WhitakerAudio J C Whitaker 20 W Stereo Audio Amplifier, Manual

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20 W Stereo Audio Amplifier
terminals, the short-circuit is removed from the Timer Reset and the auto-off system will function
as intended.
The bias voltage detection circuit is a one-stage voltage amplifier used to drive relay RYL402,
which is a part of the interlock circuit of the power management PWB. Negative voltage to run
the circuit is derived from the –60 V dc bias output on the rectifier PWB. The voltage is filtered
and held at –25 V by D405. The sensitivity of the circuit is set by trimmer potentiometer R418.
Zener diode D408 sets the minimum voltage that will trip the relay and thus keep the amplifier
running. (If low bias, or no bias, is detected, the amplifier will shut down.) With the component
values chosen, the circuit will switch on at approximately –20 V with R418 set for mid-point.
Apart from improper setting of the Bias Adj. potentiometer, the primary value of the bias
voltage detection circuit is to shut down the amplifier if the bias supply were to fail. While such a
failure would be quite unusual, it could be a potentially damaging event since the output tubes
would them be operating with no bias. Under such a condition, circuit breaker CB51 would trip,
removing power from the amplifier. The benefit of the bias detection circuit is that it operates
faster than the circuit breaker, the goal of which is to prevent—or at least limit—damage to other
components.
The interface from the bias detection circuit to the power management PWB is the SW203
terminal pair on the power management PWB. To maintain high-temperature sensing
functionality, the terminals from SW203 are routed to the expansion board for inclusion in the
interlock loop, as shown in Figure 1.4.
The interlock switch (SW401) opens the control protection loop when the bottom chassis
plate is removed. This switch can be bypass for servicing when needed. Note that when operating,
the bypass posts have 120 V ac present on each terminal. Install the service jumper only when
power is removed from the amplifier. Note also that the interlock is active only after the
protection loop is enabled (about 30–60 seconds after power is applied to the amplifier). Do not
rely on the interlock to remove dangerous voltages from the amplifier. Disconnect the amplifier
from the power source prior to servicing.
1.3 General Considerations
The front panel of the amplifier is shown in Figure 1.5. The back panel is shown in Figure 1.6.