Peavey XR500 Powered Mixer Repair

Peavey powered mixers consist of a front panel with controls and a rear panel with the amplifier.  The amplifier will generally have a different part number entirely, which you will need to know in order to find the correct schematic.

We have five input channels.  Each channel has a treble and bass control and a gain control.  A bit of that channel may be added to the effects processor input.  A bit of that channel may be sent to a monitor output.

This mixer has a reverb function and a simple equalizer.  The main, effects, and monitor outputs are line level.

The main output is amplified and available on the rear panel as shown.

This information pertains to the amplifier section, not necessarily to the mixer.

The front panel nomenclature is used on this tag.

The output transistors are mounted on the aluminum heat sink, coupled to the back plate of the mixer enclosure.

The power supply is a simple linear supply with three terminal regulators for powering the mixer and effects.  That big brown cube-looking thing is a choke, wound on a transformer bobbin and supported entirely by the large wires of the winding itself.  Not a bad idea for an inexpensive choke.

On larger powered mixers, the power transformer is mounted in the enclosure, but here, it is fastened to the rear panel.

The white diode is shorted.  This is a protection diode, often seen on high voltage power supplies, to clip high voltage transients which may occur with loudspeaker failures (or if someone disconnects the loudspeakers while the unit is running, which I suspect is what happened here.)

Shorted diode is gone.  Both protection diodes were replaced.

This resistor and surrounding capacitors make up what is called a Zobel network.  This network is intended to neurtalize the effects of the inductance in the loudspeaker voice coil from the amplifier output terminal.  When the white clamping diode shorted, this network was exposed to full high voltage, with the effects you see here.

Amazingly, the capacitors were fine even at their rated voltage, so only the resistor required replacement.

Full power testing went well.  However, the power transistors themselves became very hot.

The thermal compound and spacers under each transistor were renewed and the transistors re-torqued.  That black device with the brown wires coming from the top is a thermal relay that open circuits the power to the amplifier if it senses that the heat sink temperature is too high.

Everything is fine and this unit is ready to return to service!

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE