WhitakerAudio J C Whitaker 20 W Stereo Audio Amplifier, Manual

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141
20 W Stereo Audio Amplifier
If significant variations in the expected voltage readings are found, remove all power from the
unit (unplug the ac line cord and confirm that the power supply has discharged to 0 V) and
recheck components and wiring. Do not advance to the next step unless the voltages check
correctly.
13) At this point, the voltage at the B+ terminal on the right channel PWB should be
approximately 470 V dc; the same voltage should be measured at the B+ terminal on the left
channel PWB. Typical ac current draw from the ac line is 200 mA. The voltage at the BIAS
terminal on the right channel PWB should be approximately –30 V; the same voltage should
be measured at the BIAS terminal on the left channel PWB. Adjust the Bias potentiometer as
needed.
14) Check operation of the Bias Adjust potentiometer (R57). Measure the voltage at one of the
output tube control grids (as detailed in Table 6.2) and vary the position of the Bias Adjust
control. A voltage range of approximately –20 V dc to –50 V dc should be observed. Set the
bias voltage for –30 V dc. Note that the bias sense circuit on the power management
expansion board may remove power from the amplifier when the Bias Adjust potentiometer
is at the extreme low end of its range (less than –20 V or so).
Proper operation of the bias power supply is critical to proper operation of the amplifier. Do
not proceed further in the initial checkout unless the bias circuit is functioning correctly.
15) Check the 5Vdc connection post on the right channel PWB. A reading of approximately
+5.6 V dc should be observed. This voltage will drop to about 4 V dc when the front panel
Speaker switch is placed in the Speaker Off position.
V3, V6
6973
1 470 V
2 –
3 –30 V With Bias Adj. control at approx. mid point
4 60 V Heater
5 60 V Heater
6 –30 V
7 0 V
8 470 V
9 470 V
Table 6.2 Typical Voltmeter Readings with V1/V4, V2/V5, and V3/V6 Removed
and Power Applied