WhitakerAudio JC Whitaker 20 W Stereo Integrated Audio Amplifier, Manual

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For any SCR, typical gate trigger current/voltage parameters are specified by the
manufacturer. Within a given production run, some variations about the typical values may be
observed. If Q206 fails to close after 60 seconds, consider the following troubleshooting steps.
• With power removed, connect a high-impedance voltmeter (1 m
Ω
or greater) between
ground and the junction of R211/C207/D208. Switch the power on. The voltage will ramp
up from zero to +22 V within about 30 seconds. It should stay at +22 V.
• If the previous test is successfully passed, switch the power off and move the voltmeter
probe to the gate terminal of Q206. Switch the amplifier on and monitor the voltage,
which should rapidly climb to about +0.7 V, and about 30–60 seconds later (when the
junction of R211/C207/D208 has r22 V) the voltage should slowly increase to
+1.5 V, at which point the SCR will fire.
• which point the SCR will fire.
It is possible that, depending on the gate trigger current tolerance, the SCR will not fire after
the voltage at the junction of R211/C207/D208 has r 22 V. In this condition, the voltage
at the gate of Q206 may not increase to much more than +1 V. The simplest solution to this
problem is to add a high value resistor in parallel to R217. This can be done from the foil side of
the board without removing the PWB. A range of resistor values (1 m
Ω
to 220 k
Ω
, 0.5 W) may be
tried. Use the highest value device that will result in proper operation of the circuit. A decade
resistance substitution box is useful for this test, if available. Do not apply a value of less than 220
k
Ω
.
Note that the triggering circuit is somewhat temperature-dependent. The SCR will tend to take
more time to fire when cold than when warm. Since the exact time delay is not critical, this is not
an operational issue.
6.2 Operational Tests
The following steps should be performed to confirm proper construction and operation of the
amplifier.
– Connect speakers to the right and left channel outputs. Remove any input sources from the
Phonograph, Line, and Front Auxiliary jacks. Apply power to the amplifier