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Wharfedale pro BX1360SD powered mixer intermittent fault - preamp section OK, no output from power amp (false overtemp detection? shutting down power amp??) please supply advice, circuit diagram,schematic if possible thanks, Dennis Ruff

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Wharfedale pro bx1360sd


I have had problems with this stereo mixer amplifier which had the symptoms of no sound output from either channel .

I opened up the unit and discovered that the amplifier was correctly amplifying my input signal up to a relay which switches the output through to the speakers .

This relay did not activate at all.

When the power switch is turned ON you should hear a click approximately 2 seconds later , if no click then there is a problem with the circuitry associated with this relay .

I traced out all the connections associated with this relay and made a circuit diagram .

The relay is a 24v relay and I will explain the way that I can see that this section of the amplifier works.

It has been designed to ;

  1. Create a slow start delay ( 2 seconds ) when the power is turned ON.

This it needed because otherwise there is a current surge and sparks from the relay turning on immediately.

  1. Thermal cutout mounted on heatsink . If the ventilation fan fails the heatsink and transistors may get too hot and the cutout will operate and cut power to the relay by going open circuit.
  2. Speaker protection from blown output transistors. There are 2 transistors q802 and q803 both 2sc1815. They are connected together emitter to emitter and collector to collector to form a differential Op amplifier I.C. If there is a blown output transistor then DC voltage will be detected by q802 and q803 and the relay will be switched OFF to protect the speakers .


Q804 turns On Q805 which turns On Q806 to activate the relay .

Voltages on transistors

Q802 and Q803 2sc1815

Base 0V Collector 12.9V Emitter 0V


Q804 2sa1015

Base 12.9V Collector 0V Emitter 15.1V


Q805 2sc1815 (driver for Q806)

Base 0V Collector 7.2V Emitter 0V


Q806 2sc1815

Base 0V at Turn ON then after 2 secs increases to 0.7V

Collector 2.8V ( transistor is activated as a switch Turned Fully ON ( saturated) to turn on the Load which is the relay .

Emitter 0V


I have replaced the following transistors to fix my amplifier q805 2sc1815 and q806 2sc1815 .If you are going to use a different type of transistor it may not work and also the lead connections of a 2sc1815 for the collector and emitter are different to those such as on a 2n2222( transposed).

Posted on Aug 29, 2014

  • andrew zitterschlager
    andrew zitterschlager Dec 09, 2014

    I have made a hand drawn circuit of the section that I described below .

    If anyone would like a copy of it then please contact me at email azitters63@gmail.com

  • andrew zitterschlager
    andrew zitterschlager Dec 09, 2014

    I have made a hand drawn schematic of the section I described below .

    If anyone would like a copy then contact me at email azitters63@gmail.com

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I have a Kustom KPM8420 PA mixer and the monitor channel comes through both the monitor and main speakers and vice versa. The two channels are not separate.How do I fix that?


On the back panel there is a section labelled Patch Bay
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It would seem you have them both set the same at present. Set one to Monitor and one to Main, and use the corresponding Power Amp outputs direct to speakers.

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Overload is displayed when the amp draw is excessive, and can be caused by different faults.
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The first thing to try is hard reset. Shut down the unit for about one hour to drain power completely.
This can fix minor problems that can cause overload, for example electrostatic charge accumulation on internal capacitors.

If that does not fix the problem, then you must find the fault.

Overload can be caused by shorted speakers, wrong impedance speakers, or by a contact on speakers wiring.
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Another test can be done by unplugging all speakers, and reconnecting and testing them one at a time, at a very low volume.

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Hi, When an two channel (stereo) amplifier is connected in bridge mode the polarity of one channel is reversed, for this example say channel B. An identical (mono) signal is applied to both channel inputs.
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1) What is the impedence rating for the speakers?

2) When the unit shuts down, will it recover on its own after a little while or do you need to power cycle it to get it to work again?

3) When the unit shuts down, is it hot?

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Keep us posted on what you find.

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