Sir Jonawald . . . . .
I'm coming in waaaaay late on this post but I will give my evaluations on the best photographed board that I have marked up , and would only be able to make out more if having its foil side straight on, and about the same size and with minimal lighting reflections.
If I remember right you were initially coasting right on along until you blew/zap/banged up some of the boards foil paths and that both the marked up RED and YELLOW boxed areas are now non functioning.
The RED portion is responsible for activating a DC motor driven linear screw actuator that lifts the leg support portion of the unit up or else, lowers it down.
The YELLOW portion is responsible for the elecro-mechanics activation for the back support position.
Lets look at the YELLOW BOXED . . . .back up/down associated portion of the photo:
CN10 routes in a raw AC supply voltage in the order of ~ 12VAC, when loaded down and it passes to the left to the Full Wave Bridge of 4 discrete power diodes and they produce ~ 12VDC when properly loaded down, and since I see no filtering capacitors placed nearby, that resultant pulsating DC might work if a bit of motor hum is permissible.
The DC power flow from the FWB anodes (-)and cathodes (+) is designated with the +- DIAMONDS, over to the two relays.
Far left on the block diagram enclosure, are two relay driver transistors being actuated by the control electronics to properly activate the relays associated switching actions so that the DCpower polarity at their associated CN3 power output connector can be polarity flipped in accordance whether the motor run direction for its linear actuator needs to lift or lower the back rest.
The same action is produced by the boxed in RED area, except it is dealing with 12 VDC power output at CN4 of proper polarity to run the associated Leg Up Down motor of its linear actuator in the proper direction.
Since you can make all 3 BLUE relays make-a go clicka-clicka, but not the BLACK one you need to be sure you get your GROUNDING lead of your meter correct by connecting it to one of the common tied together emitter connections of the 4 relay driver transistors.
Then you meter the DC collector voltage of the relay driver transistor located at the side of the BLUE "B "relay" taking and logging down readings with the relay activated and then non activated.( That gives the voltage swing parameters of a good working relay)
Move to the BLACK " A" relay driver transistor just above and try for the same activation procedure, but as you said no clickee-clickee . . . .but we want to see what its unactivated voltage on its collector is, and it then MAY exhibit some degree of voltage when TRYING to activate itself.
Therefore, we want to see if it is replicating or trying to replicate the differential voltage reading swings of the "good " BLUE relay just tested.
One possibility is the relay coil, as Sir dork . . . . .I believe . . . . had marked up / by circling the FUZZY coil spec and it appears to be a 24V coil vice the others 12V coils.
Now SINCE you can easily read them, see if the sides of the BLUE ones show 12VDC coils while you confirm that the BLACK one has a 24VDC coil.
Usually a 24V one can marginally activate on 12V, but you would want a match with the other units coil wise.
The other MAJOR ingredient in the mix, is that you want to confirm that 12VAC is coming in to CN10 and CN11.
Also you want to ohm out the unplugged connectors to CN4 and CN3 down to the motors and confirm continuity of a motor being on each end.
Better yet if you have a 12VDC powered portable electric drill *** steal its power pack and use clip leads to run the motors to confirm that they are not locked down by water induced corrosion of their screw drives.
(*** Or the 12 V battery off yer Harley or from Mamma's Mercedes )
Walking on down that board, at its center, I am seeing an optisolator fed switch mode power supply with its power output.
Just below that supply is a replicate but additionally kicking out 12VDC @0.5 amp at a central 5 pin connector.
Then at the very bottom of the board, I am seeing 4 driver transistors to 4 opti-isolators to feed further over to some Triacs for a heaters temperature control, a pumps control, an AC motor and possibly some control valves .
All of those functions, I think that you said were operational.
Sooooo . . . . . . . . its just the motor driven linear actuators that are now lacking functionality.
ah . . .Mon ami . . .le Techno - Referencing:
Thasssit . . . . . . talk to me . . .and I will additionally show you how to properly repair the units botched up power foil busses,
IF its other repairs, end up being are successful.
Pee Ess . . .You certainly getting mo' bettah at your soldering expertise and particularly . . .flux clean up !
73's de Edd
.