38E: dot in upper left corner of display

[jebem]

(01-21-2020 04:18 PM)Dave Britten Wrote:  Alright, it finished running the benchmark in a little over an hour (not much slower than the 12C). The NiCd pack is reading 2.548 V. So it clearly had more than just a few minutes of runtime left in it, despite the dot appearing. I'm definitely leaning toward a voltage sensing issue now.

If sticking a 9V battery on the terminals can cause that to happen, I wonder if running a Spice from the charger with no battery pack could do the same thing. Maybe over-volting damages the voltage sensing circuit? But if that's the only thing that's damaged, I can certainly live with it.

These machines consumes a lot of current at 2.4V nominal input, surely above 200mA on average, and the running power supply lines are generated from a dc-dc converter, so the computer components are, to a certain point, protected from external overvoltage.
Now, depending on the design, an overvoltage power supply may cause damage to unprotected components, like RAM ic's on constant memory calculators.

In this kind of machines where high current demand is required to run, I wouldn't expect a 9V battery to cause such a failure, because:
- the regular 9V battery is not able to maintain 9V potential at 200mA due to its high internal impedance; I would expect to see a voltage drop by at least 4 or more volts.
- as said above, the battery is buffered to a great extent by the power supply.

Also this issue with the battery flat cable connector to the PCB is a known point of failure, and a few hundred of ohms at the nominal 200mA current consumption easily results in a voltage drop of 0.5V or above, enough to trigger the low battery led.
So, to be sure the problem is not this one, one needs to check it, before going to check the voltage sensor circuit.