HP-19C Gone Loco!

[John Garza (3665)]

Everytime I turn it on, the printer advances paper and won't stop!

Anyone seen this before?

-J

[David Smith]

Yes... been there, done that.

Could be a bad connection between the internal boards, a bad transistor on the printer board (individual or in one of the quad transistor IC's), or (much worse a problem) one of the HP custom IC's. Option 2 most likely, then 1, then 3.

[John Garza (3665)]

This seems strange, but it appears to be related to the battery. The wild printing never happens when the machine is plugged into the charger, or shortly after being disconnected. Let it set (turned OFF) an hour, and it goes loco again when turned on. Funny though, the low battery indicator never comes on, and the display seems normal during the loco printing.

My fully charged Ni-MH pack put out 5.61v, but I noticed it dropped to 5.56v within a few minutes after removal of the charger. At about 5.40v, the loco printing starts. Disassembling the battery pack, it looks like 2 cells have some internal short as they discharge much quicker than the other two. I replaced all 4 cells with alkalines (6.09v now), and all is well. The display is just perceptibly brighter, but everything works.

Very weird.

-JG

[David Smith]

A four cell Nicad/NiMH pack has a nominal volatge of 4.8V, full discharge at 4.0V, full charge at 5.2-5.6V. You might try rebuilding the pac with new cells, but it sounds like mayby something is flakey with the power supply in the calculator.

[Katie Wasserman]

The printer in the 19C (and 10) draws a lot of current when it's running. Batteries that have a high internal resistance (because they're failing) won't provide enough current for the motor to do its thing and stop. For whatever reason it just keeps running on like that. The solution is good batteries (Nicad or NiMH) and clean battery contacts. I don't think that there's anything wrong with your calculator, the 19C's that I have exhibit this same behavior with poor batteries. The low battery warning doesn't report this problem becasue it's turned off during printing when the voltage really takes a dive. However, the decimal point will often flash once or twice after a line is printed before the battery voltage recovers.

[David Smith]

Yes, the battery contacts in the 19C (and all Woodstocks for that matter) are a weak point in the design. It really helps to put a small wad of aluminium foil in the battery contact holes in the battery pack. HP service people actually made this recommendation to people who complained about 19C battery problems.

The Woodstock battery pack had three different versions. The first had the two cells connected together with only the dual coil spring. This had the possible problem of flakey connections. In reality it was probably the most reliable type.

Next they added a strap welded across the cells. The fixed the flaky contact problem between the cells, but since the cells could no longer move independently, they often made poor contact if the battery contacts in the calculator were not even with each other.

Finally they made the strap between the batteries longer with a little "U" shaped fold in it. The cells could now move somewhat independently, but often not enough to insure good contact.

In wad I trust.