Power supply of soundcraft efx12 - Soundcraft Efx12 - 12 Channel Sound Board Console Mixer

Check the inserts and pan controls.

Unit needs to be serviced by qualified technician. If you have some knowledge and experience, do this...use a Freeze spray (available at your electronic parts dealer) and freeze the effects related parts 1 at a time till they come back. No Freeze spray? When stops working, physically feel related components immediately for excessive heat.

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Yes, but it could be pointless.
Headphones are just small speakers so it is usually possible to drive a small pair of "computer/iPod" type speakers from the headphone output.
True balanced outputs should have a fair amount of drive so try connecting a speaker between pins 2 and 3. The higher the impedance of the speakers the better.
Ultimately, the results will be very quiet and the cost of a small amp or powered speakers for low level use is quite reasonable.

check it out here.. SOUNDCRAFT EFX12 EFX8 MIXER Service Manual free download schematics eeprom...

circuit diagram for soundcraft efx 12 mixer

USUALLY the Mute LED's for the switches ar operated by separate contacts on the mute switch an a resistor current limit for the LED. I could NOT find the schematic for this board, HOWEVER some Soundcraft brand mixers do NOT have illuminated Mute switches... Not sure about your particular unit.

If the LED's are level indicators, you are probably saturating the input of the effect and may cause it to hang. Effects use delay and feedback and can cause numerical overflow and the system to basically crash. I would first call the Lexicon support and ask them about this problem... I did have to return one Lexicon product myself for service under warranty.

Dirty pots and faders are common problems. Try blowing out with compressed or canned air.