WhitakerAudio JC Whitaker 20 W Stereo Integrated Audio Amplifier, Manual

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20 W Stereo Integrated Audio Amplifier
terminal #3 of TB4 on the left channel PWB should be approximately –30 V; the same
voltage should be measured at terminal #3 of TB3 on the right channel PWB. If not already
–30 V, adjust the Bias control on the chassis for –30 V.
14) 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.
15) Check the voltage between the V+ terminal on the preamplifier PWB and ground. A reading
of +24 V dc should be measured.
16) Remove ac power from the unit.
17) Set the Hum Balance control for center rotation. Set the Bias control for center rotation.
18) Install tubes for the right channel amplifier.
19) Set the volume control to the minimum (counterclockwise) position. Place the front panel
Speaker switch in the On position.
20) Connect a loudspeaker to the right channel output terminals.
21) Connect the ac line cord and switch the power on. Observe the tubes for any signs of
overheating or other unexpected behavior. The filaments should be lit. Remove power if any
problems are detected, or if noticeable hum can be heard from the speaker. Recheck wiring
and components as needed.
22) If oscillation or motorboating is observed, immediately remove power from the unit. It is
important that an oscilloscope be connected to the output terminals, in addition to a speaker
and perhaps other test equipment, when the amplifier is first powered up. It is possible that
high-frequency oscillation can occur that cannot be reproduced by the loudspeaker, or heard
by the builder. Monitoring with an oscilloscope is the only certain way of determining
whether any high-frequency oscillations are occurring (or very low frequency oscillations
for that matter). When connecting test equipment to the output terminals, remember that one
side of the output transformer is connected to ground. Incorrect placement of test leads can
short-circuit the output. Oscillation may also be indicated if differing bias voltage
measurements are observed on the grids of the push-pull output tubes (with no input
applied).
If oscillation or motorboating is observed, remove power from the amplifier and check the
following items: