Quoted from minnesota13:

With machine on:
Set your meter to measure AC volts.
Place one meter probe to any ground point.
Place the other probe to one of the sound board fuses, either end and measure.
Place the other probe to the other fuse, either end and measure.
What voltages did you read on each of the fuses?

Quoted from markmon:

The tda2030 is a 4 ohm stable amp but Williams often used 8 ohm speakers and wired them in parallel as well as 4 ohm speakers wiring those in parallel - which work under certain conditions but put good strain on the amp. If the set up works OK with the left speaker detached, this indicated that a higher impedance may run Ok in your system. Simply install the speakers in series as they should be and it will be a much easier load on the tda2030. The fact that the other tda2030 that runs the cabinet speaker is not blowing up indicates to me that this is not a transformer problem.
Wiring should go board + to left speaker +, left speaker - to right speaker +, board - to right speaker -. If the left speaker is removed from the system in this config, the right speaker will not work either. The fact that the previous owner had only cabinet and right speaker working indicates wiring was in parallel.
You can use the existing Williams wires to fix this also. You have board + and - going to left speaker and twisted with two wires going to right speaker. Detach the wires on the - post of the left speaker and leave them twisted together just not hooked up to the speaker. Now detach the two wires on the left speaker + terminal and separate them. Put one onto the + (preferably the run from the board) and the other on the left speaker - and you're done. They'll be in series bringing your impedance from an amp blowing 2 ohms to a safe 8 ohms.

Quoted from GRUMPY:

I think there is a communication problem here. Do you know the difference between a parallel and a series circuit? It's sounds like you are using two 4 ohm speakers and wiring them in parallel creating a 2 ohm load at the amp which will over heat it. Markmon is trying to get you to reconnect your speakers in series creating a 8 ohm load to the amp which will work correctly. There would be a single wire to each speaker connector forming a loop starting at the amp and going to the first speaker and then to the other speaker and back to the amp.

Quoted from markmon:

Woah read my instructions carefully. The left speaker does get a wire on both lugs. The instructions tell you to take one of the two wires that was on the left speaker + tab and move it to the left speaker - tab after removing the previous two wires that were on the left speaker - and leaving them twisted together (this step ends up being a direct run from right speaker - to the board.
Board to left speaker + then left speaker - gets wire directly to right speaker + then right speaker - to board. My steps above result in exactly this if followed. Then impedance to connector at the board results in approx 8 ohms.