Dear Vladimir,

Good a.m. and may this find you well. In regard to failed solder joints on the PCB, a break - if there is one - often is too small to be seen with the unaided eye. Indeed, it can be microscopic,or close to it.

The only way to be sure, that there is not an invisible break in a solder joint, is to reflow all of the solder connections between:
(a) the four pins of of the main relay (the small[ 3/4" / 19mm long], black plastic box close to the main connector) that connect the relay to the circuit board
(b) the 12 or 14 pins of the main connector, that go into the circuit board.

If, after reflowing the solder - use a 25-watt iron - there is no improvement, then:
(a) the problem is elsewhere in the system, see Rule 308's excellent suggestions
(b) the relay - a switch - may have failed.

Hope this helps.

Yours faithfully,

spook