HP-67 Card Reader & Sense Amp problem

[mikemorris]

Hello Everyone
This is my first thread so let me know if I'm not following appropriate etiquette.
I have an HP-67 which has been a project that keeps on giving. Lots of damage resulting from leaking batteries. I now have it working as a perfectly useful calculator with working keyboard, display etc. with an original HP logic card. However I'm having problems with the card reader.

Upon inserting a card the motor does not run and the display shows "Error". I've done some testing and I believe that the MTRS ball switch is not working properly. However, I tried forcing the line low with a jumper. Still no motor running. I applied ~3v to the motor to demonstrat that the motor does work. I then noticed that the sense amp chip is quite hot.

I then did a bunch of current measurements. With the calculator fully assembled and connected to my bench power supply, but calculator turned off, it is drawing 110mA. Turned on it draws 250mA. With the sense amp pcb disconnected from the logic card, each of these measurements are 30mA lower.

EDIT - I've disconnected the wire from the battery to the Sense PCB thereby eliminating that power draw. With the TNix logic card the device draws 0mA when switched off and 80mA when on. That makes sense to me.

I also have a TNix logic card (thanks @Tony) and performed the same tests. Similar current draw when turned off. The TNix card draws less power than the HP card when turned on. All other symptoms are the same.

I'm thinking that the power-off current draw is much too high and that current must be going through the sense amp IC (it can't go anywhere else according to the schematics). Probably explains why its hot.

Any thoughts on further testing? Could this be a problem with capacitors or am I looking for a new 1826-0322?

Any suggestions welcome!

[mikemorris]

I'm still working on this sense amp board problem (Is "Sense Amp" the right term?). I think I've found a discrepancy in the schmatics for the HP-67.

- Someone developed a set of schematics which are hand drawn, I'm not sure who the author was. Those schematics show 22uF capacitors connecting pins 4&6 (HAC, CA), and also pins 8&10 (HBC, CB). The pick up coils are shown connected directly to pins 4&5, and 8&9.

- There is another set of schematics drawn with some CAD tool making them look very professional. This set of schematics shows pins 4&6 connected to each other directly and the capacitor is in series with the pickup coil. Pins 8&10 are wired similarly.

I have not traced our my Card Reader PCB completely and I think this is not material to my current draw problem. Can anyone help clear this discrepancy?

[teenix]

It is most likely that the sense amp has failed. I think if the internal motor drive circuitry gets overloaded for too long it becomes damaged. This can be caused by heavier than normal mechanical loads on the motor, damaged motor, or the chip has simply failed from age.

Tony Duell hand drew all those circuits, amazing job, must have taken an awful lot of time. As far as I know you can trust his work.

I did some schematics, but there are a couple of errors in some of them. Eg. the 67 circuit shows the HAC and HBC pins connected to the wrong side of the capacitor. I wanted to repair them, but I was using an old WinXP 486 desktop to draw them with Protel and the hard disk partially failed (unfixable) so as soon as the disc tried to read some sectors it went bonkers and the PC had to be turned off. The circuit drawings occupied some of those sectors - go figure. With backups missing somewhere, the only way I can fix the drawings is to redo all of them, lots of time. Anyway, Tony Duell's works are so useful, I decided to delete the download.

cheers

Tony

[mikemorris]

Thanks Tony! I think your diagnosis makes sense. I'm going to try replacing the caps. I read someone's comments in which they were surprised how much doing that helped in another situation. However I also suspect the sense amp chip itself. Thanks to the Teenix logic board I don't really need the card reader, but amongst my personality defects is a strong discomfort with any machine which is not entirely functional regardless of whether I need all the functions ;-)

Does anyone have thoughts on where to find one of those sense amp chips? Presumably a donor calculator? Obviously another HP-67. Are there other models which could also be a donor?

-Mike-

[isanchez]

Hi,

I believe I have the same problem, but with an HP65...
Would it be possible to use the sense amp of an HP67 on my HP65?? they have different chip code ref., but apparently look to be the same.

Thanks and regards

Ignacio

[Hiwi]

Hi Ignacio,

yes this should work, there are several threads covering this.

Ralf

[isanchez]

thanks, I shall try and come back to confirm.

Regards

Ignacio