I/O subsystem not working on HP-41CV?



#2

Hi all,

I've just received a very good looking HP-41CV from 1983 (Singapore). I have been told it was well looked after, and it looks it, but unfortunately it doesn't work very well.

My diagnosis (possibly flawed) is that the I/O subsystem is faulty.
I deduced that from the following observations:

- Start from fresh. Batteries have been in for a while, but the calculator hasn't been turned on for hours.

- Turn on. The calc on its own (no module, no card reader, nothing inserted in the ports) works fine. I haven't found any problem with it.

- Plug in a card reader (81104A), and a CAT 2 doesn't report anything. Nothing happens if you try to insert a card in the reader (the reader works fine in any of my other HP-41's).

- Plug in a TIME module (with or without the card reader still plugged in), and the calculator goes mad, with random characters (including lots of "@" for some reason) flashing on an off, and no real keyboard control. The calculator usually cannot be turned off except by taking the batteries out.

- After a while of on/off/batteries out and messing around with the keyboard, the calc slowly "stabilises", and eventually things start to work. A CAT 2 correctly reports the TIME module and the card reader. Things look peachy. The stopwatch works, and the card reader reads cards properly.

- Turn off the calculator (and go to bed in my case).

- Turn on the calculator some time later (as short as 1/2 hour) - mayhem again...

A layman's speculation is that something needs to "warm up". I thought maybe a faulty capacitor would be the cause?

In the absence of Geoff's repair book (I'm signed up for when it comes out), I am reduced to asking for your help (and Geoff's, if he is reading this).

Any precise idea what this could be? Is that repairable?

Of course, if it's not repairable I could just "sacrifice" it to become the shell of a HP-41CL :-)

Regards,

Philippe


#3

Hi Philippe,

From the date, 1983 I am pretty sure you have a "fullnut" type of CV. It sounds like the contact between the I/O block flex circuit board and the display/keyboard is not good. This could be due to one of two problems. The contact is just dirty, or even slightly corroded, or that one or both of the top two screw posts is cracked or broken. When this happens, the screws are not tight enough to make good contact.

You will need to disassemble the calculator regardless. Hopefully it's just dirty contact, and not broken screw posts. If the screw posts are not cracked or broken, clean the I/O block flex circuit board connector and the contacts on the display keyboard, and hopefully all will be good. If the screw posts are broken, there are one or two forums postings/articles on this documenting repairs, or contact me offline about repairing it.

Cheers,
John


#4

Hi John,

Thanks for your reply and help.

Yes, the HP-41CV is a fullnut, and I am familiar with the flex connector, and posts, problem. I should have mentioned that I had thought about that. I didn't think it was a possible cause because the calc is tight all around, and there is no indication of any weakness in that area (no squeek, no play). There is no indication either that the calculator had even been dismantled (if it had, it would have been a professional job). I even put pressure at various points while testing just in case, but that didn't change anything.

Ultimately what convinced me that it probably wasn't a contact problem was the fact that *after a while* the calculator starts to behave properly. Since it's sitting on a desk the whole time, it didn't look like an environment issue (e.g. change of temperature forcing the connector to make a better contact); so I was left with the possibility of an electronic problem. I will probably open up the calculator anyway, to check.

I vaguely remember that there are two "large" electrolytic capacitors on the main board that can go bad, but I thought that these were part of the power supply control. If that were affecting the power enough to bother the card reader, I wouldn't however have thought that it would affect the TIME module.

Thanks again,

Philippe


#5

Hi Philippe,

You can't really tell if a 41 has been opened or not, as the screws are under the four rubber feet. The feet can certainly be reused after re-assembly.

You could well be right about the 41 logic board and even the capacitors, but I still think a good clean of the connectors, both the I/O flex board connection and the connectors between the logic board and keyboard etc. It's worth a shot.

You may need to test the logic board in another calc. I could help you with that if need be, and I may even have a spare CV board to match yours.

Cheers,
John


#6

Hi John,

Quote:
You can't really tell if a 41 has been opened or not, as the screws are under the four rubber feet. The feet can certainly be reused after re-assembly.

Yes, I know. I have opened 41s before, and carefully saved the feet myself.

Quote:
You could well be right about the 41 logic board and even the capacitors, but I still think a good clean of the connectors, both the I/O flex board connection and the connectors between the logic board and keyboard etc. It's worth a shot.

Indeed, so I took the plunge this afternoon and took the calculator apart.

The first thing I noticed is that it had quite possibly already been opened before. That's because one of the four feet has its backing adhesive tape missing (probable indication that the foot had been removed). Also, two of the four feet recesses had a slight but unmistakable notch; the type you could get by trying to pry up the foot using a small screwdriver. Of course, removal of the feet is no proof that the screws were taken out and the calculator opened...

All four posts were intact and solid. A minor amount of gunk was around the two halves and the spacer "U" strip - cleaned that. Some dust under the display glass - flushed it with compressed air.

The I/O block was in good condition overall. However, the narrow and long foam block that forces the connector stripe onto the PCB looked out of shape; that's not surprising really (29 years). The contact areas for that locked a bit lacklustred, especially on the I/O block. The 4 connectors to the modules looked good, but you could easily see where contact springs from modules had been - all that could go with a good clean.

I unfortunately don't have a good contact cleaner (for now), so I simply used a solvent and cotton buds and cleaned the PCB and I/O block connection areas as best I could.

I then reassembled the 41CV temporarily.

That brought some measure of success: the calculator immediately recognised the card reader this time, and I was able to feed a card; so it's working.

The TIME module also worked, but only in position #4 (that of the card reader). In position #2 it appears to work (CAT 2 is fine), but execute CLOCK and the calculator turns off. In position #3 the calculator goes mental (crazy display and piezo whining). In position #1 it "works" but you can hear a faint whine from the piezo.

In summary, I think this confirms that you were perfectly correct: it's the I/O block (contacts and connectors) that appears to be the culprit.

Once I get a proper contact cleaner (I was thinking DeoxIT D100S-2 - any comment?) I should be able to fix it properly.

NB: I left the contact strip between the keyboard PCB and the CPU PCB well alone; that worked, no need to rock the boat.

Quote:
You may need to test the logic board in another calc. I could help you with that if need be, and I may even have a spare CV board to match yours.

Thanks for the offer :-) I guess that could come in handy if a full and proper clean doesn't fix the problem entirely.

Thanks again.

Regards,

Philippe


#7

Hi Philippe,

I am glad to hear you have had some success with the CV. I think you're correct about leaving the log board/keyboard connector strips alone, they are easy to damage.

As for the cleaner, the products available to me in Australia are probably different to what's available to you, but in general I use Electrolube (U.K. made) products.

You could just use to isopropanol, often available from supermarkets, the purer the better. The one I use is 70% isopropanol, which is sold as "rubbing alcohol".

Good luck

Cheers,
John


Possibly Related Threads...
Thread Author Replies Views Last Post
  HP 41CV Repairs William Mabotja 3 562 11-20-2013, 04:45 PM
Last Post: Mike Powell
  "Packing" syndrome of my HP-41CV PGILLET 12 1,246 11-03-2013, 02:46 PM
Last Post: Hans Holzach
  HP Prime Save Power Turn Off Not Working Timothy Roche 12 1,275 10-27-2013, 01:41 PM
Last Post: Michael de Estrada
  Temporary User Mode Key Programs not working in RPN BruceTTT 7 900 10-14-2013, 01:46 PM
Last Post: BruceTTT
  HP-41CV - A Walk Down Memory Lane Tom Lewis 9 928 03-18-2013, 11:14 PM
Last Post: BShoring
  HP-97 suddenly not working Brad Barton 10 955 02-25-2013, 08:15 PM
Last Post: Brad Barton
  [41CL] Problem with PowerCL ... not working Doug (NYC) 5 663 01-22-2013, 07:18 PM
Last Post: Doug (NYC)
  42S exit key not working dwsmith 4 539 11-05-2012, 10:59 AM
Last Post: dwsmith
  HP-46 not working Ignacio Sánchez 1 337 10-24-2012, 11:17 AM
Last Post: Donald Williams
  HP-41CV power problem Don Davis 2 450 10-11-2012, 09:13 PM
Last Post: Don Davis

Forum Jump: