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D.W. FEARN
VT-15 Recording Channel
31
6.
MAINTENANCE
The VT-15 is built with only the highest quality parts and will prove to be extremely reliable.
Vacuum tubes and electrolytic capacitors, however, have a finite useful life and must be
replaced eventually.
Top Cover Removal
Removing the top cover allows access to the vacuum tubes, the VU meter calibration, and to
sidechain alignment trimpots. Eighteen 6-32 phillips-head screws must be removed. When
replacing the cover, position it so that the slotted ventilation holes are over the tubes (towards
the back).
Bottom Cover Removal
WARNING! High voltages are exposed when the bottom cover of the VT-15 is removed.
Refer all servicing to the factory or to technicians who are experienced in working on
vacuum tube equipment. Voltages over 400 are used in the VT-15. Contact with these
voltages could result in serious injury or death.
Removing the bottom cover allows access to the VT-15 circuitry, the pulse width modulator
board adjustments, and the filament voltage adjustment. Eighteen 6-32 phillips-head screws
must be removed.
Alignment
The VT-15 compressor is complicated to align and requires test equipment nor normally
found in most studios. If alignment is required, the VT-15 should be returned to the factory,
or contact the factory for detail alignment instructions. Mis-alignment of the VT-15 will result
in very poor (and possibly bizarre) performance. Do not touch any adjustments!
Vacuum Tubes
Four 6072A tubes and two 6N1P tubes are used in the VT-15. If tube replacement is needed,
contact the factory for instructions and tube layout. The 6072A mic preamp input tube and
the compressor preamp tube are quite critical for proper performance. There can be as much
as a 15 dB difference in noise level among an assortment of tubes, and the tubes used in these
stages should be carefully chosen to maintain low noise. Selected low-noise tubes are avail-
able from D. W. Fearn. Other tubes are far less critical and almost any quality off-the-shelf
6N1P will perform satisfactorily. 12AY7 tubes may be substituted for the 6072A tubes, but
with reduced performance.
Tube life is difficult to predict, but it will probably be measured in years. Catastrophic tube
failure is rare with this type of device, but a gradual increase in noise, microphonics, distor-
tion, or a reduction in headroom, should indicate the need for replacement. It is recommend-
ed that you periodically perform a quick noise and distortion check on the VT-15 and com-
pare the results to previous measurements.