Celestion csw Ij am suspecting p/s. 14 volts on collector of relay drive, If I put spkr on input of relay switch there is sound,so amp is ok. 24v relay so I think cap in ps ng?

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).