Yaesu FT-901DM Maintenance Service Manual page 85

SERVICING
14. BIAS ADJUSTMENT
The final amplifier bias must be checked to ensure
linearity and normal operating plate dissipation
for the final amplifier tubes. Adjust the BIAS
control VRy99; on PB-1708 as follows: set the
transceiver in the receive mode and allow
tubes to reach normal operating temperature.
Set the MODE switch to USB, the METER switch
to IC, and the VOX GAIN control to MOX. so
to activate the The meter will
then indicate cathode current for the PA tubes.
The idling cathode current is 50 mA if the bias
is correct (25 mA for the SD model). If the idling
cathode current is other than 50 mA. adjust the
BIAS control VRj 90, for the correct value.
the
as transmitter.
Rectifier A unit (PB-1708)
POWER OUTPUT METER ADJUSTMENT
VR,. located on the transceiver rear panel adja-
cent to the RCV ANT jack, provides adjustment
for the relative power output indication on the
front panel meter.
1S.
With the transceiver tuned at full power into a
50 Ohm dummy load, VR; should be adjusted
to indicate 1/2 to 2/3 full scale meter deflection.
with the METER switch in the PO position.
It should be noted that the PO meter indicates
relative power output, and it is not the basis for
determining actual power output.
3-30
VR; has been preset at the factory to read 1/2
to 2/3 full scale into a 50 Ohm dummy load. The
operator should not indiscriminately adjust VR,
while using an unknown load or antenna of pos-
sibly high VSWR.
(vR»)
PO Meter Adjust Accessory Plug
16. FINAL AMPLIFIER NEUTRALIZATION
When replacing the final amplifier tubes, it may
be necessary to reset the bias to give the correct
idling current, and to check neutralization. Using
the procedure outlined below will
maximum output and long tube life.
guarantee
CAUTION
HIGH VOLTAGES ARE PRESENT ON THE
UNDERSIDE OF THE CHASSIS AND INSIDE
THE FINAL AMPLIFIER COMPARTMENT, USE
GREAT CARE WHILE MAKING ADJUSTMENTS
IN AREAS OF EXPOSED WIRING.
(1) Connect a dummy load to the antenna re-
ceptacle, and set the METER switch to IC.
TCor, the
capacitor shaft, on the inside wall of the
final amplifier compartment.
(2) Locate neutralization variable
Check the final amplifier idling current as
described in part 7 of this section of the
manual (BIAS ADJUSTMENT).
(4) Tune up the transceiver at 29 MHz, using
either the 10B or 10C band, and adjust the
carrier level so that it is 150 mA. in the
TUNE condition.