HP-41C PCB Repair Question.



#2

My long time trusty HP-41C has begun giving MEMORY LOSS errors this morning when it was left off for more than 10 minutes. Each time became more difficult to turn it on, no response. While it was on, I was able to run the MEMTEST program to makesure everything was ok, worked fine, until I turned it off.

I had to pull the calculator aprat to check the curcuit. I was first suspecting from the C1 470uF capacitor which holds the memory when power is turned off. However all capacitors seems to be fine. I am not measuring any significant leakage. My next suspect is the Q3 Transistor (the most lower right in the photo), it is a 1854-0092 which is part of the Memory Reset circuit.

Is there anyone know where can I find 1854-0092 (NPN Transistor) or an equivelant?

Thanks in Advance.
Kerem

Photo

Edited: 19 Nov 2011, 2:07 a.m.


#3

Are the batteries ok?

Are the screw posts, especially the two lower ones, ok? If these screws aren't screwed in tight, the small PCB makes no good and permanent contact with the keyboard PCB, through which it gets its power.

#4

Is this alright?


http://www.talonix.com/shop/item.aspx?itemid=4386

#5

I had a similar problem - if your calculator has the "zebra strip" elastomeric connector between the CPU and the keyboard PCB that may be the problem, or it might be the connections to the LCD/switch board. Having another HP41 to swap the CPU board with is pretty handy for trouble-shooting.

This link might be of use.


Edited: 19 Nov 2011, 12:10 p.m.

#6

To assume the transistor is the problem just because it controls mem reset is a good way to waste lots of time looking for a part you don't need.

FWIW, the problem is almost always caused by a bad D/S integrated circuit. Could be a power supply IC issue but a bad memory IC is the far more common cause. Realistically, without a HP41 service rom, you're lost in the wildness at night without a compass or torch.


#7

+1. I'd give it nearly zero chance that it's the transistor. Silicon bipolar transistors that are run within their design parameters and aren't run hot last indefinitely. If it is bad, it's almost for sure because something else went south and took the transistor with it, meaning that if the first cause isn't fixed, you can replace the transistor and the replacement will blow right away too.

#8

Thank You All for your valuable comments and suggestions! Very much appreciated.

I have taken your suggestions and comments spend several hours today, troubleshooting as much as with what I had available.

- Verified Batteries are OK, contacts are fine and clean. Cleaned all gold contacts with a pink eraser very gently, and removed the entire residue.

- Zebra strip looks ok, I think the round plastic in the zebra strips is supposed to be round, but with many years of pressure on it, it is somewhat square, however I traced the lines with PCB in place and verified the electrical connectivity.

- Another reason I was suspecting the transistor, because I found a repair sticker in the back of the calculator dating 1990. Also that transistor had relatively new solder contacts and unlike other two transistors, had a non-HP part number 53-0482-952. I had this calculator since 1992 and was working OK until recently, except every key stroke resulted in display blinking with several segments turning on across the display since I owned it. I contributed this problem to the early 41C's bugs and issues. Especially how the CAT command operated... Never had to open it until now. However I understand the argument about the transistor and how unlikely it could be the cause.

- Posts are fine, no cracks, or issues.

Final Result: It no longer turns on. No response at all.

I agree with Randy that it seems like a bad D/S circuit.
I will stop wasting more time with it, perhaps I can use the original board which will become available after upgrading my 41CV to CL in the future.

Thanks and Best Regards,
Kerem


#9

Quote:
Zebra strip looks ok, I think the round plastic in the zebra strips is supposed to be round, but with many years of pressure on it, it is somewhat square
I've only seen them rectangular, as shown at http://www.fujipoly.com/products/zebra-elastomeric-connectors.html .
#10

Quote:
- Zebra strip looks ok, I think the round plastic in the zebra strips is supposed to be round, but with many years of pressure on it, it is somewhat square, however I traced the lines with PCB in place and verified the electrical connectivity.


That sounds like the older (and more reliable) printed wiring connector - looks like two clear tubes with a flat section between them? The zebra connector is a rectangular blob of pink elastomer with alternating black carbon conductive imbeds.

Complete shot in the dark: I have a 41C that had the same turn on problem (but not the memory loss) which was fixed by resoldering the interconnect cable between the LCD and the main board. Bear in mind this is an "Easter Egg" approach to trouble-shooting. Randy's advice above is the bottom line if you want to get to the bottom of the problem systematically.

You might want to sort this out before trying a 41CL transplant - if the problem you're having is not due to a bad CPU board you might be in for quite a confusing time.

Edited: 20 Nov 2011, 8:06 p.m.


#11

Quote:
That sounds like the older (and more reliable) printed wiring connector - looks like two clear tubes with a flat section between them?

It is the older type printed wiring connecter. Two clear printed wiring tubes (with an elastomeric clear tube in each)with a flat section between them. It has good contact to both board and CPU.

More I read the comments and more I think about how the problem started, It seems more to do with CPU D/S or RAM problem than connectors. Because the calculator was working fine and started giving "MEMORY LOST" errors upon turn on. It took a few presses on the ON button to get it working (getting worst each time), then it worked fine. I was able to write several MEMTEST problems to check the memory, registers, etc. If it was turned on immediately after it was turned off, no problem, came right back up. However more time it was shutdown, more difficult it got to wake up and resulted in MEMORY LOST error. Then never turned back on again... That's when I decided to open it up, began troubleshooting


#12

Quote:
If it was turned on immediately after it was turned off, no problem, came right back up. However more time it was shutdown, more difficult it got to wake up and resulted in MEMORY LOST error.

Except for the MEMORY LOST (which is obviously a big difference) that's exactly what mine was doing - it would have to sit for a couple of days before it became difficult to turn on, but once it started working I could turn it on/off repeatedly with no issues.

Regards,
Bob


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