26
A.C. Balance Adjustment:
A 60 cycle test signal has been provided so that the amplifier can be dynamically balanced. This signal is
available next to the Channel B input jack. The upper jack is the input to the Channel B a mplifier and the lower
jack is the “test signal” source.
In addition a 16 ohm, 20 watt resistor is provided to connect to the speaker terminals as a load, while ad -
justing each channel.
Perform the following adjustments periodically to insure perfect A.C. balance.
( )
Connect the 16 ohm, 20 watt wirewound resistor between ground
(
G
)
and the 16 ohm terminal on the
Channel A speaker terminal strip.
( )
Rotate the meter selector switch to the Channel A A.C. Balance position.
( )
Connect one end of a standard phono shielded patch cord
(
This is shielded lead with a RCA male plug
on either end
)
into the “Test Signal” Jack. Connect the other end of this cord into the Channel A input
jack. This should be done before plugging in the amplifier. As the 16 ohm, 20 watt resistor will get quite
hot, prolonged testing is not advisable.
( )
Adjust the A.C. Balance control located on the top of the chassis between the two KT88 output tubes
(
V9 and V10
)
with a long screwdriver. Rotate the control to the right or to the left until the meter
pointer aligns with the “BALANCE” mark.
( )
Remove the patch cord from the Channel A input jack.
( )
Rotate the meter selector switch to Channel B A.C. Balance position.
( )
Allow sufficient time for the 16 ohm, 20 watt load resistor to cool and disconnect from the channel A
speaker terminal. Connect the load resistor to Channel B speaker terminals between ground
(
G
)
and 16
ohm.
( )
Connect the phono patch cord into Channel B input jack
(
the upper jack
)
and adjust the Channel B
A.C. Balance control located on the top of the chassis between the two KT88/6550 output tubes
(
V7
and X8
)
with your screwdriver. Rotate the control as before either to the right or to the left until the
meter pointer aligns with the “BALANCE” mark.
( )
Remove the phono patch cord and the 16 ohm, 20 watt resistor and set aside for use later on.
( )
This completes the A.C. balance adjustment. Throw the meter selector switch to “OFF”.
(
The meter
should not be allowed to operate for an extended period of time while playing the amplifier for this may
result in damage to the meter movement.
)
( )
After about
½
hour warm up time, repeat this adjustment to insure minimum distortion. No further
adjustments are necessary unless any of the tubes are replaced.
It is possible that the resting point of the meter pointer
(
as seen in the “OFF” position of the meter selector
switch
)
may not coincide with the A.C. Balance mark. For optimum results, in the A.C. Balance adjustment, note
exactly where the meter pointer rests with the meter switch “OFF”, and the amplifier in its normal upright
operating position. Adjust to this point when balancing each amplifier with the test signal and the A.C. Balance
control.
This completes the A.C. and D.C. balance adjustments.
