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Dave Britten · 05-10-2019, 01:31 AM

I opened up my 41CV to clean out the 6 key via the tiny holes in the PCB behind each button, using a tiny paint brush and isopropyl alcohol. No heat-stake removal was involved, just a removal of the bottom case, the CPU board, and the black insulating sheet that lays on the rear of the keyboard PCB. It was working perfectly fine before disassembly, as I had spent about an hour writing down some programs I had in memory before yanking out the batteries.

I tried the standard stuff: various combinations of backspace, enter, and on, shorting all the battery terminals inside the calculator using some foil, trying to get Beep to do anything in case it's just a screen problem, etc. The batteries are currently reading 1.2 V each, but the calculator seemed to be working fine at that voltage right up until I opened the case.

Three of the four screws are nice and snug, but the one nearest to the divide key spins freely when put all the way in, and doesn't feel very tight. I noticed a tiny hairline crack in this screw post, as well as the post in the upper left corner (when viewing from the bottom), even though that one tightens just fine. I have some epoxy curing in those two cracks right now. I put a screw into the posts first to slightly spread the cracks so the epoxy could flow inside more easily, then removed the screws. I'm going to let it cure until tomorrow evening to make sure it's good and solid before attempting to reassemble.

Is there anything else I should try doing to revive it before I go searching for a replacement?

g8241CV · (This post was last modified: 05-10-2019 05:13 AM by g8241CV.)

Lots of more experienced people here (many great threads I researched before doing repairs on two 41's), but I would swap in known fresh batteries (1.5V) as your next step. Easy elimination of a variable for problem solving diagnostics, and you need them anyway to avoid the inevitable cell leaks from your weak existing cells. Then, before you try to close with your repaired post, suggest you try gentle cleaning of contacts (both sets, upper near batteries and lower for CPU PCB) with IPA, and use DeOxit. You're looking to eliminate a particulate contaminant on contacts that you may have missed in cleaning or dislodged during reassembly. While you're in there, suggest you use DeOxit on at least the bum key. Suggest you observe ESD precautions (e.g. use ESD strap).

Not sure what you mean by "shorting all the battery terminals" -- if this was really a short of all terminals, then no power to the calculator and that shorting may have reduced the cell voltage(s) below the operating threshold. You might post a close-up photo of the contacts since you had reason to be concerned.

Post back, don't give up too easily, there are lot's of other great threads on repairs to explore. It's a wonderful feeling when you bring a favorite calculator (CV for me) back to life.
HTH

AndiGer · 05-10-2019, 11:07 AM

Additionally: Sometimes it helps to use a little longer screw on the lower (broken) post. I once used a "left-out-from-underneath-the-label" classic's screw. That helped.
Good luck
Andi

Dave Britten · 05-10-2019, 11:23 AM

(05-10-2019 04:54 AM)g8241CV Wrote: Lots of more experienced people here (many great threads I researched before doing repairs on two 41's), but I would swap in known fresh batteries (1.5V) as your next step. Easy elimination of a variable for problem solving diagnostics, and you need them anyway to avoid the inevitable cell leaks from your weak existing cells. Then, before you try to close with your repaired post, suggest you try gentle cleaning of contacts (both sets, upper near batteries and lower for CPU PCB) with IPA, and use DeOxit. You're looking to eliminate a particulate contaminant on contacts that you may have missed in cleaning or dislodged during reassembly. While you're in there, suggest you use DeOxit on at least the bum key. Suggest you observe ESD precautions (e.g. use ESD strap).

Not sure what you mean by "shorting all the battery terminals" -- if this was really a short of all terminals, then no power to the calculator and that shorting may have reduced the cell voltage(s) below the operating threshold. You might post a close-up photo of the contacts since you had reason to be concerned.

Post back, don't give up too easily, there are lot's of other great threads on repairs to explore. It's a wonderful feeling when you bring a favorite calculator (CV for me) back to life.
HTH

Yeah, I ordered a dozen fresh N cells from Amazon just to rule that out. I've never seen a calculator run fine on low batteries, then refuse to turn on after they've been removed and reinserted, but I'm sure there's plenty of crazy things I haven't seen before. Worst-case scenario, I'll have a lifetime supply of batteries for my 19BII.

I'll probably give all the connectors a gentle scrub with some contact cleaner before I close it back up. Not sure I'd be able to use contact cleaner directly on the key, though, since the only access is a ~0.5 mm hole on the back of the PCB that I can get a couple of IPA-coated paintbrush bristles through. I'm definitely not going to pull all the heat stakes for one key that just requires slightly more force than the rest.

The battery terminal thing is often suggested with various models, in order to drain any stray charge in capacitors that might be holding the machine in some invalid (i.e. non-responsive) state. You just remove the batteries, then bridge the calculator's battery terminals with something conductive, like foil, or a coin.

burkhard · 05-10-2019, 12:43 PM

It seems unlikely to me that you've "killed" it, although there might be a bit of rewarding debug to revive it.

The loose bottom post needs fixing for sure.
While it's apart, be sure to IPA clean and DeOxit the zebra connector and the pads it contacts (or is supposed to contact...)

Keep us posted!

Dave Britten · 05-10-2019, 12:45 PM

(05-10-2019 12:43 PM)burkhard Wrote: It seems unlikely to me that you've "killed" it, although there might be a bit of rewarding debug to revive it.

The loose bottom post needs fixing for sure.
While it's apart, be sure to IPA clean and DeOxit the zebra connector and the pads it contacts (or is supposed to contact...)

Keep us posted!

Yeah, I'll definitely give it a little cleaning just to be safe. It LOOKS absolutely immaculate inside, apart from the crack starting to develop on the two screw posts. No visible evidence of corrosion or oxidation anywhere.

Dave Britten · 05-10-2019, 09:57 PM

Still nothing so far. I tried pumping a 6 V supply directly into the battery terminals via alligator clips, and I could see about 12 mA standby current (seems a bit high), but nothing happens when I press the power key. I noticed that the springs in the battery holder had a tiny bit of corrosion/oxidation on them, so I coated the two that looked bad with a bit of foil, and I was measuring much better series voltage from each pair of cells after doing that - much closer to the actual sum of the voltages from the individual cells. I've got fresh Ns coming in the mail tomorrow, so I'll see what happens with those, but I'm not expecting miracles.

g8241CV · 05-10-2019, 11:10 PM

>Not sure I'd be able to use contact cleaner directly on the key

Highly recommend DeOxit before you do anything more radical to keyboard. I found putting a tiny droplet on the hole and then exercising (depressing/releasing) the key caused the air in the dome to burp out and the DeOxit to be inhaled into the keyhole. Or depress the key, and then coat with DeOxit, and release key to inhale. Only takes a minute quantity to be effective. Mistake: if you repeatedly add the DeOxit, you may even find the fill is too much and the oil begins to migrate out of the key dome into surrounding area. BTDT. For fear of delaminating the keypad layers I burped excess oil into a clean cloth.

Further thought on investigation:

Another path to pursue: carefully inspect all the solder joints on the jumpers from the PCB to the display PCB pads. You're looking for any cold-soldered joint(s) working/worked loose.

Dave Britten · 05-10-2019, 11:14 PM

(05-10-2019 11:10 PM)g8241CV Wrote: >Not sure I'd be able to use contact cleaner directly on the key

Highly recommend DeOxit before you do anything more radical to keyboard. I found putting a tiny droplet on the hole and then exercising (depressing/releasing) the key caused the air in the dome to burp out and the DeOxit to be inhaled into the keyhole. Or depress the key, and then coat with DeOxit, and release key to inhale. Only takes a minute quantity to be effective. Mistake: if you repeatedly add the DeOxit, you may even find the fill is too much and the oil begins to migrate out of the key dome into surrounding area. BTDT. For fear of delaminating the keypad layers I burped excess oil into a clean cloth.

Further thought on investigation:

Another path to pursue: carefully inspect all the solder joints on the jumpers from the PCB to the display PCB pads. You're looking for any cold-soldered joint(s) working/worked loose.

Both good ideas, thanks, I'll try that if the fresh batteries get me nowhere. I just have to figure out where to buy DeOxit. Is it safe to squirt that stuff into the dome like that and essentially let it stay in there and potentially coat the surfaces? It wouldn't leave any harmful residues on the contacts, would it?

I might just try reflowing all the solder joints if I get desperate enough. I imagine the 41 is from before the proliferation of lead-free solder, so it should melt pretty easily.

**rprosperi** · (This post was last modified: 05-10-2019 11:37 PM by rprosperi.)

(05-10-2019 11:14 PM)Dave Britten Wrote: Is it safe to squirt that stuff into the dome like that and essentially let it stay in there and potentially coat the surfaces? It wouldn't leave any harmful residues on the contacts, would it?

Totally safe, and in fact it is recommended to leave some DeOxit on the surfaces that will make contact, so it's more than just a cleaner, it also helps to make/retain good contact. Best not to leave a pool inside as advised above, but no need at all to try to remove it all.

It's available on Amazon, eBay, all the places you might expect. For example:

https://www.amazon.com/CAIG-Laboratories...B0002BBVN2

DeOxit Red is recommended for cleaning as being discussed here, but there is also DeOxit Gold (more expensive) for truly troublesome contacts. Some folks recommend cleaning with Red then treating surfaces with Gold, but I've never found a situation where Red didn't do the job.

The small bottle it comes in seems pretty expensive, but a little bit goes a long way, so it's really not bad at all.

John Keith · 05-11-2019, 12:43 AM

(05-10-2019 09:57 PM)Dave Britten Wrote: Still nothing so far. I tried pumping a 6 V supply directly into the battery terminals via alligator clips, and I could see about 12 mA standby current (seems a bit high)...

That's more than a bit high. Sad

The standby current should be in the low microamps. Might be a mechanical issue, e.g. a key being held down by a part of the case.

Dave Britten · 05-11-2019, 01:09 AM

(05-11-2019 12:43 AM)John Keith Wrote:
(05-10-2019 09:57 PM)Dave Britten Wrote: Still nothing so far. I tried pumping a 6 V supply directly into the battery terminals via alligator clips, and I could see about 12 mA standby current (seems a bit high)...

That's more than a bit high. The standby current should be in the low microamps. Might be a mechanical issue, e.g. a key being held down by a part of the case.

Yeah, that struck me as awfully fishy. A stuck key could be a possibility; I know Pioneers can react like that, seeming like they're dead (I got a cheap 32S that wouldn't power on, and just ended up having to fix a stuck key). I can't imagine what would be sticking a key, though. I didn't remove the keyboard PCB, so it's not like something would have gotten wedged in there. Now, electrically stuck from contacts/traces getting unintentionally bridged somewhere, that I could see. Probably wouldn't be a bad idea to give it a good air dusting.

Dave Britten · 05-11-2019, 05:52 PM

Yippee! Apparently it wasn't anything a fresh set of batteries and a thorough contact cleaning couldn't fix. And the 6 key responds a lot better now too after cleaning that out (which was my original goal).

But now that I know how easy it is to remove the main PCB from a 41, it's got me thinking about things like the 41CL... Wink

grsbanks · 05-11-2019, 07:22 PM

(05-11-2019 05:52 PM)Dave Britten Wrote: But now that I know how easy it is to remove the main PCB from a 41, it's got me thinking about things like the 41CL...

Pretty easy but not without risk, so not a decision to be made lightly!

I have a fullnut 41CV that is a likely candidate for a CL upgrade. It's just the fairly hefty price tag stopping me from mashing that "buy" button right now...

Dave Britten · 05-11-2019, 08:13 PM

(05-11-2019 07:22 PM)grsbanks Wrote:
(05-11-2019 05:52 PM)Dave Britten Wrote: But now that I know how easy it is to remove the main PCB from a 41, it's got me thinking about things like the 41CL...

Pretty easy but not without risk, so not a decision to be made lightly!

I have a fullnut 41CV that is a likely candidate for a CL upgrade. It's just the fairly hefty price tag stopping me from mashing that "buy" button right now...

Yeah, if it were like $100 from Amazon, I'd already have one on the way. Big Grin

**rprosperi** · 05-11-2019, 10:12 PM

(05-11-2019 07:22 PM)grsbanks Wrote: I have a fullnut 41CV that is a likely candidate for a CL upgrade. It's just the fairly hefty price tag stopping me from mashing that "buy" button right now...

and

(05-11-2019 08:13 PM)Dave Britten Wrote: Yeah, if it were like $100 from Amazon, I'd already have one on the way.

All things have a price and an associated value.

I agree the price of a CL is not low, however the value of the CL is extremely high, even when considering the not-so-low price. If you like using a 41, just mash the order button ASAP; you will never regret it. I have never heard any CL owner mention a moment of regret once they had their CL working. It really is that good.

grsbanks · 05-11-2019, 11:07 PM

(05-11-2019 10:12 PM)rprosperi Wrote: If you like using a 41, just mash the order button ASAP; you will never regret it.

I fear my bank manager will mash me if I do that right now Smile

Dave Britten · 05-11-2019, 11:46 PM

(05-11-2019 10:12 PM)rprosperi Wrote: All things have a price and an associated value.

I agree the price of a CL is not low, however the value of the CL is extremely high, even when considering the not-so-low price. If you like using a 41, just mash the order button ASAP; you will never regret it. I have never heard any CL owner mention a moment of regret once they had their CL working. It really is that good.

Oh yes, no argument there, but for someone like me that doesn't have 40 years of experience with - and attachment to - the 41, I have to weigh it against all the other things I could spend ~$250 on. But knowing that it's pretty easy to swap the circuit board, and that my CV is of a compatible type, maybe some day.

mfleming · 05-12-2019, 12:50 AM

(05-11-2019 11:46 PM)Dave Britten Wrote: But knowing that it's pretty easy to swap the circuit board, and that my CV is of a compatible type, maybe some day.

My thought exactly about the FRAM71 Sad

burkhard · (This post was last modified: 05-12-2019 01:33 AM by burkhard.)

(05-12-2019 12:50 AM)mfleming Wrote:
(05-11-2019 11:46 PM)Dave Britten Wrote: But knowing that it's pretty easy to swap the circuit board, and that my CV is of a compatible type, maybe some day.

My thought exactly about the FRAM71

Exactly my sad dilemma on the FRAM71. I learned from that and now have (and love) my 41CL.

If there's risk associated with opening it up for a 41CL swap, I'm not sure what it would be, unless you drop it on the floor while doing so or maybe spill your Coke on it. Many (most?) of them would benefit from getting their cracked posts repaired before it is a bigger problem, anyhow.

If you wait too long on a 41CL, it will be assuredly too late. The electronic components these sort of projects are based on are not produced forever. When they are discontinued, the party is over. Or at least the music has stopped and anybody who didn't grab a seat is out (non-US readers... a reference to a children's party game here).

Get while the gettin's good! Tick tick tick...