Yamaha EMX 5014 problem

[ARI]

Sometimes, when it's switched on, SMPS protection gets activated and it switches off automatically.
I have checked every power transistor, they are okay but there is DC voltage on output A and B. (normally, there is no DC on output or speaker unless output transistor is damaged.)

Sometimes it's switched on without a problem. Also, both A and B sides are same.

(I don't know technical terms about electronics, so I apologize if I have mistaken. I am writing it for someone else, who has this problem and wanted me to post it here.)

Thanks in advance, if anybody can help.

[Enzo]

Well, why not invite that person to join us and discuss it directly? I mean this place is free after all.

HOW MUCH DC is on those outputs? 2v? .2v? .02v? 20v? Does it go away when a load is present?

Without me looking it up, on some Yamaha PAs, the power amp input circuit grounds are separate from the power grounds. The input connector carries the ground from th earlier stages into the power amp inputs. Lose the solid ground through that cable, and the input stages of th PA start to float, and that causes offsets. SOmething to consider.

Monitor mains current draw. Is it higher than expected in general? Is it higher the times it shuts itself down than when it runs OK?

[ARI]

Thanks for the reply, Enzo!

That person's English is limited that's why I am posting for him. I'll tell him what you said and come back with his answer.

[sonicmember]

i have the same problem! did it get solved by anyone??

[Enzo]

anyhow, ive disconnected the power amp board and now the the mixer doesnt switch off, it was just coming on then switching straight off(the fans came on for a few seconds) OK, i downloaded the schematic and checked the output transistors but almost everything reads s/c which i know isnt right, or is it?

If your output transistors appear to be shorted, then remove them and retest them while not connected to the circuit.

It is entirely possible the whole row of them failed. You will have to find out.

If any of your outputs have shorted, then check ALL resistors associated with them, AND check the driver transistors.

[HutchAmpMan]

Where do you find the insulating tape used on the heat sink? It's very thin and stuck (mostly) on the transistors.

[Enzo]

Start by looking up "SILPAD".