Yamaha emx512SC protection circuit

[Enzo]

First we would be looking for shorted output transistors, not output shorted to ground.

Every component lead is a test point.

[Captain Pete]

Hello to all and thanks for having this thread. I have found it very useful in the past and now I have a questions about this specific SMPS. Specifically the bridge rectifier circuit: I have a Yamaha EMX512sc version U as specified on the PCB. As I look at the jumpers for this version (USA 100V) I see that jumper J401 is in place along with J402. Now when I look at the schematic as well as the actual PCB, I see that the two AC line inputs for the Bridge Rectifier D410 are shorted together. Although I was trained on Electronics back in the early 1970's, I certainly have never seen a bridge used in this way. The incoming line (110V AC) is now driven across both AC inputs and the Neutral has no way to complete this circuit that I can see. Can anyone help me understand this? It must be something simple that I am missing as I believe in the motto (when you hear hoofbeats think Horses, Not zebras). By the way, the original problem was the owner blew out the power amp somehow. I have been working backwards from there. I have the Power amp rebuilt and as I was working back through the SMPS, I replaced some defective parts and when I looked at the bridge circuit, I have become fixated on this as it looks like it CANNOT WORK!!!!

Help Please!

Pete

[Enzo]

J401,402 are installed, but J403 is not. Neutral comes in through J402 to the point between the two stacked filter caps. Absent J403, there is no place else for it to go. Hot goes through J401 to the bridge AC terminals, basically putting the two halves of the bridge in parallel. The positive corner goes to the V+ top of the stacked caps, and the negative corner to the V- end of the stacked caps. This is a voltage doubler, so you should have something like 340vDC across the pair of caps.

[keyfixer]

THANK YOU ! THANK YOU ! THANK YOU !

I too have been working on one of these and after repairing the SMPS and replacing power transistors was also at my wits end, having the 3 second relay shutdown problem...GRRRR

Thank goodness I decided to see if there was anyone else suffering the same fate...and here I found the magic answer...I also was trying to operate it without the front panel earth in place, read about it, took a short length of cable with a crock clip on each end, hooked it up and Hey Presto, SORTED!

Thankyou once again for this information, I wonder how many other souls are out there pulling their hair out?