WhitakerAudio 60 W Stereo Power Amplifier, User And Assembly Manual

Page: 92 / 154

92
WhitakerAudio
15) Using a dc voltmeter, measure the voltage present at Terminal 2 on the right channel PWB,
and then at Terminal 2 on the left channel PWB. This test measures the output of the bias
power supply. With the Bias Adjust potentiometer (R61) set to the approximate center
position, the voltage on Terminal 2 of each amplifier board should be in the neighborhood of
–40 V dc.
Note that a short time after the amplifier is switched on (30 seconds is typical), you will hear a
“click” sound. That is the sound of relay RYL51 closing. This is normal.
16) Switch the amplifier off and remove the ac plug from the power source. Install the bottom
cover and turn the amplifier right-side-up.
17) Reconnect the power source and switch the amplifier on.
18) Check the operation of the bias supply. Using a dc voltmeter, measure the voltage on the
control grids of the power output tubes (pins 5 and 6 on each of the four output tubes). To
make this measurement, connect one lead of the voltmeter to the ground terminal on the
back panel and carefully probe the tube sockets from the top of the chassis. The voltage at
the measured pins (5 and 6) should be approximately –40 V dc.
19) Check operation of the Bias Adjust potentiometer (R61). Measure the voltage at one of the
output tube control grids (as detailed in the previous step) and vary the position of the Bias
Adjust control. A voltage range of approximately –20 V dc to –60 V dc should be observed.
Set the bias voltage for –40 V dc. Move the voltmeter to the Bias Test pins (black is ground
and green is hot). The voltage measured here should agree with the voltage measured at pins
5 and 6 of the output tubes.
2
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.
20) Check the amplifier tube filament voltages. Using an ac voltmeter, measure the voltage
between pins 4 and 5 on V1, and between pins 2 and 7 on V2 and V3 on the right and left
channel amplifiers. A reading of approximately 6.3 V ac should be observed.
21) Check operation of the Hum Balance control. Using an ac voltmeter, measure the voltage
between pin 2 of V2 on the right channel amplifier and ground. Vary the Hum Balance
control (R60). A voltage of 0 V to 6.3 V ac should be observed. Now measure the voltage
2. Note that a high impedance voltmeter must be used for this measurement, and the others detailed in this section.