53152 PA

voltage across the supply capacitors to drop. The meter can now be removed and a fuse installed in the negative supply linę.

Re-apply power and check that the DC voltage at the amplifier output is close to zero; ie, no morę than ± 100mV. Typically, the voltage will be around ± 10mV or even lower.

The action of the protection circuitry should also be checked.

The relay should puli in a few seconds after power is applied and drop out again when power is removed.

Now connect a 1.5V battery across the first 47^F bipolar capacitor, ie at the junction of the two 22kQ resistors. This should cause the relay to drop out.

Reverse the battery connection and check that the relay again drops out.

Finally, connect a loudspeaker and check for the presence of hum or any other signal. With no signal applied the amplifier should be absolutely quiet. Touching your finger to the input should then cause the speaker to emit a smali “blurt”. If so, all is well.    £

39

ELECTRONICS Australia, October, 1985