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voltmeter of the amplifier, M801.
Next, place switch, 3801,
which is the high voltage- low voltage switch of the amplifier
in the low voltage position.
Next,
reduce the driver plate vol-
tage by depressing button marked " lower" on the amplifier power
cabinet control panel.
This powerstat will be set to nearly maximum on receipt from th
factory.
It requires approximately 15 seconds for the motor
driving the powerstat to take the powerstat from full maximum
to minimum.
Depress button marked " lower" for approximately
15 seconds,
or until " Low " Limit"
light operates.
The power-
stat should then be at minimum.
Then depress the " Raise"
button for approximately 4 seconds.
This should give 600 to
700 volts on the driver as read on the driver plate voltmeter,
when the driver high voltage switch,
3506,
is depressed.
Provision is made on the first IPA driver stage for metering
the grid bias voltage of the IPA driver by means of a test
point on the panel.
A meter such as a Simpson Model 260,
or
equivalent, may be used.
With negative meter lead plugged
into this test point and the positive lead on ground,
a rise
in grid voltage will be observed as the Exciter comes up to
operating temperature.
This is a good check on the exciter
operation.
Place the switch,
S512, which is a test meter switch located
on the bottom center of the driving amplifier panel, in the
grid position or extreme counterclockwise.
If the exciter and
IPA are functioning properly,
about 20 ma,
of driver grid curr-
ent will be indicated.
If this is not the case a bit of trim-
ming of the previous tuned circuits will be necessary.
When
this has been achieved initial tuning can proceed.
When originally checking the driver portion of the FM-20Bcare
must be exercised in keeping the driver plate voltage to a low
setting.
High plate voltage on the driver will result in ex-
cessive screen current due to rectified grid voltage on the
amplifier.
Press the high voltage " ON" button,
on the driver control pa-
nel,
and immediately check the plate voltage on the driver.
Ad-
just as necessary for 700 to 800 volts of plate voltage on the
driver.
Increase the plate voltage on the driver to about
1,000 volts and check driver plate resonance with control
marked " plate tuning."
At this time P.A.
grid current will
be indicated.
There should be about 50 MA.
depending on how
much drive is applied and how much fixed bias is being used.
It may be necessary at this point to reduce the fixed bias to
about 200 volts.
The grid tuning control to this time may need
to be trimmed for maximum grid current.
Before pressing high voltage " ON" button on the amplifier con-
trol panel, reduce the amplifier screen voltage,
as controlled
6/22/61
-10-
FM- 20B
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