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4.1 THEORY OF OPERATION
4.1.1 Audio Amplifier
The Model 2770AV and 2780AV Signal Electronics provide signal processing and control for the entire system. They are divided into four separate circuit boards which are: the Master Audio board, the Bias Oscillator board, Master End of Program board and Copier End of Tape board of which there are two.
The 2780AV does not have a Master End of Program board but rather three End of Tape boards. The Master Audio board in the 2780AV differs slightly from the one used in the 2770AV.
4.1.2 Master Audio and Bias Oscillator Boards
As both channels are functionally identical only channel one will be discussed.
The heads detect the magnetic changes on the tape and feed --them to the preamplifier. Q501 and Q502 form the high gain, direct coupled head preamplifier. Their output is developed across the auto/manual potentiometers.
The auto/manual potentiometers allow for either a predeterrriined or an adjustable level input to the power amplifier stage.
The power amplifier stage contains a predriver stage (Q503) that builds the signal from the auto/manual controls to a higher level.
The driver stage (Q504) provides sufficient level to drive the complimentary output pair of Q505 and Q507 to provide a 3 VRMS level (normal maximum record level) at the input to the bias trap. Q506 provides quiescent biasing for the output pair.
Diode D501 rectifies this signal for application to the VU meter for operator display.
The bias trap serves to prevent the 500 kHz bias frequency from being fed back into the signal electronics, eliminating any possibility of modulation.
At the output side of the bias trap, the audio and bias signals are mixed for application to the copier head.
The Bias Oscillator is a free running multivibrator with a nominal output frequency of 500 kHz. Operation of the circuit is controlled by Q603 that provides Vcc. Q603 is only turned on when a cassette has been loaded in the copy well and the play solenoid for that position is energized.
The Bias Level control determines bias level to the heads as a function of the rear panel selector switch setting. Normal record head bias level is 15 volts rms. The Bias Level control allows this level to vary from 14.0 to 25.OV rms (new issues and 2780) depending upon switch position.
The following operational sequences are provided to afford the reader a better understanding of the automatic functions.
4.1.3.1 Manual Rewind
With the AUTO REWIND switch in the Manual position, +30 vdc from D6 is applied to the anode of D104 on the copier and Master boards, forward biasing the diodes. The level through the forward biased diodes will activate Q103 and hence Q104. With Q104 turned on the Rewind solenoids are activated.
As SW2 is a spring loaded switch it will return so that contacts two and three are connected. Once this occurs the actuating voltage for Q103, Q104 and the Rewind solenoid is removed. However, during contact closure C107 is slightly charged. This charge causes Q103 and Q104 to remain in conduction for about one second. During this period if no information is received by the level detector the solenoid drops out and the system reverts to stop.
If during this one second period, the inserted cassette causes the shutter wheel to rotate, the level detectors charge will remain sufficiently high to hold the solenoid engaged through Q1 03 and Ql 04.
The shutter wheel is mechanically attached to the take-up spindle assembly. With a cassette loaded, rotation of the take-up spindle causes the shutter wheel to rotate. The shutter wheel is physically mounted between two inductively coupled coils. The disc portion of the shutter assembly interrupts this coupling.
The two coils are part of the regenerative feedback path of an oscillator, Q101. The normal output frequency is 320 kHz. When the two coils are exposed by the cut away section of the wheel oscillation is allowed. However, when the wheel covers the two coils oscillation is interrupted. The operation/interruption sequence produces an amplitude modulated waveform with a 320 kHz carrier and a modulation frequency dependent on wheel rotational velocity.
Q102 acts as an AM detector removing the carrier and leaving only the shutter wheel modulation portion. D103 and C107 serve as a level detector for the modulation portion. The resulting DC charge on C 107 is used to keep Q 103 in conduction.
Q1 03 drives Q 104 which drives the Rewind solenoid holding it in. At tape end shutter operation ceases and the charge on C 107 dissipates. At discharge of C 107 both Q 103 and Q104 are turned off dropping out the Rewind solenoid.
Q2 senses current through the Rewind solenoid and turns on, illuminating B4, the start/tape motion lamp. The lamp will remain illuminated until all Rewind solenoids drop out.
The start switch SW9 provides two commands on two lines. One line furnishes +23.5 vdc while the other furnishes a ground. One portion of the switch applies ground to the anode of SCR Q301 on the master endof-program PCD insuring its turn off. The other portion provides +23.5 vdc to Q105 and Q106 on both the master and copier boards.
The +23.5 vdc to Q105 and Q106 of the Master end-of-program PCB will actuate the Play Solenoid provided the "master present" switch SW4 is closed through SW2B The increased current through R9 will cause the Start/Tape motion lamp to illuminate.
As soon as tape motion actually starts the shutter wheel will commence rotation. Through the Q101, Q102, D103 and C107 path Q103 and Q104 will be operated to provide hold in current for the play solenoid.
The circuitry of the Copier EOT board is similar and therefore, will not be discussed.
The area of tile Copier EOT board that does differ from the master EOP board is the warning and record lamp circuitry. The warning lamp circuit illuminates the lamp in event that the cassette being copied in that position stops before the master. Should this occur SW7 will return to its original position causing C108 to discharge through R124, R301, R303 and R304 to ground. The resulting positive voltage across R304 will gate on Q301 and illuminate B5. This condition will continue until the troublesome cassette is removed, and SW8 (master present switch) removes the +23.5 vdc from the lamp.
The Record lamp circuitry consists of Q201, a lamp driver and R201 and R202 a mixing network. This circuitry provides to the operator visual assurance that audio is being processed and applied to the copy record heads.
A bypass stage, Q 107, provides an inhibit function. If a cassette is not loaded in the well, +23.5 vdc is applied to Q 107 keeping it turned on. With Q107 on, ground is applied to QI 05 and Q 106, inhibiting their operation. Since these two stages provide initial play solenoid activation, it would be impossible to operate that position unless a cassette were in the well.
4.1.3.3 Duplicate Mode with Auto Rewind and End of Program
In order to have this capability the EOT/EOP switch must be in the EOP position.
With the switch in the EOP position three changes are brought about. These are:
1. The 320 kHz oscillator is inhibited by an AC short of oscillator feedback circuit, provided by C I I I and C108.
2. Audio from the master cassette channel I is fed to QI01 via R16 the threshold control.The start to copy sequence is identical to that previously described in paragraph 4.1.3.2.
The audio from the master cassette is amplified by QIOI and Q102. The output of which is fed to a level detector.
As long as there is audio present the level from the detector is positive. This level keeps Q 103 and Q 104 turned on and the play solenoid held in.
Should audio cease for longer than 2 seconds the charge on C 107 will dissipate; Q 103 and Q 104 will cut off and the play solenoid will drop out.
With solenoid drop out on the master module SW6 and SW7 are returned to their rewind/stop position. C 108 will discharge through R123 causing Q103/QI04 to conduct and activate the rewind solenoid.
With the rewind solenoid pulled in SW5 is opened and the copier play solenoid has its B+ removed. Thus the copier is pulled out of the play mode.
Upon copier play solenoid dropout, copier SW6 and SW7 return to rewind/stop position.
The copier shutter wheel is still rotating thus Q I 03/Q 104 are still biased to conduct. With the return of SW6 to rest, the rewind solenoid will be activated, and the copier will start its rewinding function.
The stopping of the master and copier at end of rewind have previously been discussed in paragraph 4.1.3. 1.
Two types of power supplies have been used in the 2770AV. Serial numbers below 4000 have the components mounted on terminal boards located in the rear case area. Serial numbers 4001 and above have a power supply board which contain these components.
The primary power is controlled by the main power switch located on the main frame of the 2770AV.
Closure of the switch applies 115 VAC via F-1, a 2.5A Slo Blo fuse, to the power transformer T-1.
T- I steps the 115 VAC down to a 23 VAC in its secondary, and feed a full wave diode bridge rectifier.
The rectifier provides +30 vdc unregulated to a ripple filter and series regulator.
The series regulator Q I drops the +30 vdc to a regulated +23.5 vdc for use by the control circuits and signal electronics.
All solenoid power flows through D6 and R9, the resulting voltage drop (approximately 0.8V with all solenoids on) is applied to Q2's base through R4 turning on Q2 illuminating B4.
4.1.4.2 The 2780AV Power Supply (See Figure 4.4)
The 2780AV is powered by its own power supply. This supply is switched on or off by a relay in the primary circuit of T I.
Relay actuation voltage is supplied by the companion 2770AV through the patch cable. This voltage is provided whenever the 2770AV power is on.
The step down transformer ratio on the 2780AV is identical to that of the 2770AV. The rectifier provides +30 vdc to the ripple filter and series regulator.
Five additional stages are used in the 2780AV supply that are not on the 2770AV supply.
Since Q2, Q3 and Q4 are functionally identical to Q5, Q6 and Q7, only the Q5, Q6 and Q7 chain will be discussed.
The base of Q6 is tied to +27 vdc so long as the rewind selector switch is in the auto or manual position. This level keeps Q6 and hence Q5 turned on.
Q5 furnishes +28 vdc solenoid power to all rewind solenoids in the 2780. If any rewind solenoid is actuated current flow will be sensed by Q7. The turn on of Q7 will cause the start indicator lamp to illuminate thereby indicating tape motion.
The only difference in the operation of Q2, 3 and 4 and Q5, 6 and 7, is the +27 vdc control from the 2770. This control voltage for Q2, 3 and 4 comes from the EOT/EOP switch in the 2770.
1. If the switch is set to EOT the +27 vdc is always present.
If the switch is set to EOP the +27 vdc is only present when the master cassette is not loaded. If the cassette is loaded and the cassette is not being rewound the +27 vdc will be present.
4.1.5.1 The Model 2780AV signal electronics are identical to those of the Model 2770AV with few differences.
We shall cover only the differences.
The electronics boards are identical in that the following boards are used.
1. Master Audio PCB.
2. Bias Oscillator PCB.
3. Copier End of Tape PCB of which there are three as opposed to two in the 2770AV.The Master Audio PCB in the Model 2780AV is similar to that of the Model 2770AV. The basic difference is that the preamplifier stages have been removed.
The 2780 has no manual level adjust only an automatic level adjustment, which is set for unit gain input to output. 5.There are no metering components, high frequency compensation controls or headphone monitoring facilities.
The power amplifier stages are functionally identical to those used in the 2770AV.
The input level from the 2770AV will be approximately 3 VRMS when it is being operated in the auto record level mode. This will vary depending on levelThe auto level controls on the 2780 master audio PCB should be set to produce 3 VRMS at the output of the final power amplifier stage (maximum record level).
The Bias Oscillator PCB is identical to that of the 2770AV. An additional set of components have been added to accommodate the third copier position. Bias frequency and output level are identical to that of the 2770AV.
The operation of the Copier EOP PCB are identical to those used in theThe following is a list of equipment that will be required to perform all the following electrical tests and adjustments.
1 . I kHz test tape (81-00862690-8) 1/2 track tape. 5.4.2.2 Playback Level-Manual
1. Set the REWIND selector in the MANUAL position.a. Follow disassembly sequence as described in paragraph 2-3-2.
b. Adjust the RECORD LEVEL control to a IV reading on the VTVM.
c. Now calibrate the meter adjust pot (R528) to center the needle on the diamond.
7. To verify track two, turn the tape over, remove and re-insert the VTVM from Channel I to MONITOR Output jack, Channel 2.4.2.3 High Frequency
High frequency response is adjusted and set at the factory and no further adjustment should be attempted.
4.2.4 Bias Frequency and Level
1. Bias frequency is not adjustable.
NOTE
Bias level adjustment may only be required after Record head replacement.3. Manually cause the number I copier to operate by moving the slider plate to its duplicate position. The oscilloscope will now display the 490 kHz waveform.
4. The level must be 16V rms, if not adjust C473 to obtain this reading. Step Bias Selector knob through all five positions. Refer to Table I for correct levels.
TABLE I3. Place the AUTO REWIND SELECT switch in the END PROGRAM position.
4. Rotate the Master module shutter wheel so that the metal is centered between coils L101 and Ll 02. Refer to paragraph 3.2.1.
NOTE
The SCR Q301 must be off during this test to prevent a false DC feedback.a. Turn threshold pot (R16) fully clockwise. Set channel 2 Record Level control full counterclockwise but not to Auto.
b. Cause the 2770AV to go into a copy condition and adjust the Channel I Record Level control for 200 mv as indicated on an AC VTVM connected to the Channel 1 monitor jack.
C. Turn R 16 counterclockwise just to the point where the master and both copies go into rewind and cease further counterclockwise rotation of R16.d. Repeatedly go into a copy mode while slowly adjusting R16 clockwise. Continue in a clockwise direction until master and both copies remain in operation. Cease further adjustment of R16.
e. Rotate Channel I RECORD LEVEL control to the number FIVE position.
f. Cause the system to operate and rotate chanel 1 record level control counterclockwise and observe VTVM reading. Master and copiers should not go into rewind as VTVM reading drops to between 275 and 300 mv.4.2.6 Record check
Perform the test ad described in paragarph 2.2.6. Adjust the azimuth screw until you obtain a maximum reading on the VTVM.
4.2.8 Azimuth Adjust - Copier Module 2770AV and 2780AV
The following adjustment procedure is identical for all Copiers. Upon completion of adjusting the azimuth on one Copier, it is necessary to re-align the head of the other Copier(s).
1. Follow disassembly sequence as described in paragraph 2.3.2.
2. Unplug the Master module head cable and the Copier head cable from their sockets.
3. Plug the pin jacks of the adapter cable (Accessory No 81-28196240-7) into J401 and J501 on the audio board. Insert the Copier head cable plug into the adapter sockets.
4.
Remove the wires at J601, J602 and J603. This
"kills" the bias oscillator.
NOTE
To prevent unwanted oscillation, route the adapter cable from J401 and J501
up over the top of the unit and then down to the head cable. Place the record
level control, of the channel not being used, in manual mode at minimum output
level.
5. Insert a 10
kHz azimuth test tape.
6. Connect a VTVM
to the MONITOR Output jack,
Channel 1.
7. Depress the
POWER and START buttons.
8. Adjust the azimuth
screw until you obtain a maxi mum reading on the VTVM.
9. Replace the
wires removed in step 4.
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