38E: dot in upper left corner of display

[Dave Britten]

I just got a 38E, and it appears to be working fine (after soldering the crummy solderless display connection), but there's a little dot displayed in the upper left corner, above where the negative sign appears. A quick skim of the manual didn't uncover any clues. It doesn't seem to be linked to the BEGIN/END switch position (which is working - I get different results calculating PMT), and as far as I know these are fresh batteries. Anybody know what it means, if anything?

[Dave Britten]

Here's a photo of the dot, with -8 entered in the display. It's hard to tell from the photo, but it kind of looks like a tight cluster of 4 dots arranged 2x2.

https://i.imgur.com/R8Cz7AE.jpg

[David Hayden]

That's the "low battery" indicator. From page 127 of the 38E manual:

Quote:Low Power

When you are operating from battery power and the batteries get low, a raised decimal is turned on at the far left of the display to warn you that you have between 1 minute and 25 minutes of operating time left.

[Dave Britten]

(01-15-2020 07:45 PM)David Hayden Wrote:  That's the "low battery" indicator. From page 127 of the 38E manual:

Quote:Low Power

When you are operating from battery power and the batteries get low, a raised decimal is turned on at the far left of the display to warn you that you have between 1 minute and 25 minutes of operating time left.

Ahh, okay. I was wondering if that's what it might be, but couldn't find that paragraph in the manual. Thanks.

Strange, the batteries are freshly charged, as far as I can tell. I wonder if there's an issue with the charge-level-sense circuit. I'll keep running it and see what happens.

[David Hayden]

It might also be oxidation on the battery or terminals. Try cleaning them with deoxit if you have some. Otherwise rub with a pencil eraser and then wipe clean with isopropyl alcohol.

[Dave Britten]

I'm a bit suspicious of oxidation on the power switch, actually. The dot flickers very slightly when monkeying with the switch. I gave the PCB a quick scrub with alcohol on a swab, but it still looked fairly ugly. I may open it up again tomorrow and see if I can get it any cleaner.

[teenix]

(01-16-2020 03:22 AM)Dave Britten Wrote:  I'm a bit suspicious of oxidation on the power switch, actually. The dot flickers very slightly when monkeying with the switch. I gave the PCB a quick scrub with alcohol on a swab, but it still looked fairly ugly. I may open it up again tomorrow and see if I can get it any cleaner.

The actual switch contact may have crud build up as well. Possible, but unlikely, the contact tension might be less than normal. Easy to tweak if so.

cheers

Tony

[Dave Britten]

No dice. I cleaned off the PCB underneath both switches with a combination of Deoxit, alcohol, and pencil eraser, but the dot persists. It doesn't seem to fluctuate as much when jiggling the power switch, though. I'll have to check the voltages at the PCB end of the ribbon cable and see if there's a voltage drop happening somewhere between the batteries and the components. Every time I Incredible-Hulk the case open I'm sure it shortens my lifespan, though...

[Rogier]

How is your battery doing? My 33E switched off a few minutes after the dot appeared. Now I run it on modern rechargeable batteries and it behaves differently. The dot is there much longer. Probably an effect of different properties of these batteris.

[Dave Britten]

(01-19-2020 09:56 PM)Rogier Wrote:  How is your battery doing? My 33E switched off a few minutes after the dot appeared. Now I run it on modern rechargeable batteries and it behaves differently. The dot is there much longer. Probably an effect of different properties of these batteris.

I haven't had a chance to run it too hard yet, but I did more cleaning and testing. Voltage looked fine along the ribbon cable, and resistance was only a couple of Ohms, so I'm not really sure why it thinks the batteries are low.

[jebem]

(01-19-2020 10:23 PM)Dave Britten Wrote:  

(01-19-2020 09:56 PM)Rogier Wrote:  How is your battery doing? My 33E switched off a few minutes after the dot appeared. Now I run it on modern rechargeable batteries and it behaves differently. The dot is there much longer. Probably an effect of different properties of these batteris.

I haven't had a chance to run it too hard yet, but I did more cleaning and testing. Voltage looked fine along the ribbon cable, and resistance was only a couple of Ohms, so I'm not really sure why it thinks the batteries are low.

Remove the batteries.
Check your multimeter by shorting the two probes in low ohm scale and take note of the reading (this value is to be subtracted from the next procedure readings).
If the resistance measured between each battery contact and the corresponding PCB pad is above 0.3 ohm, there is a conductivity issue that needs to be addressed.
In this case, use a external battery holder (or power supply) to apply the voltage directly to the PCB, to confirm the issue.

[Dave Britten]

(01-20-2020 08:43 AM)jebem Wrote:  Remove the batteries.
Check your multimeter by shorting the two probes in low ohm scale and take note of the reading (this value is to be subtracted from the next procedure readings).
If the resistance measured between each battery contact and the corresponding PCB pad is above 0.3 ohm, there is a conductivity issue that needs to be addressed.
In this case, use a external battery holder (or power supply) to apply the voltage directly to the PCB, to confirm the issue.

Excellent tips, thanks. I will definitely do that if I get brave enough to remove the plastic spine and let all the chips fall loose. (Mine isn't a later soldered model.)

Annoyingly, the power input pads on the PCB are buried under the plastic spine so it's pretty much impossible to test them or solder replacement wires without really tearing the thing apart.

[jebem]

(01-20-2020 02:11 PM)Dave Britten Wrote:  

(01-20-2020 08:43 AM)jebem Wrote:  Remove the batteries.
Check your multimeter by shorting the two probes in low ohm scale and take note of the reading (this value is to be subtracted from the next procedure readings).
If the resistance measured between each battery contact and the corresponding PCB pad is above 0.3 ohm, there is a conductivity issue that needs to be addressed.
In this case, use a external battery holder (or power supply) to apply the voltage directly to the PCB, to confirm the issue.

Excellent tips, thanks. I will definitely do that if I get brave enough to remove the plastic spine and let all the chips fall loose. (Mine isn't a later soldered model.)

Annoyingly, the power input pads on the PCB are buried under the plastic spine so it's pretty much impossible to test them or solder replacement wires without really tearing the thing apart.

You are right, Dave.
Last time I opened one of these beauties, I managed to crack the case near the battery compartment, it broke my heart.
Sometimes it is better to just leave it as it is, to preserve these nice classic machines in good looking shape in our collection.

[burkhard]

I have a really nice looking 34C with the same problem. Even with freshly-charged batteries which run my other Spice models fine, this one keeps the low battery light on. In my case, I suspect the problem is how it was treated by the guy I bought it from: He "checked it out" by powering it by jumpering a 9V battery up to the battery contacts. Why would someone think 9V is close to 2.4V? Ugh.

Anyhow, it did not blow the ACT chip, because otherwise it works fine, but it does have that low battery light stuck on. If someone understands how the voltage sense circuit, that is probably the place to scour around. With a lot of other projects on the plate, it has been low on my list of priorities. Perhaps we have the same issue?

[Dave Britten]

(01-21-2020 01:50 PM)burkhard Wrote:  I have a really nice looking 34C with the same problem. Even with freshly-charged batteries which run my other Spice models fine, this one keeps the low battery light on. In my case, I suspect the problem is how it was treated by the guy I bought it from: He "checked it out" by powering it by jumpering a 9V battery up to the battery contacts. Why would someone think 9V is close to 2.4V? Ugh.

Anyhow, it did not blow the ACT chip, because otherwise it works fine, but it does have that low battery light stuck on. If someone understands how the voltage sense circuit, that is probably the place to scour around. With a lot of other projects on the plate, it has been low on my list of priorities. Perhaps we have the same issue?

It sure sounds like the same issue, but it's hard to say for sure without being able to easily test the voltage at the PCB pads, and the resistance between those pads and the battery terminals. It seems to work fine otherwise, though I don't know yet if it will give out at a higher voltage than it would normally. I should probably just key in the 12C benchmark program and let it run, then check the voltage when the display shuts off.

EDIT: I popped in a reasonably fresh NiCd pack rated for 1000 mAH, typed in the 12C n-queens benchmark program, and it's been happily running for about 36 minutes now, with the low-battery dot illuminated the whole time. I'm not 100% sure the program will produce the same/correct results on the 38 - STO Nj isn't a valid operation, so I replaced it with just Nj. When I compared the two functions on my 12C, I couldn't see any obvious difference between them. I'll report back when it either finishes or runs out of gas...

[Dave Britten]

Alright, it finished running the benchmark in a little over an hour (not much slower than the 12C). The NiCd pack is reading 2.548 V. So it clearly had more than just a few minutes of runtime left in it, despite the dot appearing. I'm definitely leaning toward a voltage sensing issue now.

If sticking a 9V battery on the terminals can cause that to happen, I wonder if running a Spice from the charger with no battery pack could do the same thing. Maybe over-volting damages the voltage sensing circuit? But if that's the only thing that's damaged, I can certainly live with it.

[jebem]

(01-21-2020 04:18 PM)Dave Britten Wrote:  Alright, it finished running the benchmark in a little over an hour (not much slower than the 12C). The NiCd pack is reading 2.548 V. So it clearly had more than just a few minutes of runtime left in it, despite the dot appearing. I'm definitely leaning toward a voltage sensing issue now.

If sticking a 9V battery on the terminals can cause that to happen, I wonder if running a Spice from the charger with no battery pack could do the same thing. Maybe over-volting damages the voltage sensing circuit? But if that's the only thing that's damaged, I can certainly live with it.

These machines consumes a lot of current at 2.4V nominal input, surely above 200mA on average, and the running power supply lines are generated from a dc-dc converter, so the computer components are, to a certain point, protected from external overvoltage.
Now, depending on the design, an overvoltage power supply may cause damage to unprotected components, like RAM ic's on constant memory calculators.

In this kind of machines where high current demand is required to run, I wouldn't expect a 9V battery to cause such a failure, because:
- the regular 9V battery is not able to maintain 9V potential at 200mA due to its high internal impedance; I would expect to see a voltage drop by at least 4 or more volts.
- as said above, the battery is buffered to a great extent by the power supply.

Also this issue with the battery flat cable connector to the PCB is a known point of failure, and a few hundred of ohms at the nominal 200mA current consumption easily results in a voltage drop of 0.5V or above, enough to trigger the low battery led.
So, to be sure the problem is not this one, one needs to check it, before going to check the voltage sensor circuit.

[Dave Britten]

(01-22-2020 06:33 PM)jebem Wrote:  These machines consumes a lot of current at 2.4V nominal input, surely above 200mA on average, and the running power supply lines are generated from a dc-dc converter, so the computer components are, to a certain point, protected from external overvoltage.
Now, depending on the design, an overvoltage power supply may cause damage to unprotected components, like RAM ic's on constant memory calculators.

In this kind of machines where high current demand is required to run, I wouldn't expect a 9V battery to cause such a failure, because:
- the regular 9V battery is not able to maintain 9V potential at 200mA due to its high internal impedance; I would expect to see a voltage drop by at least 4 or more volts.
- as said above, the battery is buffered to a great extent by the power supply.

Also this issue with the battery flat cable connector to the PCB is a known point of failure, and a few hundred of ohms at the nominal 200mA current consumption easily results in a voltage drop of 0.5V or above, enough to trigger the low battery led.
So, to be sure the problem is not this one, one needs to check it, before going to check the voltage sensor circuit.

I opened it up again because I needed to clean the 0 key, which was being a bit fussy. So I figured I might as well dismantle the internals for more testing. Voltage looked fine at the pads on the far end of the ribbon cable, and I only measured 3.0 Ohms on the positive trace, and 0.5 Ohms on the negative one. I cleaned all the contacts on the "board" with Deoxit, then wiped the whole thing down with alcohol, and wiped it dry with a paper towel. After closing it up, the dot is still there. So unless there's some massive internal resistance happening after the ribbon cable, I'm leaning toward a voltage sense issue.

I would test the machine with a fresh set of alkalines (3.2 V), but they aren't flat-tops, and I don't want to force them in and break the battery contacts. I'm going to type in the 12C n-queens benchmark and run it again to put another solid hour of run time on the NiCd pack and see where it ends up.

[jebem]

(01-23-2020 01:54 PM)Dave Britten Wrote:  

(01-22-2020 06:33 PM)jebem Wrote:  These machines consumes a lot of current at 2.4V nominal input, surely above 200mA on average, and the running power supply lines are generated from a dc-dc converter, so the computer components are, to a certain point, protected from external overvoltage.
Now, depending on the design, an overvoltage power supply may cause damage to unprotected components, like RAM ic's on constant memory calculators.

In this kind of machines where high current demand is required to run, I wouldn't expect a 9V battery to cause such a failure, because:
- the regular 9V battery is not able to maintain 9V potential at 200mA due to its high internal impedance; I would expect to see a voltage drop by at least 4 or more volts.
- as said above, the battery is buffered to a great extent by the power supply.

Also this issue with the battery flat cable connector to the PCB is a known point of failure, and a few hundred of ohms at the nominal 200mA current consumption easily results in a voltage drop of 0.5V or above, enough to trigger the low battery led.
So, to be sure the problem is not this one, one needs to check it, before going to check the voltage sensor circuit.

I opened it up again because I needed to clean the 0 key, which was being a bit fussy. So I figured I might as well dismantle the internals for more testing. Voltage looked fine at the pads on the far end of the ribbon cable, and I only measured 3.0 Ohms on the positive trace, and 0.5 Ohms on the negative one. I cleaned all the contacts on the "board" with Deoxit, then wiped the whole thing down with alcohol, and wiped it dry with a paper towel. After closing it up, the dot is still there. So unless there's some massive internal resistance happening after the ribbon cable, I'm leaning toward a voltage sense issue.

I would test the machine with a fresh set of alkalines (3.2 V), but they aren't flat-tops, and I don't want to force them in and break the battery contacts. I'm going to type in the 12C n-queens benchmark and run it again to put another solid hour of run time on the NiCd pack and see where it ends up.

You do not want to see 0.5 or 3 ohms in the battery flat cable. On led machines with just one digit lit you may expect to see at least 20 mA. That results in a voltage drop of 10mV to 60mV for just one digit. With more digits it is easy to see 0.6V voltage drop. On nicd batteries that 0.6V alone may trigger the CPU low battery indicator.
The more digits are lit, the higher the current and voltage drop.

There is no low battery sensor circuit in these machines. The CPU chip handles this task.

What I have seen is that a defective power supply will prematurely trigger the low battery sensor.
A noisy dc-dc converter would cause this issue, even when there is no voltage drop in the battery connector/flat cable set.
A scope here will show it.

Another way is to assume thar the electrolityc capacitors in the power supply are defective and replace them, as these can fail with passing time even when they looks good on a regular multimeter (you need a esr meter as well).
If you are brave enough to use a solder on miniaturized components on a delicate pcb easily destroyed if handled less than very very carefully, remove the caps, get new ones with the same capacitance values (voltage equal or above), and insert them.
That could fix the issue.

Edit:
While you are at it, check the diodes as well for leakage. Please note that one of them can be a zenner, I do not have access to the schematics at the moment.

[AndiGer]

From time to time I read on the forum that tantalum caps degrade, often measure right values but high ESR.

What do you suspect to be "high ESR"?

I bought new 2.2µF/16V tantalum caps and they measure values between 3.6 and 5.6 Ohms ESR (Vloss between 0.3 and 0.6%). Measured by LCR-T4 transistor tester.

So 15 Ohms I measure in an old one is high ESR or where is the border you call it high ESR?