Help with HP 15c

[edwincool2]

Hi everyone, I need help repairing an HP 15c that belongs to my dad.
Bit of a backstory... When I was in college I needed a scientific calculator and my dad introduced me to his trusty hp 15c, he had the manual lying around and it was certainly a pivotal moment learning RPN with that beautiful wire-bound manual. Being new to matrices I was in awe that such a perfect unusual device could perform matrix operations. I specially liked the landscape format.
Of course my next step was dropping a significant amount of cash on the flashy new hp 50g, but I digress.
Recently my dad closed up his auto shop, and in the moving out we came across the 15c. Naturally I just ordered a set of Energizer 357 batteries but the calculator won't turn on anymore.
I read a few posts which were helpful, like identifying it is a later model with the coil spring, and rotating it as the contact between the positive terminal to the PCB may be faulty, but still nothing.
There were a couple of small springs that go to the back plate, what is their function? Would it help to improve the connection to the back plate with aluminum foil?
I opened up the calculator but as far as I can see it looks to be in good state with no corrosion. I already ordered some Deoxit D100 just in case.
I will attach a picture of the open calculator, perhaps you could help me diagnose any problem. I noticed that I left a little aluminum foil on the negative terminal.
   
From what I read on some other posts on the forum by removing the PCB and therefore the heatstakes makes the calculator very unstable, so I was hoping to avoid that.
Any help is appreciated. Thanks!

[lyuka]

Hi, edwincool2

The story of 15C with your father is a good story,
but 15C that doesn't turn on seems sadly,
The inside is kept as beautiful as it was,
So let's bring it back to life to make another story.

First, to check for poor contact at the power terminal, insert the batteries in the correct orientation and measure the voltage across the 22u / 10V tantalum capacitor at the end of the PCB with a digital multimeter.

If the voltage is about the same as the battery voltage, the contact is normal and the processor or something may be defective. There's not much you can do in this case, but it's possible that the 180uH microinductor (marked as 181) is scratched and may stop oscillating due to a wire break in that inductor. To check this, measure the resistance across the inductor with the battery removed. Normally it should be around 17Ω, but in case of disconnection it should be several hundred kΩ or more.In this case, it is worth asking someone with rework skills to replace the microinductor with an equivalent.

If the voltage is near 0V, measure the current of the power supply current (although it is a little difficult to measure). If the current is almost 0mA, it is possible that the power supply terminal has poor contact. You may need to do something about the aluminum foil on the negative terminal.

If the current at power off is more than 100uA, it may be broken in the short circuit mode of the tantalum capacitor due to the reverse insertion of the battery in the past. To check this, measure the resistance at both ends of 22uF / 10V (marked as | 22 10V) with the battery removed. At this time, the + side on the PCB should be the plus side (usually the red lead wire) of the multimeter.

If the resistance across the tantalum capacitor is less than a few tens of Ω, then the tantalum capacitor is probably failing in short circuit mode. In that case, it may protect the processor as a result. In this case, it is worth asking someone with rework technology to replace the tantalum capacitor with an equivalent one.The inside of the 15C looks beautiful enough.

First, to check for poor contact at the power terminal, insert the batteries in the correct orientation and measure the voltage across the 22u / 10V tantalum capacitor at the end of the PCB with a digital multimeter.

If the voltage is about the same as the battery voltage, the contact is normal and the processor or something may be defective. There's not much you can do in this case, but it's possible that the 180uH microinductor (marked as 181) is scratched and may stop oscillating due to a wire break in that inductor. To check this, measure the resistance across the inductor with the battery removed. Normally it should be around 17Ω, but in case of disconnection it should be several hundred kΩ or more. In this case, it is worth asking someone with rework skills to replace the microinductor with an equivalent.

If the voltage is near 0V, measure the current of the power supply current (although it is a little difficult to measure). If the current is almost 0mA, it is possible that the power supply terminal has poor contact.

If the current at power off is more than 100uA, it may be broken in the short mode of the tantalum capacitor due to the reverse insertion of the battery in the past. To check this, measure the resistance at both ends of 22uF / 10V (marked as | 22 10V) with the battery removed. At this time, the + side on the PCB should be the plus side (usually the red lead wire) of the multimeter.

If the resistance across the tantalum capacitor is less than a few tens of Ω, then the tantalum capacitor is probably failing in short mode. In that case, it may have been protected the processor as a result. In this case, it is worth asking someone with rework skills to replace the tantalum capacitor with an equivalent one.

Cheers,
Lyuka

P.S.
About nominal current and power trace, see my 15C's page below.
HP-15C LC resonator replacement for triple speed 15C
Note : The replacement of a tantalum capacitor to MLCCs is NOT recommended for everyone. As it lose a sort of reverse voltage protection.

[BillBee]

The two little springs (one long, one short) are for protection from static electricity IIRC. Be careful not o lose them - carpet seems to love those.

[edwincool2]

Hi Lyuka,

Thank you for your kind and quick response. I was able to obtain a digital multimeter to follow your procedure. I apologize for this delay.

I have measured the voltage at the golden ends of the battery terminals as 4.74 V. Also the voltage at the other end of the calculator at the tantalum capacitor marked 22 10V is also exactly 4.74 V.

The microinductor labeled 181 just below the battery compartment looks a bit smudged with something or burnt, such that it only reads "18". I measured the resistance at the soldered ends of it and it is reading out 5.6 Ω (on the 200 Ω mode of the multimeter). I think this is visible on the original attached picture, but let me know if you need additional detail that I could photograph.

Do you think it is worth it to replace the tantalum capacitor? Finding out that it could be a defective processor has certainly made me sad. In your opinion do you think there is any hope for this calculator? Please let me know.
Thank you.

P.S. Also thanks to BillBee. I will take special care in ensuring the little springs will not get lost.

[lyuka]

Hi, edwincool2

The normal voltage across the tantalum capacitor indicates that it is not failing in the short circuit mode of the tantalum capacitor.
So I don't think it's necessary to remove it at this time.

Maybe it's just bad contact with the ON key. It once happened on my 15C.
Try contacting the ON key from the back of the PCB with the batteries installed, and if the 15C turns on, you're lucky.

[hp 15C Tripple speed modification, with protection diode]


If it turns on, as a next step, on suspicion of contamination, how about cleaning the contacts, by blowing an air duster through the throughhole on the back of the ON key?

If it does not work, the next move is to physically clean the contacts.
For my another 15C, where the ON key had poor contact, I slide a 3M Lapping Film (.3MIC 3M261X SHEET 8.5X11, White, 0.3 Micron Grade, 3mil (76.2mm) thickness) cut to a width of 0.2 inches (5mm) through the gap between the PCB and the contact metal cover, then I pulled it out while pressing the ON key lightly, cleaned it, and revived. Fortunately, the ON key is located at the very end, so it was possible.

There still is hope. I wish you good luck.

[edwincool2]

Thank you, Lyuka for all your wisdom! I am very excited to work towards a solution.

OK, I will not replace the tantalum capacitor. With the back plate off and the calculator PCB exposed I am not able to reach behind the PCB to press the ON button from inside. I tried with different paper clips, staples, and a curved pick but I am not able to reach the button.

I have tried cleaning the outside of the calculator and it was really dirty with some kind of oil substance. I will attach a picture. I used the loop end of a sewing needle and a wet wipe (I think the brand is lysol), to get in the little gaps between the keys. I did manage to pull some of the grime, but not completely.
   

I was not familiar with 3M lapping films. I may be able to order some through amazon, but I did not understand the procedure you outlined. Currently the bottom part of the PCB is protected by a tape. Should I remove it? I tried with a piece of paper but it is not stiff enough and it bends before it goes in.

Alternatively, I have access to a small ultrasonic cleaner (I use it to clean bicycle components), and I read other threads where they mention cleaning using distilled water. In your opinion do you think this would work?
I feel we are so close Lyuka! Thank you for all your help.

Greetings,
Edwin.

[edwincool2]

Good news!

I was able to follow your advice, Lyuka. I used a breadboard solderless wire as a pin and inserted it at the ON switch and voila! The calculator now lives!!
I continued using alcohol trying to clean the contacts and a small air compressor discharge at these holes and now ON is working normally.

I was very quick to play with it by trying to SOLVE functions and performing integrals, I have nothing negative to say, everything works fine. If anything a little slow, but it is expected for such a marvelous device of this age.

However the key for point "." still needs hard pressing, enough to annoy me that I need to always look at the screen to see if it registered it.
The division "÷" key does register very well but it seems after a press and while unpressing the key makes a small noise as if it is getting stuck (?) this does not affect functionality as it never registers twice, so just a minor detail.

Thank you so kindly for everything!

Please let me know if you think I should do an ultrasonic cleaning or should I proceed with the 3M lapping films.

[lyuka]

Hi, edwincool2
I'm glad to hear that.

The sticky feeling may be that sugar or something is on the switch putton. If you can input normally, there may be some gum in the plastic button from the front side.

When a number key of my 15C went sticky, may be due to a drop of soft drink, I used a small oil-paint brush dipped with small amount of rubbing alcohol from the front side to clean the gaps in the button and the panel. The dirt on the brush was wiped off with kitchen paper, and then cleaned with a small amouint of alcohol again. After several times of brushing the sticky condition was eliminated.

If it is difficult to register a key, contamination to the contacts is suspected, so add a very small amount of solvent -- try with absolute alcohol first -- from the through hole on the back of the contacts and gently blow it off with gus duster.
If you've tried it several times with ethanol (or isopropanol) and it doesn't work, then try using a small amount of xylene-based parts cleaner.

In the current situation, I don't think it is necessary to use lapping film, and the risk of ultrasonic cleaning is too high, so it is better not to do it.

Good luck
Lyuka