Allen & Heath Xone:92 (DJ mixer) no output on main out & magic smoke

[jaifraic]

Hello,

I have a DJ mixer here, an Xone:92 by Allen & Heath.

When I turned it on, only magic smoke came out, so I turned it off again.
An optical troubleshooting has brought me quite fast to the
source of the smoke: two resistors look disproportionately black (see first pic)

Those are R73 and R74 on the master PCB. See second pic for the corresponding page of the service manual.
Behind those resistors are five OP-Amps (U3, U4, U5, U6 and U9).

My thought is that one (or more) of those is defective. I've measured through and there is no short circuit between the pins of those.

Does anyone see another reason for the error? Or has an idea on how to troubleshoot further?

I have attached the complete service manual.

Best regards and many thanks already,
Jaifraic

[Agent24]

Re: Allen & Heath Xone:92 (DJ mixer) no output on main out & magic smoke

It does look like only those op-amps are powered through those resistors, so aside from C58/C60 (which would both have to go shorted at the same time, and is unlikely) it is most likely one of the opamps that shorted.

It's possible you can't find a short because the opamp that shorted blew open circuit due to the fault current.

You could try one of two things - replace the resistors and power up. If the opamp is now open circuit, then trace the audio through all of the ones in question, and you will find the audio signal disappears at some point. Then you know which opamp is bad.

You could also apply power at the +/-VA rail from an external (current limited) supply and slowly increase current. With just 5 opamps powered, you should have pretty much no current draw. If the current is significant, then at least one of the opamps should heat up and you can touch each one and find whichever is bad.