Russco charger malfunction (again)

Hi all

My 3-year old russco 24-120 charger seems to be acting up again.

Last time it was a blown bridge rectifier. That was an easy and cheap fix fortunately.

This time the symptoms are a bit different:

The charger is working, but if you turn the current adjustment knob all the way up (as I normally have it set), the charging current jumps to about 14-15 amps (normally with my 126V battery it would be about 11 amps) and the output current is unstable, it oscillates and causes the needle on the current gauge to bounce around, and the charger makes some funny noises (bumping and popping sounds) while doing it. (It's not the fan, which is working fine). Eventually, the output fuse will blow if I let it continue like this.

For now I have turned the current adjustment knob down and this seems to be allowing the charger to behave itself, but I would like to know what is going on.

I don't know if it is related or not but this behavior started after some particularly cold (for around here, anyway) days. (15-20 degrees farenheit) The charger is located inside the cabin of the car so it is well protected from the elements so I would not expect moisture intrusion to be at fault.

Clearly whatever regulates output current has developed an issue; possibly it is just an internal adjustment that has (suddenly) gone out of whack. Any ideas?