HP-29C Low-battery warning, help needed please!

[AndyWalter]

Here in UK, I bought my beloved 29C in 1978, and have been using it regularly since. I swapped the nicads for Nimh ages ago, but even with new ones I'm seeing the warning flashing DP appear within about 5 minutes of removing the mains charger. The battery pack is still fully charged at this stage, and the internal power-supply board seems to have decent voltages (very many thanks to Jim Johnson's for his HP 29C Calculator Diagnosis and Restoration PDF). I suspect the 1820-1983 POR chip used in 22, 25, 25C & 29C may be failing, but I don't know what signal/voltages etc it feeds into the ACT (F1 signal line?) to indicate this. If I know this, it might be straightforward to bypass that chip, but at the same time I'd really like to check the other voltages around to make sure they're within tolerance.

Here are the voltages I have, compared to Jim's doc page 14:
V batt-in Jim: 2.4V Mine: 2.441V
V ss 6.0V Mine: 6.163V
V disp 4.37V Mine: 4.369V
V ram 6.5V Mine: 6.535V
V gg -12.0V Mine: -11.62

These look okay to me, but what's the tolerance especially on the -11.62 ?
Does anyone here know what the problem is please, or does anyone have a Service Manual for this superb machine?

Also, Does anyone know how to contact Jim Johnson? I think there's an error in the circuit diagram page 12 he's published in his PDF. Resistor 33Ohms connected to POR pin 1 is in fact 3.9 Ohms, it's on page 8, Orange White Gold Gold just below the bright orange tantalum capacitor.
Andy

[jjohnson873]

Hi Andy...Jim Johnson here. I haven't done an exhaustive study on the low voltage warning, but will try to make some measurements this weekend using a bench power supply feeding the battery inputs and measure the low voltage threshold. My guess is pin 3 input to the ACT is the low battery warning signal. I have my doubts about the POR/Low Battery IC degrading. I looked again at one of my HP-29C's and found a 33ohm, 5% resistor (orange, orange, black, gold). However, I looked at my presentation from 2013 and saw that the HP-29C had a 3.9ohm resistor (orange, white, gold, gold) on the pc board. I'll have to do some more investigation of why HP used two different values for the POR circuit. If you're having problems with the low battery warning, you might look at the tantalum cap. While tantalums usually don't degrade too much, your calculator is probably 40 years old by now. The capacitor may have become leaky. Also, make certain you have good NiMH batteries. I've had issues with some that have failed. If in doubt, get a second battery pack. Remember to never charge your battery pack inside of your calculator. The battery charging circuit has no protection should the battery not be properly connected to the charger. The incoming charging voltage can destroy your ACT if the battery isn't properly connected in the circuit!
The Vgg voltage for the ACT chip shouldn't be too critical. This bias voltage for the ACT is an artifact of the process technology at the time. If you check your PHI1 or PHI2, you should see a good voltage swing, around ~19vpp on a scope.
I've yet to find anyone who has an HP-29C service manual. If we had one, we'd make an electronic pdf and include it in the HP Museum CD/Flash drive.
~Jim J.~

[PANAMATIK]

(03-12-2018 12:09 PM)AndyWalter Wrote:  V batt-in Jim: 2.4V Mine: 2.441V

A fully charged NiMh Battery is > 2.6 Volt. The ACT pin 3 threshold for Low Bat is around 2.4 V. I assume you simply have to renew your battery pack.

Bernhard

[jjohnson873]

I measured the "low battery warning" threshold on three (3) different HP-29C calculators using a Fluke 87V DMM and a bench power supply where I could vary the voltage to the calculator. Two of the HP-29C calculators were made in America, one in Singapore.
The POR/Low Battery IC is very consistent in measuring the battery voltage between the three HP-29C's. I entered all "8's" in the calculator display for a worst case current. At 2.28 vdc, the decimal point did not flash (low battery indicator). At 2.27 vdc, all of the HP-29C's indicated low battery. I could vary the voltage back and forth between 2.27-2.28 vdc and the low battery signal to the ACT was very consistent. The current draw on the battery pack increases as your voltage decreases and the calculator current was around 192mA at 2.27 vdc. When I entered a single "1" in the display, the calculator current dropped to 160mA at 2.27 vdc. Larger currents will deplete the battery faster and, of course, leaving the calculator on will deplete the battery's charge.
Once the low battery warning indicator began to flash, I continued to lower the voltage until I noted the display starting to flicker. With all "8's" in the display, the display began to flicker between 1.90-2.00 vdc. I haven't done a study on power use between the low voltage threshold indication and where the calculator begins to fail, but I'm guessing you still have quite a bit of time before the battery pack gets to 2.00 vdc.

I wondered how different NiCd and NiMH battery discharge curves looked but found that the initial "fully charged" voltage of between 1.2 - 1.4 vdc/cell is very similar between battery technologies. NiCd and NiMH perform quite similarly with regards to discharge curves with the voltage/cell remaining around 1.2 vdc for 85% of the discharge curve. A fully charged, 2 cell, NiMH battery pack should be around 2.4 - 2.6 vdc. If there's a load on the battery pack, the charge drops to just above 2.4 vdc and stays there for most of the discharge time. If your NiMH batteries are good, they should stay above the 2.28 vdc low battery warning threshold for quite some time before they need to be charged again. Of course the NiMH batteries don't exhibit the memory charging issues that the older NiCd batteries were famous for. (I know I'm conjuring up Katie Wasserman's wrath by mentioning batteries since she's the guru in this area).

The batteries are the main factor that should be investigated for a single bad cell or both cells not holding a charge at 1.2 vdc each for a long time. Another possible issue is battery contact resistance, either with the springs in the back of the Woodstock battery pack or the calculator contacts. Use some sandpaper or a small metal file to clean these contacts. You can use distilled white vinegar to clean off any corrosion.

If you really want long battery life for your HP-29C, get Panamatik's low power replacement IC kit and replace your ACT with a very low power HP-29C emulator micro that will give you weeks of battery time! I haven't used my converted HP-25LP much but the battery is measuring 2.54 vdc after 2 weeks of turning it on once a day. The auto shutoff feature really saves battery power by turning off the display after a period of no activity.

[AndyWalter]

I started doing some tests yesterday. I'd already swapped the Orange 47 microFd cap for a MLCC suface mount one on prongs in case that was the culprit, but it tested fine & the swap didn't solve the problem. So I've replaced it now for originality. I've already swapped the 22 microFd electrolytic cap at battery-input to psu board as electrolytics tend to have a short life (40 years so far!), and that swap will stay.

I've always thought the original Nicads were 1.2V, and on that basis the 2.441V I was measuring at battery-connection to the power-supply board meant my cells were fine. I was using Duracell 2650 mAh nimhs and they aren't that old. The way my flashing low-power warning was getting progressively worse made me think it was the calculator, not the batteries. And if I left it for a few more minutes, the display would start to break up & obviously the cpu etc was struggling. Battery pack contacts, springs etc were all clean, and I've had the batteries in & out & back in the pack, so I didn't think it was connections.

Since swapping the 2 batteries, it's charged ok (it always did seem to charge ok), but most importantly it's now holding charge overnight & not flashing at me the moment I power-on. So it's deffo the batteries, and the 2.441 I was seeing would not have flashed the led, but I was testing without the display+keys board attached as it's easier to measure that way! No doubt it would have dropped below the 2.27 Jim found, if I'd attached the display.

I've now charged the dodgy batteries overnight in separate charger, also 2 others, and am now running the set of 4 in series on a torch bulb to stress them a bit & see what happens. So far they charged to 1.493 & 1.484V, and the set is happily putting 0.470A through a bulb so looking good, but it's only been running 25 minutes.

So exactly what is/was wrong about the batteries I don't quite understand. More info if/when I have some.

Very many thanks to everyone for the immense help & support here - now I must go & recharge my super-large-and-reliable battery using my solar panels 'coz the sun's up, courtesy of the Chevvy Volt Mk1 I drive (& love to bits!). Known as Vauxhall Ampera here in UK. Rather like the best HP Calculators, they're rare, very wonderful, and you can't buy them new any more! But that's another story!!
Andy

[PANAMATIK]

Thanks for these accurate HP-29C measurements. My higher threshold voltage was derived from HP-25 hardware, which doesn't have the POR chip and doesn't apply to this problem, sorry.

It should be easy to measure the threshold voltage at pin 3 of this particular HP-29C, perhaps it is higher than normal. Possible solution is to connect pin 3 directly to Bat+ instead of POR chip.

Bernhard

[PANAMATIK]

I made some measurements with my HP-25C.

The white circular trimmer below the LED display near the anode driver adjusts the "Low Batt" threshold voltage. I never have seen a Woodstock calculator without this trimmer. Its middle pin is connected to pin 4 of the anode driver, which contains a voltage comparator. It compares battery voltage at pin 17 and outputs a high signal at pin 7 if battery is above the threshold. Pin 4 should be adjusted to 1.20 Volt, which sets the threshold to approx 2.35V in my calculator. If turned to the far left (anti clockwise) pin 4 shows 1,27 V and the threshold is below 2.0V, the HP-25 calculator never shows "Low Batt" dots. If turned to the far right (clockwise) pin4 shows 1,17V and the threshold is at about 2.8V, which shows the dots all the time. Pin 7 is connected to digital input pin3 of the ACT chip. The HP-29C firmware shows a blinking dot instead of all dots on, when ACT pin 3 is at low level.

If your HP-29C board has a Power On Reset chip, I assume it has nothing to do with measuring the battery voltage.

In your case you should adjust the trimmer a little bit anticlockwise.




Bernhard