Yamaha O2R repair

[Rachie5272]

I'm trying to fix a Yamaha O2R mixing board.  One day, it simply decided not to turn on.  Probing around revealed the obvious problem: one of the +5 V rails is dead.  Visual inspection revealed a scorched transformer connector.

The device uses a linear power supply.  The two blue wires in the image go to one of the secondary windings of the transformer.  The respective PCB header leads to a bridge rectifier, then a big 4 A linear regulator.  The transformer reads around 9 VAC, with or without the plug connected.  The DC output of the rectifier reads less than a volt.  Nothing heats up as far as I can tell.

I tried injecting 5 V with a bench power supply, but it still doesn't work.  A relay starts clicking, but nothing else.  The logic chips only get about 2 V, leading me to think there might be a short somewhere.

Any ideas?  I have the service manual with schematics, but it neglects things like test points, voltage readings, or troubleshooting.

[poot36]

I would start by checking the resistance between ground and the 5V line on all of the caps as well as the chips that use this line.  It should help you find the area of lowest resistance and then you can start removing components to check them individually for shorts.  How much current does it draw when you have it connected to your 5V bench supply?  If it is more then what the regulator can put out then that is a problem.  If you can remove the board and check the back side of it for damage that would help as well.  Also if possible disconnect any other boards from the main board to rule them out as a problem.

[Rachie5272]

I found the problem.  The transformer winding is bad.  It confused me, because it shows 9 VAC even when connected to a load.  Injecting power at the transformer connector works.  The original transformer is a custom multi winding toroidal, so I'll have to add a second one.  It's a good thing there's so much empty space inside.

[Anks]

I repaired one of these awhile ago. Much different type of repair and just be glad you didn't have to take the entire thing apart. Here is a link just in case you fancy a look. https://krisanks.wordpress.com/2013/06/03/repairing-a-o2r-digital-mixer/

[Rachie5272]

I fixed it.  As it turns out, the transformer was fine all along.  The only problem was a bad connector.  I measure the voltage from the top of the connector, which explains the anomalous reading.  I guess I needed to be a bit more thorough.