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TonyDuell · 21st Jun 2022, 5:09 am

I'd start with some DC resistance tests -- using the normal ohms range of your multimeter -- with the unit unplugged from the mains.

The tags on top of the transformer are almost certainly the secondary windings as they are connected to the logic board. It looks to be a pair of orange wires, a pair of red wires, and one other (black). This suggests a simple winding with 2 wires and a centre-tapped one with 3.

With the connector unplugged from the logic board (as in your photo) and the unit unplugged from the mains :

Measure the resistance between the 2 orange wires, should be a few ohms I would think.
Ditto between the 2 red wires.
Now test from the black (?) wire to each of the others. I would expect a low resistance to either the red ones or the orange ones.

Now, still with the connector unplugged from the board, turn on the power switch (mains plug unplugged from the wall) and measure the resistance between the live and neutral pins of the mains plug. This circuit consists of the mains cable, the switch , the fuse, and the mains transformer primary winding. The resistance will be a bit higher, maybe even a few thousand ohms, but it should not be 'infinite', a total open-circuit.

21st Jun 2022, 5:09 am	#2
TonyDuell Dekatron Join Date: Jun 2015 Location: Biggin Hill, London, UK. Posts: 5,208	Re: Dead commodore 4040 disc drive I'd start with some DC resistance tests -- using the normal ohms range of your multimeter -- with the unit unplugged from the mains. The tags on top of the transformer are almost certainly the secondary windings as they are connected to the logic board. It looks to be a pair of orange wires, a pair of red wires, and one other (black). This suggests a simple winding with 2 wires and a centre-tapped one with 3. With the connector unplugged from the logic board (as in your photo) and the unit unplugged from the mains : Measure the resistance between the 2 orange wires, should be a few ohms I would think. Ditto between the 2 red wires. Now test from the black (?) wire to each of the others. I would expect a low resistance to either the red ones or the orange ones. Now, still with the connector unplugged from the board, turn on the power switch (mains plug unplugged from the wall) and measure the resistance between the live and neutral pins of the mains plug. This circuit consists of the mains cable, the switch , the fuse, and the mains transformer primary winding. The resistance will be a bit higher, maybe even a few thousand ohms, but it should not be 'infinite', a total open-circuit.