current flowing through R58A comes from ZD7A. To turn the LEDs on, the amplifier is required to
produce an output voltage approximately 5V above the reference, at which point ZD7A is no longer in
breakdown and the current flowing through R58A comes from the output stage via D13A, LED1A and
LED2A. Thus the “peak” LED threshold and the “Clip Limiter” threshold vary with the +HT voltage and
thus the output loading conditions.
FAULTS IN THE OUTPUT STAGE
Output device failure usually leaves you with two or more of the Collector, Emitter, Base shorted together. Decid-
ing which of the output devices is at fault is relatively simple. Use a desoldering tool to remove the solder from
the device pins (Base and Emitter) and move the pins so that they are clear of the pads. Do this for all suspects.
You can now measure each output device separately with a multimeter set on the diode tester. Remember to test
for all possibilities. A good output device will measure like this:
Probe connections:
For MJ15024 (npn): red (Volts/Ohms), black (COM).
For MJ15025 (pnp): reverse the red and black probes
Correct reading
Base > Emitter …………………> 500mV
Emitter > Base …………….…..Open Loop or no reading
Base > Collector……………….> 500mV
Collector > Base ………………Open Loop or no reading
Collector > Emitter………….….Open Loop or no reading
Emitter > Collector……………..Open Loop or no reading
No special selection is required when replacing output devices.
If an output device has failed, it is very likely that other parts of the output stage will also have been
damaged, especially Q9A - Q12A and associated resistors/capacitors and Q21A, Q22A and other parts of the V-I
limiting circuitry.
Other Causes of Apparent Output Stage Faults
Output DC offsets can be caused by faults outside of the output stage itself. In the event that no dead output
devices are found, check the driver stages for correct operation. The fault-finding routine should start at the feed-
back point: check that the DC fault is reflected at this point. Then progress through the driver stages: check that
the relevant current sources are operating correctly, and that the feedback is attempting to correct the fault.
Premature clipping may also be caused by faults not connected with output device failure. Most often the fault
lies within the V-I limiting circuit. This can easily be confirmed by removing D7A and D8A from the circuit board
and observing if the fault clears. Note that, counter-intuitively, a faulty D8A will show as a problem on the nega-
tive half-cycle, and a faulty D7A will show as a fault on the positive half cycle. Faults within the V-I limiting cir-
cuitry can also cause some extremely strange and gross distortion, especially off load. Note that R44A-R51A, the
'current sharing' resistors are part of the V-I limiting circuitry.
