Paccomm Tiny-2 Mk2 capacitor polarity problem

Many Paccomm Tiny-2 Mk2 TNCs have a polarised capacitor installed incorrectly. They become leaky and it degrades performance.

Let’s look at the datasheet for a MAX231 and review the charge pump and related filter capacitors.

Note especially the capacitor connected from pin 3 to ground, it is an electrolytic with -ve terminal to pin 3 and it develops the -ve voltage for the true RS-232 line driver.

Now let’s study the Tiny-2 schematic.

Note C23 is the same capacitor, and its symbol shows +ve to pin 3 (though a + sign near the ground symbol).

The silk screen is wrong, and the factory installation is wrong (though it agrees with the silk screen).

The reliable test is to check whether the -ve terminal connects to pin 3 of the MAX231.

If it is installed incorrectly, remove it and replace it with a new capacitor (years of use with reverse current would not be good for the capacitor). Inspect all the other capacitors in that region and replace them if signs of bulging or leakage.

Above is a correctly installed C23, note the white stripe on the side of C23 is to the right (factory installation on most is to the left). The silkscreen + on C23 is in the wrong place. (The capacitor is different to the others in the cluster as it was replaced, no point reversing the existing capacitor which has had reverse current flowing in it for a substantial time.)