your answer. I was afraid of that....a pity. I shall try to find one from a 97.
You mention that it controls the low battery led...in my case, after replacing the cathode, it in the 91, the low battery indicator is always on. I have read in the service manual, that it is associated to he pair cathode / R2 resistor ... In the logic schematic, R2 is not shown...I suppose it is the R* shown as variable... I should then change the R2 resistor to reduce the indicator sensitivity...
Kind regards and many thanks
Ignacio
RE: cathode for HP91 - Katie Wasserman - 07-31-2015 05:09 AM
(07-31-2015 04:59 AM)isanchez Wrote: You mention that it controls the low battery led...in my case, after replacing the cathode, it in the 91, the low battery indicator is always on. I have read in the service manual, that it is associated to he pair cathode / R2 resistor ... In the logic schematic, R2 is not shown...I suppose it is the R* shown as variable... I should then change the R2 resistor to reduce the indicator sensitivity...
In the 97 service manual (page 4-23) it's marked as R1* and the parts list shows that it can have values of between 2.87K and 9.09K ohms.
RE: cathode for HP91 - PANAMATIK - 07-31-2015 08:04 AM
(07-30-2015 06:29 AM)isanchez Wrote: I would appreciatte very much your help. It is really a pity to have this out of order just for the cathode.
Although it would need much less work than the new ACT and the concept would be rather easy, the development of a microcontroller based cathode driver is too expensive concerning SMD manufacturing costs for small quantities.
Bernhard
RE: cathode for HP91 - isanchez - 07-31-2015 08:27 AM
Hi Bernhard,
maybe we need to launch a crowdfunding initiative! ;-D the thing is to coordinate to all people affected by cathodic failures.
Thanks Katie!, I think you are talking about the card reader board. The display /keyboard circuit board list of components is in table 4.9, where R2* is listed but I can't see in the diagram of page 4.23. I believe in this page it is mentioned as R1*, instead of R2*...but maybe I am wrong.
may I kindly ask you what should I do to reduce the low battery indicator sensitivity (as it is always "on"): to increase or decrease the resistor value?
Kind regards
Ignacio
RE: cathode for HP91 - Thomas Radtke - 07-31-2015 08:34 AM
(07-31-2015 08:04 AM)PANAMATIK Wrote: Although it would need much less work than the new ACT and the concept would be rather easy, the development of a microcontroller based cathode driver is too expensive concerning SMD manufacturing costs for small quantities.Maybe an Arduino Nano or similar would do. I guess you'd need 14 IOs for the display and keyboard and some control lines (3?).
RE: cathode for HP91 - Katie Wasserman - 07-31-2015 04:04 PM
(07-31-2015 08:34 AM)Thomas Radtke Wrote:(07-31-2015 08:04 AM)PANAMATIK Wrote: Although it would need much less work than the new ACT and the concept would be rather easy, the development of a microcontroller based cathode driver is too expensive concerning SMD manufacturing costs for small quantities.Maybe an Arduino Nano or similar would do. I guess you'd need 14 IOs for the display and keyboard and some control lines (3?).
The problem would be the socket. It's a 22 pin DIP using 18 pins for i/o plus Vcc and ground (2 pins are apparently unused acording to the schematic). I'm pretty certain that no modern microcontroller can match the pin configuration of this custom IC so you're stuck with using an SMD chip on a carrier-like board or wiring a 22 conductor ribbon cable in place and locating the circuit elsewhere inside the calculator. This is a possibility in that there's a good about of free space inside a 91/92/97. If you do that you could use almost any micro with enough i/o (and many an ADC for low voltage detection).
RE: cathode for HP91 - Thomas Radtke - 07-31-2015 05:34 PM
(07-31-2015 04:04 PM)Katie Wasserman Wrote: [...] you're stuck with using an SMD chip on a carrier-like board or wiring a 22 conductor ribbon cable in place and locating the circuit elsewhere inside the calculator.This is what I was thinking of. Programming a serial RAM emulator shouldn't be too hard. Most of the work is soldering the wires. Common anode should also be possible by switching the IOs to input w/o pull up. Problem might be the current. 40mA is probably not enough per digit (all segments plus dot), so it would take a driver. Anyway, piece of cake for our EE-wizards.
RE: cathode for HP91 - Etienne Victoria - 08-02-2015 08:35 AM
Hi Ignacio,
You have mail.
Best
Etienne
RE: cathode for HP91 - isanchez - 08-04-2015 07:21 AM
Hi,
This is to thank very much Ettienne Victoria for his offer to send me a working display board for my hp97 for free....even a printer frame to replace mine.
This forum has always been a place of wise and generous people.
Thanks again, Etienne
Best regards
Ignacio
RE: cathode for HP91 - d b - 08-04-2015 02:25 PM
(08-04-2015 07:21 AM)isanchez Wrote: Hi,
This is to thank very much Ettienne Victoria for his offer to send me a working display board for my hp97 for free....even a printer frame to replace mine.
This forum has always been a place of wise and generous people.
Thanks again, Etienne
Best regards
Ignacio
Yes, Ettienne is an asset to our community in many ways. Being a master tinkerer with a box of spare parts are only two.
BTW: He cooks with dangerously hot sauces from small bottles and likes obscure or local ones that he can't find in France. (hint mine, not from him)
RE: cathode for HP91 - TASP - 08-05-2015 05:22 AM
Beats the dead battery packs I've been inflicting on the regulars here !!
