Heathkit IB-5281 - Bias Adjustment

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BIAS ADJUSTMENT
Refer to Pictorial 4-3 (Illustration Booklet, Page 5) for
the following steps.
( | Position the RLC Bridge as shown.
NOTE: Use a small-bladed screwdriver to make the
following adjustments. If you do not obtain the proper
results during the bias adjustment, refer to ‘In Case of
Difficulty” to help you correct the problem. Do not
proceed with any other steps until the problem is
corrected,
(/) Preset controls R9, R14, and R33 to the center of
their rotation.
( Place the POWER switch to ON.
NOTE: You will need a voltmeter capable of measur-
ing both AC and DC for the following steps.
(4 Set the meter to the iowest range capable of
measuring 5 volts DC.
( ‘” Connect the common lead of your voltmeter to
the chassis.
(.} Touch the voltmeter probe to circuit board
point A and adjust control R9 (Bias Adj A) for 0
+.1 volts. Note: Adjust R9 very slowly and wait
for the reading to stabilize each time you stop.
Then adjust the control further if necessary to
obtain the correct reading.
(+) Set your meter to the lowest range capable of
measuring 5 volts AC (rms).
(j Touch the voltmeter probe at circuit board
point A and adjust control R14 (Feedback Adj)
for a reading of 3 volts AC.
( Y,) Set your meter to the lowest range capable of
measuring 5 volts DC.
(“} Touch the voltmeter probe to the collector (©) of
transistor Q8 and adjust control R33 (Bias Adj
B) for a reading of 4.5 volts.
(“) Repeat the steps after the second NOTE:
( ) Place the POWER switch to OFF. Disconnect
the voltmeter leads.
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“Dial Calibration.”
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