Casio FX-602P

[PerranOak]

I have this wonderful programmable calculator that I bought in about 1981 or 82, I think.

Ever since then it has been my "go to" calc until it got forgotten somehow and left in a drawer. Anyway, I found it the other day, replaced the batteries and hey presto! Nothing.

At times it will show random digits but the buttons never work and the batteries drain away in no time. It's a shame as it worked perfectly for 35 years. 

I know it's all very vague and a long shot but are there any "typical" things I can check for or is it a lost cause?

Cheers.

[Whales]

it will show random digits

Is the screen zebra-stripped?  Apply gently pressure to the back of the LCD screen + strip + board sandwich, see if the digits come alive.

Tried it outside?  Random digits might be corruption/soft-locks from interference from something in your room.


Otherwise: are you putting the batteries in backwards? 

Finally: depending on how much power the batteries are putting out: an alcohol evaporation test might help you pinpoint which part is causing the issue.  Douse the board (*gack* this thing has a screen -- might not be possible) in methylated spirits or similar and put the batteries in, see which part of the board it evaporates from the fastest. 

[Ero-Shan]

Yeah, I love my FX-602P also. A damn fine calculator (and you can easily speed it up!)

If you don't want to bathe yours and have access to a thermal camera, you could try that. By chance I had one on loan when I was asked to look after a clamp meter with a huge power drain. The culprit could be easily identified (but repair was impossible).

Hope you can find the problem and fix.

[PerranOak]

Thanks both.

There is no area that is hotter than the rest … even the chips!

When it's turned on it displays a sort of "H" shape then when I press a load of keys I get the other pattern. Nothing else happens.

I made sure that the batteries are in right. I love the old machine, it's such a shame.

[SeanB]

Remove the main board and clean it with IPA and dry it thoroughly, then try again. The fault most likely is either a conductive film from the old CR2032 batteries dying, contaminating the zebra strip to the LCd, or a zebra strip that has relaxed. Take the display off, clean the contacts on the LCD with a Q tip and gentle rubbing, ands the same for the 2 zebra strips. Install the same way as they came out, and be careful with the display clips. Nice calculators, I have one as well, on it's 4th set of cells from new, they last a long time

[HighVoltage]

I had similar problem with a Casio calculator, took the zebra strips out, cleaned both sides carefully with alcohol cleaned also the glass with alcohol and it worked perfectly afterwards.
 

[Whales]

I had similar problem with a Casio calculator, took the zebra strips out, cleaned both sides carefully with alcohol cleaned also the glass with alcohol and it worked perfectly afterwards.

^ this

Please also don't skip my suggestion of gentle pressure on the LCD + zebra + board sandwich. 


Bad connections to LCDs make them display all sorts of interesting patterns.  I've seen this cause everything from "static corrupt patterns" to "valid individual digits, wrong overall number".   The latter was on one of my multimeters.

[shakalnokturn]

No experience on this peculiar model.

Looking at photos the "zebra" is probably the last thing I would handle. It looks like a glued ribbon is used rather than a rubber strip -much more of a risk to handle.
I've never seen a CR2032 spew of old-age so you're really out of luck if yours have managed to destroy the zebra at the opposite end of the pcb.

Your calculator appears to have tantalum drop capacitors, considering age and that the batteries drain too fast these are the first things I would be looking at (not always easy to test... I'd throw in MLCC equivalents without even testing).
Also check switch on and off contact resistances, then maybe look for leaky diodes/transistors if no improvement.

[rsjsouza]

I also have no experience with this model. However, my old fx-39 (VFD, bought new in 1982) suffers the plague of the resistive membrane keyboard that was very common across Casio product lines at the time. This membrane becomes brittle over the years and causes the keyboard to stop working. Since then I got my brother's fx-39 and another one from eBay; all with the same issue.

[drknob]

My HP42S ( a fabulous programmable RPN calculator discontinued in 1995 ) was going through batteries at a high rate. Solved by replacing some leaky tantalum caps. Works like new....

[PerranOak]

Wow! Thanks all, can't wait to try these out.

Here's hoping!

[PerranOak]

So, just to be sure. By "clean the zebra strips" this means pulling them apart, yes?

[HighVoltage]

So, just to be sure. By "clean the zebra strips" this means pulling them apart, yes?

NO!
Just remove the zebra strips from the glass and from the PCB
Then clean the two surfaces on the zebra strips and the surface on the glass and the surface on the PCB

[PerranOak]

Sorry to be a pain but to be crystal clear - rip this strip (see photo) completely off the PCB?

How do I stick it back down?

[rsjsouza]

I usually don't call this a Zebra strip, but instead those rubbery rows of contacts. I have seen it fail at a times, but I would be wary of disassembling this particular one.

[HighVoltage]

That is not a typical zebra stripe and should not be disassembled
You can measure continuity on each connection but they are probably ok

[Whales]

Ooh no, don't pull that off.  That's not a zebra strip.  That's a flexible PCB/connector that has been solderes/glued on.

Zebra strips feel and look like flat bits of rubber sandwich, often pink and black.  They typically have 'too many' conducting rubber wires inside of them so that you can use them on any pattern of pads; they don't need customising to your particular pad pitch.

https://en.wikipedia.org/wiki/File:Zebra_connector.jpg

Notice the black conductive 'filaments' are narrower than the pads.

A real zebra strip will just 'fall out' of a calculator with the gentlest of knocks or tugs.