HP-33C strange power / weak battery indicator problem

[btreguier]

Hi everyone,

I'm currently repairing my second HP-33C, and I'm confronted with a strange problem: the "weak battery" indicator is constantly on even with fully charged batteries, and the calculator also has an erratic behaviour.

I then replaced the batteries with a lab power supply, and gradually went up with the voltage, starting at 2.6V, to see at which point the indicator would vanish, and it did, but only around 3.08V, at which point, when the "STO ENTER" test is attempted (and all the display segments are on), the power consumption is about 150mA. The calculator behaves normally at this point.

As I had a few spare parts, I replaced some components one by one to see whether I could fix the problem, so here is what I replaced:
- the power board
- the processor (the one in the 33C was a 1820-2105-B, which I replaced with a 1820-2105-A from a 33E)
- the LED display

All to no avail... :-(

In fact, the only active component that I did not replace (as I had no spare part for that one) is the little 8-pin chip, which I believe is the memory (but not sure about that). So it sure might be that last component which is faulty, but I may have missed a few points, so if anyone out there has an idea about what else can cause such a problem, I'd be interested.

Thanks in advance!

Bruno

[teenix]

Check for voltage drops along the power supply ribbon cable. There could be corrosion under the rivets that hold it to the battery terminals on the small charger board or cracks in the ribbon cable.

If so, this may be reducing the voltage to the rest of the calculator fooling the battery monitor circuit inside the CPU chip.

The 8 pins chip is for RAM memory and shouldn't be a problem. If so, it may get warm if it has a fault.

cheers

Tony

[btreguier]

Hi Tony, thanks for your suggestion and all the information provided.

So I was right about the 8-pin chip, it is the RAM indeed. I wasn't really sure.

What I don't understand is that I swapped the power board, including the ribbon cable attached to it, but it didn't change anything. I'll double check that though, and will try to measure the voltage on the power board itself, maybe by soldering 2 wires so that even when inserted in its plastic recess, it is still accessible.

I also wondered if the voltage drop could be due to a problem on the main printed board, the one on which the chips are "press-fitted". It is not an active component as such, but it participates in the power distribution, and that one hasn't been swapped as I have no spare part for it. :-(

Thanks again and best regards,

Bruno

[teenix]

Check the connections to the main CPU chip and make sure none are corroded, especially the power supply pins (12 and 13). I think all the voltage testing is done inside this chip and the low volt drive comes out via pin 37.

cheers

Tony

[btreguier]

Thanks again Tony. :-) I'll keep you informed about my findings.

By the way, where did you get the processor pinout? Is it on the "SLP" document on panamatik.de (http://panamatik.de/SpiceLPManual.pdf), or is there something else available out there?

Best regards,

Bruno

[teenix]

Sorry, lots of good info compiled by Tony Duell here

cheers

Tony

[btreguier]

Once again thanks very much Tony, I wasn't aware of this site, which seems to contain a lot of valuable info!

Best regards,

Bruno

[btreguier]

Hi again Tony and other possible readers, a little update:

The 33C isn't totally reassembled yet, but it seems the problem has been solved, by replacing the main board by another one taken from a dysfunctional 33E. As the part number is the same, I guess there is no difference. The "STO ENTER" test succeeds, and now the weak battery indicator only begins to show around 2.35V.

Pretty normal, so. ;-)

Next step: trying to find what is faulty in the original main board, if possible...

Thanks for your help and information!

Best regards,

Bruno

[btreguier]

Hi, new update: the calculator is reassembled and now works perfectly!

Best regards,

Bruno

[Mike T.]

(08-22-2021 07:57 AM)btreguier Wrote:  Hi, new update: the calculator is reassembled and now works perfectly!

Best regards,

Bruno

Nice to see another HP33C brought back to life. They are my favourite model, not the most powerful or even most useful, but very usable.

Out of interest was it a soldered or non soldered version (I'm assuming the former).

Mike T.

[btreguier]

Hi Mike,

Yes, good news! :-)

It was a non-soldered version.

The only soldered version of an HP SPICE calculator I ever had here is an HP-34C. All the other ones (9) were non-soldered, although I've seen mild variations from one to another (notably the number of "small" ICs: 1, 2 or 3 depending on the version and manufacture date).

I really like the 33C as well. I used to own a 33E when I was in high school in the late 70s - early 80s, but I hated it when I had keyed in a "long" program and had to turn the calc off! :-)

Best regards,

Bruno

[tangdfx]

I have the same problem on my HP33C.

And I find the root cause the the contact problem about the on off key.

After i clean the contact. The low power indication disappeared.

[btreguier]

Hello, thanks for your update!

I tried that as well, but I think my problem is somewhere else, probably the power supply board itself. Thanks anyway! When I can get hold of another one I'll cross-test that.

Best regards,

Bruno

[teenix]

The battery low LED is controlled by an internal voltage monitor circuit built into display module and monitors the battery voltage when the calculator is switched on.

The LED goes on/off at around 2.18 - 2.20 battery voltage.

There could be a drop in voltage anywhere from the battery and connections to the power board and then through the CPU board and then to the display. These connections are not soldered so there could be a higher than normal resistance join. The battery connector and ribbon cable connection is rivetted and may be high resistance also especially if there are signs of corrosion.

The battery is monitored via the outer most pins either side of the display.

cheers

Tony