HP 32SII success and woes

[Sanjeev Visvanatha]

Here is the interesting side of the 32sii PCB, only exposed when you remove it from the unit.

   

Similarly, the 10B PCB looks like this:

   

[Sanjeev Visvanatha]

OK, the capacitor seems bad based on some resistance checks I made with an analog ohm meter. I get a small constant resistance when the positive lead is on the right of the capacitor and an infinite resistance when the leads are reversed.

Now, the 10B capacitor also behaves in exactly the same way. So, I think there is still a problem internally within the SoC.

I don't have a lab power supply. I can try to see if I can borrow one just to see at what voltage it starts to misbehave.

[Katie Wasserman]

(02-20-2015 01:50 AM)Sanjeev Visvanatha Wrote:  OK, the capacitor seems bad based on some resistance checks I made with an analog ohm meter. I get a small constant resistance when the positive lead is on the right of the capacitor and an infinite resistance when the leads are reversed.

Now, the 10B capacitor also behaves in exactly the same way. So, I think there is still a problem internally within the SoC.

I don't have a lab power supply. I can try to see if I can borrow one just to see at what voltage it starts to misbehave.

It's not uncommon for chips to stop working at voltages near the limit of their design. I suggest you use all 3 cells in series but install a series silicon diode to drop the voltage by about .6 volts. That will net you about 3.9 volts and keep the low battery indicator from turning on and the cells tight in the battery compartment. The low battery indication is set to about 3.5 volts in the 32sii.

[Sanjeev Visvanatha]

(02-20-2015 02:22 AM)Katie Wasserman Wrote:  It's not uncommon for chips to stop working at voltages near the limit of their design. I suggest you use all 3 cells in series but install a series silicon diode to drop the voltage by about .6 volts. That will net you about 3.9 volts and keep the low battery indicator from turning on and the cells tight in the battery compartment. The low battery indication is set to about 3.5 volts in the 32sii.

Thank you for this suggestion. I'll have to explore where to place the series diode and do some practice soldering before embarking on replacing the capacitor as well.

[jebem]

(02-20-2015 12:16 PM)Sanjeev Visvanatha Wrote:  (...) do some practice soldering before embarking on replacing the capacitor as well.

Hi, Sanjeev,

I would say the best way to check a capacitor is to replace it with a new one, assuming we do not have easy access to professional measuring equipment.
Testing DC characteristics is not enough, we would need to test AC as well.

But tantalum caps last for ages and when they fail, they usually do it in a catastrophic way by going into short circuit state.
But from your test description, that is not the case.

Your analog multimeter when set to continuity mode, will present its battery internal voltage to the probes, where the positive probe will have a negative voltage (polarity is reversed).

So to test the cap, first remove it from the circuit.
Then connect the multimeter negative probe (positive voltage) to the capacitor positive lead and the other probe to the negative lead.

If all is well, you should be able to see a fast reading towards the zero ohm in the dial (initial rush current) and then gradually the reading will increase towards infinite (it may never reach that point, depending on how large is the capacitor value and the used technology - electrolytic have leakage current all the time, for instance).

If the cap is defective by current leaking, you will get a intermediate reading. if the cap has lost the capacitance, it will not have a large initial rush current, if at all.
The best way is to shop for some new tantalum caps and compare.

One additional test, is to leave the capacitor to rest for a minute or so after doing the above test procedure.
Then use your multimeter in voltage mode to read the charged cap.
You should be able to see a fast voltage rising in the dial and then it will drop slowly (or not, it will depend on your multimeter internal impedance and cap value).

Edited to correct some language errors, as I'm not a native English.

[Sanjeev Visvanatha]

(02-20-2015 03:39 PM)jebem Wrote:  I would say the best way to check a capacitor is to replace it with a new one, assuming we do not have easy access to professional measuring equipment.
Testing DC characteristics is not enough, we would need to test AC as well.

Thank you. I will likely be replacing it anyways.

For the silicon diode, I have seen this one:

1N4148FS-ND

For the Capacitor, I have selected this one:

Kemet T491C106K016AT

I am pretty sure the capacitor is the correct one. However, I am not so sure of the silicon diode (see its forward voltage, for instance). Would anyone care to weigh in with a recommendation for a silicon diode with a 0.6V forward voltage, suitable for this application?

EDIT - I am not experienced in selecting this sort of thing, so bear with me!

Thank you!

[Katie Wasserman]

(02-24-2015 01:29 AM)Sanjeev Visvanatha Wrote:  I am pretty sure the capacitor is the correct one. However, I am not so sure of the silicon diode (see its forward voltage, for instance). Would anyone care to weigh in with a recommendation for a silicon diode with a 0.6V forward voltage, suitable for this application?

EDIT - I am not experienced in selecting this sort of thing, so bear with me!

Thank you!

Pretty much any silicon diode will do just fine, since they all have about a 0.6v forward voltage drop and any one will easily handle the few mA of current. A 1N4148 is a good choice. I doubt that the capacitor is causing the problem you have.

[Sanjeev Visvanatha]

(02-24-2015 02:44 AM)Katie Wasserman Wrote:  Pretty much any silicon diode will do just fine, since they all have about a 0.6v forward voltage drop and any one will easily handle the few mA of current. A 1N4148 is a good choice. I doubt that the capacitor is causing the problem you have.

Thank you for the confirmation. I do not think I will get another chance at taking this apart without damaging the twisty clips, so I may elect to replace the capacitor as a preventive measure.

[Sanjeev Visvanatha]

After purchasing the silicon diodes, some test leads and a new temp controlled soldering station, I embarked on repairing this 32SII. I was confident enough that the capacitor was alright.

The first step was to find a place to mount the silicon diode. The negative battery terminal was one spot I had considered, but the metal terminal there is thick, and it is not easy to work on that side. The positive terminal is connected to the PCB, and is very springy, so I decided to place it there.

The positive terminal has two small holes in the arm leading to the PCB, so I decided to cut a section between those holes and solder the silicon diode to the holes.

   

After cleaning the zebras, centering the LCD and reattaching the PCB (Twisties worked!), I was able to turn on the calculator and see positive signs of life. No more garbage display, and the self test passes with flying colours. I see about a 0.6V drop after installing the silicon diode. I read about 3.5V, a bit lower than what Katie had quoted. No low battery indicator either!

   

   

Last step is to close the case off. With the addition of the diode, I will have to enlarge the slot in the back case that goes over the positive terminal.

[Sanjeev Visvanatha]

I really owe each and every individual who helped in this thread a Thank You! It was a real learning process for me. I sincerely appreciate your time, advice and patience.

[rprosperi]

(02-28-2015 03:52 AM)Sanjeev Visvanatha Wrote:  ...I was able to turn on the calculator and see positive signs of life. No more garbage display, and the self test passes with flying colours. I see about a 0.6V drop after installing the silicon diode. I read about 3.5V, a bit lower than what Katie had quoted. No low battery indicator either!

Great news, congratulations Sanjeev, job well done. Thanks for taking us through the diagnostics and repair process. Certainly there must be a strong sense of accomplishment here, which you will appreciate every time you pick up this 32SII to use.

[Bruce Larrabee]

Excellent trouble shooting!