selection of either left, right, or both outputs is made by the AUDIO MONITOR switch, S2, on the IS-4
board. It then goes to the MONITOR OUT and TV CONNECTOR.
Broadcast television is monoaural. When the input select switch is set to TV, you use only one
audio channel, Ch-2 (r). Since the two audio circuits are identical in most sections, the description that
follows deals with the right channel.
Playback signal from the audio head feeds through the normally closed contacts of RY-1 to IC-2
(PB Amp). A bias trap made up of L1 and C23 removes the AC BIAS (70 KHz) after equalization of
the signal using R42. The audio signal then goes across the emitter of Q10 (muting) and collector of Q9
(20 KHz trap) to R47 (PB level). The signal then routes through the bias trap, L3 and C26 to IC-3 amp.
The output of IC-3 splits into several paths, one to the line out jack on the rear of the machine, to
R68 and through Q16 to the audio monitor switch and the audio meter. The headphone jack provides
two output levels selectable with the phone level switch. Muting for the audio playback comes from the
system control board to Q11 during threading and unthreading.
Audio may be recorded from the TV, MIC, or LINE IN connectors. TV audio goes past Q1,
which turns on only when TV is not recording, to Q3. The output of Q3 goes through the audio level
control on the front panel to pin 2 of IC-3. The output of IC-3 follows the same path as the playback
audio. In addition the output of R68 goes to the record amplifier, Q13, past Q13, with no muting in the
record mode, to the audio head for recording on the tape. C52 L5 make up a bias trap to prevent the 70
kHz AC bias from feeding back into the record circuits.
The power supply (PW-1A) will accept 100V or 120V, 50 or 60 Hz. It supplies all regulated and
unregulated voltages required for proper operation of the machine. There are three regulated voltages
and three unregulated voltages. Only the regulated +6.5, +12, and 24 volts lines are adjustable.
Since all the regulators operate basically the same, we will discuss only the regulated +12V
supply. The 18 VAC enters the power board at pin 7. D4 and D5 rectify this voltage to provide a DC
voltage. The rectified and filtered voltage goes to Q30005. The output of Q30005 at the emitter goes to
pin 1-5 of CN-2.
Q5 (error detector), Q4 and Q30005 (series regulator) accomplish regulation. When the
regulator +12 volts increase because of a change in load, the voltage at the base of Q5 also increases
because of the voltage divider network. Q5 will conduct more, causing its collector voltage to decrease,
thereby decreasing the emitter current and the forward bias of Q4. Q4 conducts less, decreasing the
emitter current and the forward bias on Q30005. Q30005 conducts less, increasing its internal
resistance. When the resistance increases, the voltage at the output will return to +12 VDC. The three
stages will operate in the opposite manner if the output should decrease.