Theory:
With No DC applied, the IN relay is open and RF goes through the toroid to all antenna ports. Removing an antenna from that mix requires that the control box key the appropriate of the 1, 2, 3, relays on. That will drop that antenna out.
When just one antenna is selected, the IN relay is keyed on as is 1, or 2, or 3 according to the desired antenna.
W0CG Procedure:
Connect all three 50 ohm dummy loads to the
antenna ports
Connect box to the test cable with the switch on the other end.
Connect switch to DC power with PowerPole connector.
Diagnose.
Look for bad diodes, relay, or toroid, based on the above theory of operation.
The IN, 1,2, and 3 relays have 270 +/- 10% ohms. Measure the coils from either your cable end if the StackMatch is up on the tower, or take it down to measure.
You should measure from each wire or terminal to the ground wire or terminal. Remember to figure the amount of resistance in your cable.
The relays will pull in at 9 VDC. They will draw 40 ma each. But try to use cable that will deliver 11-13.8 VDC to each relay.
Set the control to pull in a single antenna at a time. This will apply 12V to the IN relay and one of the antenna relays (1,2,or 3) This will take the torroidal transformer out of line.
You can measure a DC continuity resistance to each antenna. For instance antenna 1 position should have 0 ohms from its SO239 port to the feedline port. The other antennas will be at DC ground through the torroidal transformer.
Measure each antenna port for continuity while connected.
You may also use a dummy load to test each antenna port for 1:1 or close to it SWR.
Power off is means you have RF to the full stack. Three dummy loads in each antenna port should deliver a SWR of less than 1.2:1 at the user frequency.
Test each combination for SWR to be below 1.2:1 at each combination of antennas.
With a 12V DC power supply and some jumper wires. Test the StackMatch relay box for proper operation without the controller. Use three dummy loads.
Test single antennas by placing 12V on the IN relay to ground and any one of the 1,2, or 3 antenna relays. You should see an approx. 1:1 SWR
Test multiple antennas selection by no voltage on any relay, or full stack, You may now place 12V on any antenna relay 1,2,or 3 to pull it out of the stack. SWR should remain good.
If not the controller may be defective. Test the controller output voltages to correspond to the above single and multiple antenna function. Use your schematic to derive which diode may be bad.