VTL IT-85, Owner'S Manual

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IT-85 Integrated amplifier
Owner’s Manual
VTL 23
reading on the multimeter should stabilize after a second or two and
indicate a reading between 275 to 300 millivolts DC (0.275 to 0.300 Volts.)
This reading may fluctuate a little due to variations in the AC lines. (It is
not unusual for a 120V rated AC line to vary between 115V and 122 V.)
6. Caution: If the voltage reading is 50
millivolts or more below or above the
acceptable range then adjust the
trimpot in the direction required (clockwise for increase and
counter- clockwise for decrease) and ensure that the reading
is changing as desired. If the reading does not respond or
continues to climb rapidly, or fluctuates with large swings
back and forth then you should turn off the IT-85 immediately
and contact the VTL factory service department for
assistance.
7. In general a 10 - 15% variation around the 275-300 millivolts range is an
acceptable current measurement. When the measurement is outside this
range, you will need to adjust the bias setting by using the following
procedures:
1. Keep the positive probe from the meter in the same measurement
point where you found the out of range bias reading. Locate the bias
adjustment point whose number corresponds to the bias measurement
point. See Figure 2 for the locations of the measurement points.
2. Insert a 1/8” flat tipped screwdriver (with a plastic shaft or a properly
insulated handle) into the bias adjustment trimpot that corresponds to
the out of range measurement point. Rotate the screwdriver slowly to
see the changes on the multimeter reading. Try rotating the screw
slightly in both directions to see the effect of the change. Adjust the
setting of the bias to the proper range, i.e. 275 to 300 millivolts DC.
3. After you’ve completed the bias adjustment changes, it is a good idea
to go back and check the bias measurements for all the other tubes
since changes made to one tube will slightly affect the setting of the
others.
4. Place the IT-85’s protective cage firmly back onto the IT-85 after you
are satisfied that the measurements are stable.
Caution: If your observations do not conform to the
above instructions, or if you do not feel confident
about performing the bias procedures yourself,
please consult with the VTL factory service
department. Improper setting of the bias will affect the current draw of
the output tubes and affect the performance and/or lifetime of the tubes.