WEBSTER CHICAGO 79 - Page 34

WEBSTER CHICAGO
BULLETIN 79P038
MODEL 79 AND MODEL 80
Amplifier Check No. 3 — With output switch in
No. 3 position, volume and tone controls fully ad-
vanced, and signal applied as in above, the out-
put voltage shall be 6 Volts minimum, undistorted.
If greater than 6 Volts, but distorted, the volume
control may be retarded until the output wave is
undistorted, at this new setting the output voltage
should be at least 6 Volts.
Controls — Check all controls for correct operation
and noise.
Hum and Noise Voltage — With the grid of the
6SJ7 tube grounded, the tone and volume controls
fully advanced, the output switch in position No.
2, the hum and noise voltage shall be . 010 Volts
maximum measured at the output jack. With the
output switch in position No. 3, the hum and noise
voltage should be . 060 Volts maximum.
RECORD- LISTEN SWITCH IN
RECORD POSITION
Output switch in No. 3 position, tone and volume
controls fully advanced, . 01 volts at 1000 cycles is
fed into the first amplifier stage thru the input re-
ceptacle. The output voltage at the output jack
should be 7.0 volts minimum, undistorted. If the
output voltage is greater than 6.0 volts, but dis-
torted, the volume control setting may be retarded
until the output wave shape is undistorted. The
amplifier will be considered to be alright if, at this
new setting, the output voltage is at least 6.0
volts.
The erase frequency should be 35 to 45 kilocycles.
The erase voltage should be 4.8 to 5.2 volts.
erase current should be . 9 to 1.0 amperes.
(Be sure to use an R.F. Ammeter).
The
R.F.
Volume control setting is retarded to a position
such that exactly seven volts are measured with
all other conditions identical to those of test above
and a 1000 cycle note recorded on wire.
RECORD- LISTEN SWITCH IN
LISTEN POSITION
The wire is rewound and the recording made
above is played back and checked according to
the following:
With output switch in position 1, the 1000 cycle
note is listened to for excessive " Wow -.
With output switch in position 2, volume and tone
controls fully advanced, the 1000 cycle voltage
across an external 3.2 Ohm load shall be 1.2 volts
mimimum, undistorted. If the output voltage is
greater than 1.2 volts, but distorted, the volume
control setting may be retarded until the output
wave shape is undistorted. At this new setting of
the volume control the output voltage should be
at least 1.2 volts.
With output switch in position 1, recorded speech
and music is listened to for audible distortion,
speaker rattle, microphonic tubes, excessive noise
and hum.
ABILITY TO ERASE
With the record- listen switch in the record posi-
tion, a portion of the recorded wire is run thru
in the forward direction with the volume control
in the extreme counterclockwise position. This
portion is then rewound and played back for
check of adequate erasing.
31