Tascam 122 Mk.II - Addendum to Service Manual

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PART 6 - ELECTRICAL CHECKS AND ADJUSTMENTS

6.1. PLAYBACK PERFORMANCE

Initial Settings

DOLBY NR Switch:OUTOUTPUT Switch:REPRO
MPX FIL Switch:OUTOSC Switch:OFF
TAPE Switch:NORMALADJ Switch:OFF

1) Playback Azimuth Connect the Left and Right outputs to a double beam CRO or other means of checking phase between channels. Play Azimuth Test Tape and adjust the "P/B Head Azimuth Screw C" (Fig.6-4-2) for maximum amplitude from both channels and minimum phase difference between them.

2) Playback Output Level

a) Connect an a.c. voltmeter to TP11 /TP12. (Fig 6.2) Play the Standard Level Test Tape and adjust R115/R215 (Fig 6.2) to obtain 245 mV at each Test Point. Note: This sets the level into the Dolby Decoder, and this voltage (245 mV) must be set when replaying the Standard Level Test Tape to be used as the Standard (0dB) regardless of the actual level chosen as Standard. This should be done whether or not it is intended to use Dolby as it sets a standard replay level for the remainder of the adjustments. (Note: The signals on TP 11 and TP 12 fluctuate independently with variations of the signal off tape. If possible check they are close to identical over a period of time by such means as a double beam CRO.)

b) Connect the a.c. Voltmeter to the Left channel output (RCA jack). Play the Standard Level Test Tape and adjust the Output Control (Front panel) to obtain 316 mV. This is the nominal input/output level. (-10 dBV), (-7.8 dBu approx). However, under this condition the meter will not read 0 – ignore this! Leave the Output Control at this setting for the remainder of the checks and adjustments. Adjust R201 to obtain the same level from the Right channel output, (R201 is on the board called VR PCB Assembly in Fig 6.1, Volume PCB Assembly on Circuit Diagram.)

3) Playback Frequency Response. Replay the Frequency Response Test Tape and adjust R116/R216 to obtain the minimum difference between the Reference Frequency (315 Hz, 400Hz, 1 kHz) of the tape being used, and 10 kHz. Check that the response is within the limits of fig. 6.7.

Note 1: Take into account any correction figures for the calibration of the Test Tape.

Note 2: The response at 10 kHz may be set a little lower than at the reference frequency, provided it is still within specification, if this is necessary to avoid levels much above 0dB at the mid frequencies (2 - 6 kHz) or at the high frequencies (12.5 - 20 kHz).

6.2 MONITOR PERFORMANCE

Initial Settings

DOLBY NR Switch:OUTOUTPUT Switch:INPUT
MPX FIL Switch:OUTOSC Switch:OFF
TAPE Switch:NORMALADJ Switch:OFF
INPUT Switch:REARINPUT SELECT Switch (Rear):RCA IN

4) Minimum Input Level. Connect Audio Oscillator to the INPUT connectors (Rear, RCA). Apply 126 mV and check that an output of 224 to 447 mV can be obtained.

5) Nominal Input Level. Apply 316 mV at the reference frequency. Adjust the Input Controls (Front Panel) to obtain 316 mV at the outputs. Check that the same level is obtained at both channel outputs. Leave the Input Controls at this setting for the remainder of the checks and adjustments. Note. Although the two Input Controls are ganged together by a belt, they can move relative to each other if sufficient force is applied. It may be necessary to adjust their ganging to satisfy the above test.

6) Meter Level. With 316 mV applied to the Inputs, adjust R106/R206 (Fig 6.1) so that the meters read 0 VU. (NB. These are the R106/R206 on the Meter PCB Assembly, NOT the ones on the Mother Board!)

Note: When the OUTPUT switch is switched to “Input”, the meter is actually switched to the monitor output of the Dolby Decoder, which is before the “OUTPUT” control. When the OUTPUT switch is switched to “Repro”, the meter is switched to the actual output. Therefore, the meter does not always read what one would expect! Follow the procedure given here and the levels will work out all right, and when operating in the “Adjustment” mode, the “Input” and “Repro” levels will read 0dB for the correct level in and out.

7) Internal Oscillator.

a) Remove Input signal. Switch the OSC switch on. Check that the VU meters read 0 VU ± 2 VU.

b) With the OSC switch on, operate the 400Hz/10kHz switch. Adjust R325 (Fig 6.1) for minimum difference between readings. (Remove bottom cover for improved access to R325).

8) Phones Output Level. With 316 mV applied to the Input (Rear, RCA), check that a level greater than 891 mV (-1 dBV) can be obtained for each channel of the Phones output (with specified load).

9) Bias Osc. Frequency. Place Deck in Rec mode. With no Input signal, adjust U126 (Fig 6.1) to obtain 100 kHz at pin 2 of Connector P103 (Fig 6.1). Do not remove P103 from socket during this adjustment.

10) Bias Trap. Connect CRO or meter to TP13/TP23 (Fig 6.3). Place Deck in Rec/Pause mode. Adjust L154/L252 (Fig 6.3) for minimum bias leakage.

11) Front Input. Place Deck in Rec/Pause mode. Apply 316 mV to Input (Front, RCA). Check that 316 mV ± 1 dB (355 to 282 mV) is obtained at the output (Rear, RCA).

6.3 RECORDING PERFORMANCE

Initial Settings

DOLBY NR Switch:OUTOUTPUT Switch:REPRO
MPX FIL Switch:OUTOSC Switch:OFF
TAPE Switch:NORMALADJ Switch:OFF
INPUT Switch:REARINPUT SELECT Switch (Rear):RCA IN

Note 1: For GRN use it is only necessary to set up the recorder for "Normal" tape. Use a tape of the type for which it is desired to set up the recorder, and ensure that the TAPE switch is always in the "NORMAL" position. If for some special application it is necessary to use CrO2 or Metal tape, carry out this procedure with the appropriate TAPE switch operated, using the appropriate tape, and the corresponding adjustment points.

Note 2: Although in GRN the recorder will normally be used with the ADJ switch "On", the adjustments must be done in accordance with this procedure ( ADJ Switch "OFF") and then the ADJ switch set to "On" because the Record Equalization is in operation regardless of whether the ADJ switch is "On" or "Off".

12) Bias 1: Not applicable - Ignore!

13) Record Azimuth: Check that the Playback adjustment is correct before proceeding with this adjustment. Connect Audio Oscillator to the INPUT connectors (Rear, RCA). Apply a 6.3 kHz signal in the range 32 - 56 mV (15 - 20 dB below 316 mV) so that it is not affected by any limitation on the level, which can be recorded at high frequencies. Connect the Left and Right outputs to a double beam CRO or other means of checking phase between the channels. Adjust the "REC Head Azimuth Screw D" (Fig 6.4.2) for maximum amplitude from both channels and minimum phase difference between them.

14) Record Bias. Connect a suitable measuring instrument to the output. Apply a 6.3 kHz signal in the range 32 - 56 mV (15 - 20 dB below 316 mV). Start recording and set R101/R201 (on the motherboard Fig 6.1) to the extreme Counter Clockwise setting. Carefully adjust them Clockwise past the point of maximum signal output to the point where the output is 4 dB below the maximum value.

15) Record Level. Apply a 316 mV at the reference level to the input. Adjust R107/R207 (Fig 6.1) so that the output is 316 mV. i.e., the Recorder is recording at 0 dB Reference level.)

16) Record Equalization. Apply a 1 kHz signal to the input in the range 32 - 56 mV (15 - 20 dB below 316 mV). Start recording and adjust R104/R204 (Fig 6.1) so that the level of the output, when the input frequency is changed to 10 kHz, is the same as the output for 1kHz. Record and replay a range of frequencies at this level to determine the Overall Frequency Response.

Note: This assumes that the replay characteristic is flat. If not, the Overall Frequency Response should be adjusted, as far as possible, to take account of the extent to which the replay characteristic departs from flat. This will give a record characteristic which is as flat as possible, which will give the best interchangeability between recorders.

17) Compliance with Specification. Check the remainder of the Characteristics listed in the specification to ensure the Recorder is operating satisfactorily.

Note: In the case of the overall frequency response it is more important to achieve a reasonably flat frequency response without excessive peaks in the range up to 10 k Hz than to be within the 4 dB tolerance to 16 kHz. If necessary, sacrifice an extended frequency response in order to achieve the smoothest response up to 10 kHz, with a falling response at higher frequencies.

OPERATION

Standard GRN practice is to use the Tascam 122 Mk. II in the "ADJ" mode. This allows the recordist to optimise the recording characteristics for each individual cassette. First, the recorder must be set up as explained above. Then the following Operational procedure should be used.

Initial Settings

POWER:ONINPUT LEVELL & R BOTH "0"

OUTPUT LEVEL

:8OUTPUT Switch:INPUT

MPX FIL Switch

:OUTDOLBY NR Switch:OUT

TAPE Switch

:NORMALADJ Switch:OFF

1) Put the deck in the Record/Pause mode by pressing the Record and Pause buttons.

2) Press the OSC button and set the signal to 400 Hz, then press the ADJ switch.

3) Set the INPUT level controls so that the VU meters read 0VU

4) Start recording by pressing the PLAY button.

5) Set the Output selector to REPRO.

6) Turn the BIAS trimmers fully Counterclockwise

7) Slowly turn the BIAS trimmers Clockwise so that the VU meter needles deflect to the peak value, adjust one at a time.

8) Turn the REC LEVEL trimmers so that the VU meters read 0VU.

9) While recording, alternate the oscillator signal between 10 kHz and 400 Hz, and adjust the BIAS trimmers by rotating Clockwise so that the VU meters show the same level at both frequencies.

10) Set the oscillator signal back to 400 Hz and adjust the REC Level trimmers so that the VU meters read 0VU.

11) Release the OSC button.

Note 1: This procedure should be carried out for each cassette to be recorded, even though they are of the same type.

Note 2: If a different type of tape is used, it may be necessary to readjust BIAS and possibly RECORD

EQUALISATION

As described above.

CIRCUIT OPERATION

OPERATION OF SOLID STATE SWITCHES

1) "Digital Transistor RT1N241S" (e.g. U138) Ref. P45 of Tascam Service Manual
"Digital Transistor RT1P241S" (e.g. U139) Ref. P45 of Tascam Service Manual

Tascam call these “Digital transistors”, i.e. switches. They are used extensively in control functions.

Consider Operation of S101 -"OUTPUT - Repro/Input Switch", on the Monitor Switch PCB Assy. The moving contact on the RH side as drawn on the circuit is connected to earth via pin 4 of J102/P106. The contact which is made in the "Input" position is connected via R101 and J105 to LED D105, the other side of which goes, via J105 and pin 2 of J102/P106 to +5V, which causes the LED to light.

The moving contact on the LH side is connected to +7.5 V via pin 3 of J102/P106. The contact which is made in the "Input" position connects this voltage to U138 via pin 1 of J102/P106. The + 7.5 V is also connected to U137 via D117. These are both RT1N241S "Digital Transistors" i.e., switches. (See above). Consider the operation of U138. The emitter stays on - 6.8 V (nominally -7.5). The base goes from - 6.8V to + 6.8 V when the (nominal) + 7.5 V is switched to it by the switch going from "Repro" to "Input". The collector goes from + 6.9 V to - 6.8 V. In the case of U137, the change on the base is from -6.8 V to + 5.7 V because of the drop across the junction of diode D117, but the collector still goes to -6.8 V. Hence the outputs of both U137 and U138 change state when Switch S101 moves from the "Repro" position to the "Input" position

U138 controls pins 6 and 5 on U119, causing the contact between 8 & 9 and 3 & 4 to close when Switch S101 moves from the "Repro" position to the "Input" position. Similarly, U137 causes the contact between 8 & 9 and 3 & 4 to close when Switch S101 moves from the "Repro" position to the "Input" position. However, U138 also controls U140, and U137 controls U139. As these are "P" rather than "N" devices, they will invert the switching signal, and so when U138 causes U119 8 & 9 to close, U140 will cause U119 10 & 11 to open. The base of U137 is also connected via D118, pin 8 of P106/J102, pin 1 of P106 (on Monitor switch PCB Assy.) to Adjustment Switch PCB Assy., to + 12 V from the "ADJ" switch when it is operated. This causes U137 to also operate when the "ADJ" switch is operated, but not U138.

2) Solid State Switch LC4066B. (e.g. U119) Ref. P45 of Tascam Service Manual.

These take the form of a 14 pin DIL package which contains 4 solid-state switches. Tascam uses them to switch audio connections. A positive voltage on the control pin (i.e. pin 6) causes the connection (in this case between pins 9 & 8) to be an open circuit. Application of a negative voltage to the control pin causes a low resistance path (typically 50 – 60 ohms) between the other two pins.

3) Operation of U119 and U120 Solid State Switches.

OUTPUT
Switched TO:
U119U120
Pins 8 to 9Pins 10 to 11Pins 8 to 9Pins 10 to 11
REPRO{"ADJ In"ONOFFOFFON
{"ADJ Out"ONOFFONOFF
 
INPUT{"ADJ In"OFFONOFFON
{"ADJ Out"OFFONOFFON

SIGNAL PATHS

A. Summary:

1) "ADJ" OUT

a) Output switched "REPRO” Meter reads level after “Output” control, as it is connected to Output. Output Signal is off tape, "Output" control in circuit. Output of Replay head, through Dolby, through U119 (8-9) (Switch), output control, U125 (Amp) to RCA output. From U125 (amp) output via U124 and U123 to the XLR OUTPUT (Balanced output).

b) Output switched "INPUT" Meter reads input signal via Input level control and monitor output of Dolby encoder, not affected by Output Control. Output is monitor output of Dolby encoder, via "Output" control, U125 (Amp) to RCA output. From U125 (amp) output via U124 and U123 to the XLR OUTPUT (Balanced output). Output signal is not what goes to Meter.

2) "ADJ" IN (Operation in "Adjustment" mode)

a) Output switched 'REPRO" Meter reads signal off tape before Output level control via U119 (8-9) and U120 (11 - 10). Output signal is via Output control, not what goes to meter. Output of Replay head, through Dolby, through U119 (8-9) (Switch), output control, U125 (Amp) to RCA output. Balanced output via U124 and U123 to the XLR OUTPUT.

b) Output switched "INPUT" Meter reads input signal via Input level control and monitor output of Dolby encoder. Not affected by Output Control. Output is via Output control U 125(Amp) to RCA output. Balanced output via U124 and U123 to the XLR OUTPUT

c) OSC switched IN Operating the "OSC" switch disconnects the input from U122 and instead connects the built-in oscillator, which provides the "input" for setting up the "Adjustment" mode of recording. The "OSC" switch also adjusts the sensitivity of the meter circuit to match the output of the Oscillator (approx - 20 dB)

B. Details of Signal Paths.

Note: In some places connections appear to be reversed L to R on circuit drawings. Also, there is a lack of consistency in the numbering of plugs and sockets on the boards. Most of the detail is for Left channel only, details of the Right channel have to be inferred.

1) Input Signal.

a) RCA input to pins 7&8 on P113 on Mother board, into Bal Amp PCB Assy, through RCA/XLR switch, back to Mother board via pins 4&5 of P113.

b) XLR input via P101/P201into Bal Amp PCB Assy, through RCA/XLR switch, into Mother board via pins 4&5 of P113

c) Input Jack on front panel connects via Headphone PCB Assy and pins 1&3 of P111 to Motherboard, then via pins 5&11 of P106 and J102 to Rear/Front Switch.

d) From pins 4&5 of P113, the rear panel input goes to Monitor Switch PCB Assy via pins 7&9 of P106 and J102, to Rear/Front Switch. From rear/front switch, whichever input is selected goes to Adjust PCB Assy via pins 5&3 of P106? (Presumably a different P106 - this one on Monitor Switch PCB? Apparently mating with P101) Then to Adjustment Switch PCB Assy via unnamed connectors, apparently pins 3&2 of "top" connector on circuit drawing. These go to normally made contacts of "OSC" switch. Corresponding levers go back to pins 4&1 of "top" connector, then 4&2 of P101 on Monitor Switch PCB Assy, then 6&10 of J102 &P106 to Mother board. Pin 6 goes to input of U122 (an amplifier). Pin 10 presumably goes to a corresponding Right channel amplifier, presumably U222.

e) The output of U122 goes via C152 to pin 1 of P108, to contact "L3" of Volume PCB Assy, then to high end of R103 (Left Channel Input Level Control). Moving contact of R103 goes to "L2", then to pin 2 of P108, back to the Mother board to pin 3 of P120 and J154 ("L line in") of Record Amp PCB Assy. Then to pin 3 on U156, the Dolby encoder.

f) From pin 21 of U156, the “Encode out” goes via C174 to pin 1 of J154 and P120 back to the Mother board, then to pin 5 of P101, to pin 3 of 3pin connector on Adjustment VR PCB Assy, to top of R303, "L Level" adjustment. Moving arm of the pot. goes to pin 4 of P301 and J301 into Joint PCB Assy. From there via pin 17 of J302 and P302, into Adjustment Switch PCB Assy, to the normally open contact of the "ADJ IN" switch. The normally made contact comes from a fixed resistive divider across R303 via pin 3 of P301 and J301 via pin 18 of J302 and P302. Lever of "ADJ IN" switch goes to pin 1 of "lower" socket on Adjustment Switch PCB Assy, shown on the circuit diagram as going to pin 2 of P101 on the Motherboard. However, this is labelled "R", so it perhaps should be pin 3? The pair of pins on P119 and J153 labelled "Rec Level (L)" and the pair labelled "Rec EQ" are connected to one half of Op ampU152. Also connected are three level adjustments, one for each tape type and three EQ adjustments, one for each tape type. These are selected by solid-state switches in response to the logic circuits and front panel switches.

g) The signal then goes to the time constant networks where solid-state switches controlled by the logic circuits apply the appropriate characteristic together with U151. The signal then passes thru the bias trap, has the bias added, and goes to the Record Head. The bias is generated in, and the level controlled by, U160, in accordance with the “HX PRO” characteristic. The nominal setup level for various tape types, and for operating in the “ADJ” mode, are set by d.c.voltages applied to U160.

2) Monitoring of Record (Input) Signal

a) Pin 2 of P120 and J154 where "Record Amp PCB Assy" connects to the Motherboard is labelled "Monitor L". Pin 8 is labelled "Monitor R". Presumably these are the Monitor outputs from the Dolby encoder. (See Block Diagram).

b) Pin 2 of P120 connects to pin 11 of U119 (solid-state switch) which is normally open. When closed ("OUTPUT" switch to "INPUT") the signal appears on pin 10. From there it splits, one path going to pin 3 of P112, into the headphone amplifier. The other path goes to pin 4 of P109, to "L3" of the Volume PCB Assy, to the top of R102 ("OUTPUT LEVEL" Control). The moving arm of this control goes to "L2" and then to pin 1 of P109, then to U125 (amplifier) via C115. The output of U1splits. One path goes to pin 8 of U120 (solid state switch). It goes no further under this condition, U120 only being operated for "OUTPUT" switch to "REPRO". The other path goes via C116 and R142 to the RCA OUTPUT. It also goes to pin 7 of U124 via C111. U124 and U123 provide a balanced output and drive the XLR OUTPUT.

3) Replay of Tape (REPRO)

a) Signal from Replay head enters the "Play Amp PCB Assy" via pin 1 of P101. It passes through a pre-amplifier consisting of Q101, Q102, 1/2 U101. Cuing/ muting/ time constant adjustments are provided by Q103, Q104 which are switched by the logic circuits. Low Pass Filtering is provided by U104. R115 adjusts the level (measured at TP11 - pin 11 of U102) and R116 the frequency response (mid./high frequencies). The signal then goes via C110 to the Dolby decoder U102. The output is taken from pin 9 of U102 via C111 and pin 1 of J103 and P122 into the Motherboard. Other control lines switch "Dolby In/Out" and "Dolby B/C".

b) From pin 1 of P122 it goes to pin 8 of U119 (solid-state switch) which is operated for "OUTPUT" switch to "REPRO". The signal comes out on pin 9. There is a connection to pin 10 of U119 and pin 11 of U120, but both these switches are open for this condition. It then splits, one path goes to pin 4 of P109. This goes to "L3" of the "Volume PCB Assy" and then to the top of R102 ("OUTPUT" level Control) via R101. The moving contact of R102 goes to "L2" of the "Volume PCB Assy" and back to pin 1 of P109 on the Motherboard. It then goes via C115 to U125 (Amplifier). The output of U125 goes via C116 and R142 to the RCA OUTPUT. It also goes to pin 7 of U124 via C111. U124 and U123 provide a balanced output and drive the XLR OUTPUT. As well, the output of U125 goes to pin 8 of U120 which is operated for "OUTPUT" switch to "REPRO" so signal comes out on pin 9 of U120 and goes to "Meter PCB Assy" via pin 7 of P110.

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