WhitakerAudio JC Whitaker 20 W Stereo Integrated Audio Amplifier, Manual, Page: 146 / 196

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• The output transformer primary connections to the amplifier PWB. Confirm that the leads
go to the correct positions.
• The output transformer secondary connections to the speaker PWB. Confirm that the leads
go to the correct positions.
• The input and feedback connections to the amplifier PWB. If the 4-pin Molex connectors
are swapped (RCA input jack to Conn1 and the feedback signal from the speaker PWB to
Conn2), the amplifier will oscillate.
If oscillations persist and the above checks have been made, install C3 (3.3 pF, 1000 V
ceramic) on the right channel PWB. This component may be used to suppress high frequency
oscillations, if needed. The device is typically not required. There is a small impact on high
frequency response with C3 installed, slightly lowering the –1 dB point on the high end.
23) Switch the amplifier off.
24) Install the left channel amplifier tubes. Connect another speaker to the left channel output
terminals and apply power to the unit. Observe for any unexpected behavior as detailed
previously for the right channel. If oscillations persist, install C9 (3.3 pF, 1000 V ceramic)
on the left channel PWB. The device is typically not required. (See the cautions above for
the right channel equivalent, C3.)
25) If problems are experienced, disconnect power and recheck the installation of all
components, particularly the input stage (V1) of the amplifiers. With no audio input, the
amplifier should draw about 800 mA from the ac line.
26) Remove all power from the amplifier. Turn the chassis over to provide access to the PWBs.
Note: The following step is optional and intended only for experienced technicians.
Dangerous voltages exist.
27) Confirm that a speaker (or a resistive 8
Ω
load of 10 W or more) is connected to each
speaker output. Apply power and check the operating voltages detailed in Table 6.3. Check
the right channel amplifier and then check the left channel amplifier. It is normal to observe
small differences between readings for each amplifier. Any large deviations should be
investigated further. Note that the input line voltage will have a significant impact on the
voltages measured. An ac input of 120 V ac is assumed. Note also that the bias setting will
have an impact on the voltages as well. Set the bias as follows: