Revox B77 Service Manual

REVOX service

6.4.3. Gain of input amplifier

Adjust operating controls as per fig.6.4.- 1.

Connect audio voltmeter to OUTPUT 51 (pot-meters LEVEL [50] fully open).
Connect audio generator to feed the sockets AUX INPUT [48] in parallel. Select

frequency of 1000 Hz and adjust generator level until a reading of 0.775 V is
obtained on the audio vol tmeter .

The required input level should read 20 mV approx.
Note:

At all positions of the mode selector (MONITOR 17) the difference in output level
must not exceed 0.5 dB.

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6.4.4. Testing the mixing facility with a single

Adjust operating controls as per fig. 6.4.-2.

Audio generator adjusted as for previous test ( 1000 Hz. 0.775 V output level) and
connected to AUX INPUT [48]

Both VU-meters indicate 0 VU.

Switch record pre-selector REC R [22] to ON.
Only the left-hand VU-meter indicates 0 VU.

Turn potentiometer INPUT LEVEL LEFT fully counterclockwise to position "0".
Turn potentiometer INPUT LEVEL RIGHT fully clockwise to position "10"
Only the right-hand VU-meter will read 0 VU.

Switch record pre-selector REC L to position OFF.
Both VU-meters indicate 0 VU

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6.4.5. VU-meter calibration

Set operating controls as per fig 6.4 -3.

Audio generator still adjusted to same level and frequency (1000 Hz. 0.775 V on
output [51] )

Adjust potentiometers METER CAL L and R (fig. 6.4.-4) to obtain a 0 VU
deflection on the respective VU-meter.

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6.4.6. Checking the overl oa d indicators

When raising the signal level by more than 6 dB, the overload indicators in the VU­meters must become illuminated.

The difference in triggering level between the two indicators must not exceed

0.5dB.

When exceeding this tolerance. alter the values of resistors R37 or R34
respectively in the monitor amplifier 1.177 260.

On newer versions, resistors R34/R37 are replaced by additional trim-pots R30.
R38

6.4.7. Frequency response "be f ore tape"

Set operating controls as per fig 6.4.-3.

Connect audio generator to both AUX INPUTS [51] in parall el .
Connect audio millivolt meter to OUTPUT [48]

Sweep generator from 30 Hz to 20 000 Hz and check frequency response by
observing the level variation on the audio mill i volt meter.

Permissible tolerance+ 0/-1 dB.

Check frequency respons e on OU TPU T L and R.

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6.4.8. Reproducing head azimut h, coarse adjustment
(Level maximum)

Double check:
Check to make sure that the head gap is centered inside the wear pattern on the

head.

With grease pencil make horizontal mark on the head face, run tape for several
seconds.

Then check position of the head gap inside the polished area. lf necessary adjust
head position to bring gap into center of wear area.

Set operating controls as per fig. 6.4.-5.
Connect audio millivolt meter to OUTPUT L.

Load recorder with calibration tape 19 cm/s and run azimuth adjusting secti on
(10 kHz) by pressing button PLAY.

Rotate screw “P” to obtain maximum output voltage.

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6.4.9. Reproducing head azimut h, f ine a djustment

(Phase check method)

When using the phase check method for align ment of the head azimuth, it is
essential that the azimuth is first adjusted for level maximum (section 6.4.8.) to
avoid the possibility of a phase error in excess of 90°

Switch mode selector MONITOR to position MONO.
Connect audio millivolt meter to both outputs OUTPUT L + R @ in parallel.

Load recorder with calibration tape 19 cm/s and run azimuth section
(10 kHz) while rotating screw “P” until a pronounced maximum in output voltage is

obtained.

6.4.10. Reference level from calibration tape

(REVOX test tape no. 46001)

Set operating controls as per fig. 6.4.-6.

Connect audio millivolt meter (1 V range) to OUTPUT L .
Search for reference level 257 nWb/m on the calibration tape and run that

section by pressing the button PLAY.

Adjust trim-pot REPR LEVEL L (fig.6.4.-7) to obtain an output level of 0.775 V.

Connect audio millivolt meter to OUTPUT R.
Adjust trim-pot REPR LEVEL R to obtain an output level of 0.775 V

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6.4.11. Reproducing system frequency response

(measured with calibration tape)

Set operating controls as per fig. 6.4.-6.

Search for the beginning of the frequency response section on the calibration
tape.

Connect audio millivolt meter to OUTPUT L + R in parallel.
Start recorder in PLAY and measure frequency response relative to 1000 Hz.

The frequency response of both channels may be checked in one run by alternating
the setting of the MONITOR mode selector between the positions LEFT and RIGHT

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6.4.12. Checking oscillator frequency and RF voltages

Set operating controls as per fig.6.4.-8.

Load recorder with blank tape (REVOX 631) and start in the recording mode.

Erase voltage:
Use electronic 'Voltmeter to measure the RF-voltage on the erase head:

approx. value: 30 V ... 32 V

Oscillator frequency:
Measure the oscillator frequency on the erase head with a digital counter.

The frequency must read 150 kHz ± 5 kHz.

When measuring with an oscilloscope, the display must consist of 15 cycles on the
100 µsec time base.

If the frequency deviates beyond the above limits. adjust the slug in Tl on the
oscillator board .

For this adjustment, the extension board 1.177.241 is required in order to gain
access to the coil.

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6.4.13. Adjustment of bias trap in recording amplifier

Set operating controls as per fig. 6.4.-8.

Connect millivolt meter to the test point “X” of the respective recording channel
fig.6.4.-9 Refer to circuit diagram 1.177.230.

Load recorder with blank tape and start in the recording mode.
Adjust the controls BIAS TRAP L + R to obtain a minimum voltage indication

(<350 mV). fig 6.4.-10.

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6.4.14. Recording head azim ut h c oar s e adj us t m e nt

(Level maximum)

Set operating controls as per fig. 6.4.-11.

Connect audio millivolt meter to OUTPUT [51] and select 100 mV range.
Connect audio generator in parallel to AUX INPUTs . Select frequency of 10 kH z.

Load recorder with blank tape (REVOX 631 ) and start in the recording mode.

Adjust generator level to obtain an out put r ea di ng of 0.775 V.

Rotate screw R (fig. 6.4.-12) to obtain maximum output level. Because there is a
time lag before the effects of any alteration in record head azimuth will be

shown on the meter, screw “R” should be rotated very slowly.

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6.4.15.Recording head azimuth fine adjustment

(Phase check method)

In order to avoid the possibility of phase errors in excess of 90° , it is essential that
the azimuth adjustment as described under section 6.4.14. is first completed.

Turn mode selector MONITOR to position MONO.
Connect audio millivolt meter to both sockets OUTPUT in parallel.

Connect audio generator to AUX INPUT in parallel.
Select frequency of 10 kHz and adjust to a level of 20 dB below 0VU.

Load recorder with blank tape (REVOX 631 ) and start in the recording mode.
Adjust screw R (fig. 6.4.-12) until a sharply rising. well defined maximum in output

voltage is obtained.

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6.4.16. Bias adjustment

Since tape magnetization varies with bias current, a sharply defined maximum
output is exhibited at high frequencies when gradually increasing the bias current
from zero upwards.

This relationship may be utilized to determine the optimum operating point
(6 4 -13)

Set operating controls as per fig 6.4.-14.

Connect audio millivolt meter to OUTPUT L or R respectively .
Connect audio generator to AUX IN- PUTs in parallel.
Select frequency of 10 kHz and adjust to a level of 20 dB below 0 VU

Load recorder with blank tape (REVOX 631 ) and stat in the recording mode

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The potentiometers:

BIAS ADJ LEFT (SLOW+ FAST)
BIAS ADJ RIGHT (SLOW+ FAST)

Are to be adjusted at their respective tape speeds by turning them from their fully
counter clock wise position in clockwise direction until the signal level, as
measured on the recorder's output, reaches its maximum (mode selector
switched to either LEFT or RIGHT as required)

After having reached the maximum, slowly keep on rotating the respective trim-pot
in clockwise direction until the 10 kHz output signal has dropped by the amount
shown in table 6.4 -15

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6.4.17. Recording level

Set operating controls as per fig. 6.4.-16.

The levels of the reproducing system must already be adjusted as per section

6.4.10.

Connect audio generator in parallel to AUX INPUTS
Connect audio millivolt meter to OUTPUT
Load recorder with blank tape and start in the recording mode.

Select frequency of 1000 Hz on audio generator and adjust its level until a reading
of 0.775 V is obtained on the audio millivolt meter .

Switch MONITOR selector to position TAPE.
Adjust the trim-pots REC LEVEL, LEFT or RIGHT respectively to obtain an output

level of 0.775 volts.
Turn mode selector to either LEFT or RIGHT as required.

Double check:
There must be no level difference when switching the monitor selector between the

positions TAPE and INPUT.

At the tape speed of 9,5 cm/s, a level difference of 1 dB is permissible

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6.4.18. Adjusting the recording equal ization

Set operating controls as per fig. 6.4.-17.

Connect audio millivolt meter to OUTPUT
Connect audio generator in parallel to both AUX INPUTS

Signal condition:
12 kHz, level 20 dB below 0 VU

Load recorder with blank tape and start in the recording mode.

While recording the 12 kHz signal, adjust the trim-pots EQ SLOW. FAST at the
respective tape speeds until the signal level at the recorder's output falls within the
range from 0 dB to + 1 dB relative to 1 kHz (77,5 mV / -20dB).

Turn mode selector to position RIGHT and adjust equalization for the right channel
as well.

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6.4.19. Frequenc y respons e "overall"

Set operating controls as per fig. 6.4.-18.

Output level of audio generator remains set as for previous test
(20 dB below 0 VU).
Load recorder with blank tape and start in the recording mode.

Calibrate audio millivolt meter for frequency response measurement by adjusting
the audio generator level at 1 kHz to obtain a convenient mid-scale deflection on
the meter (e.g. -5dB on the 100 mV range).

Check frequency response at the two tape speeds. Both channels may be checked
in one run by alternating the mode selector between the positions LEFT and RIGHT
at each individual frequency .

For permissible response deviations check section 2. Techni cal Specifications
of this manual.

Frequency Response:

(measured via tape, at -20 VU)

At 9.5 cm/s 30 Hz ... 16 kHz + 2/-3 dB

50Hz ... 10kHz ±1,5dB

At 19 cm/s 30 Hz ... 20 kHz + 2/-3 dB

50 Hz ... 15 kHz ± 1,5 dB

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6.5.1. Distorsion via tape

Set operating controls as per fig. 6.5.-1. Connect audio generator to AUX INPUT
Frequency: 500 Hz
Level: 0VU + 6 dB

Connect distortion factor meter to OUTPUT
Load recorder with blank tape and start in the recording mode.

Measure distortion on both channels. Switch MONITOR mode selector between
positions LEFT and RIGHT as required.

The permissible amount of distortion (pure third harmonic) at the respective
tape speeds may be seen from table 6.5.-2.

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6.5.2. Weighted and unweighted noise m easurements via tape

Noise voltage measurements are taken with reference to the peak level
modulation of 514 nWb/m (output level 1.55 V).

In order to avoid erroneous test results due t o bias frequency pick-up, al l
measurements are taken by running tape. which has been erased on the recorder
in a previous run.

Set operating elements as per fig. 6.5.-3.

Connect audio millivolt meter to OUTPUT L .
With previously erased tape on the recorder, start the machine in PLAY.
Read noise voltage of the left channel through ASA A weighting network.

Switch mode selector to position RIGHT.
Read noise voltage of the right channel through ASA A weighting network

Measure noise voltage at the second tape speed as described above The RMS
signal to noise ratios (weighted as per ASA A) can be seen from table 6.5.-4.

If the signal to noise ratios are less than those shown in the table, carefully
demagnetize the heads and tape guides and repeat the test

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For the following measurements

- Erase depth

- Crosstalk MONO

- Crosstalk STE EO

An audio millivolt meter with a narrow band-pass filter of less than 100 Hz
bandwidth (wave analyzer) will be required.

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6.5.3. Erase efficiency

To measure the erase depth, a sig nal of appr oximately 1000 Hz has to be recorded
at peak level and that same recording has to be erased again in a subsequent run.
The residual level of that signal will then be measured.

Set operating controls as per fig 6.5 -5.

Connect wave analyzer to OUTPUT
Connect audio generator in parallel to both AUX INPUTS

Select 1000 Hz and adjust level to obtain peak level indication on the VU-meter
( 1000 Hz. 0 VU+ 6 dB) .

Load recorder with blank tape and start in the recording mode.

Establish reference reading on wave analyzer.

STOP and rewind to the beginning of the recording.
Close fader INPUT LEVEL LEFT to position "0".

Start the machine again in the recording mode and measure the residual signal level
on tape.

The magnetic recording remaining on tape must be attenuated by at least 75 dB
below the above established reference (typical value 80 dB).

Switch mode selector MONITOR to position RIGHT and repeat the above
measurements.

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6.5.4. Crosstalk MONO

It is advisable le to use virgin (or bulk erased) tape for this test.
Record a signal of 1000 Hz at peak level for approximately 1 minute on the left

channel only. then record the same signal again for 1 minute on the right channel.

In making that recording the preselector switch of the unused channel is switched
to the position OFF

Set operating controls as per fig. 6.5.-6.

Select 1000 Hz on audio generator and adjust for peak level recording.

Load recorder with tape and start in the recording mode as described above.
After approximately 1 minute switch recording preselector REC-L to position

OFF.
Turn potentiometer INPUT LEVEL LEFT to position 0.

Switch recording preselector REC-A to position ON.

Turn potentiometer INPUT LEVEL RIGHT to position 10.
Continue to record for approximately 1 minute. then press button STOP and

rewind to the beginning of the recording.

Connect wave analyzer to OUTPUT R. Start recorder in PLAY .
Measure crosstalk L – R (>60dB).
Wind forward to the recording on the right channel.
Connect wave analyzer to OUTPUT L and switch mode selector MONITOR to
LEFT. Start recorder in PLAY. Measure cross- talk R - L (> 60 dB).

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6.5.5. Crosstalk STEREO

To measure the stereo crosstalk performance connect audio generator to AUX
INPUT L.

Set operating controls as per fig. 6.5.-7.

Crosstalk L .- R.

Connect wave analyzer to OUTPUT R

Select 1000 Hz on the audio generator and adjust for peak level recording.
Start the recorder in the recording mode.
The signal level appearing on OUTPUT R must be 45 dB below that of a peak level

recording.

Crosstalk R .- L.

Connect audio generator to AUX INPUT R.
Connect wave analyzer to OUTPUT L. Close potentiometer INPUT LEVEL
LEFT to position "0".

Open potentiometer INPUT LEVEL RIGHT to position "10".
Continue to rerord 1000 Hz at peak level.

Measure crosstalk. The signal level appearing on OUTPUT L must be 45 dB below
a peak level recording.

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RADIO

2.8 mV

open

60dB

MIC Hi

2.8 mV

200Ω

70dB

MIC LO

0.17 mV

200Ω

50dB

LINE AMP

-

Input level pot

80dB

Output voltages from peak level recording

OUTPUT

LEVEL min.

70mV

1.55 v

RADIO

LEVEL min.

70mV

PHONES

max. 5.6 V AL= ∞

PHONES

max. 2.8 V AL = 220Ω

6.5.6. Audio Measurements "Before tape"

Input

AUX

*B = 20 kHz linear

OUTPUT

Input voltages

for peak

level

recording (0

VU + 6 dB)

38mY

Terminating

resistance for

noise

measurements

10 kΩ

pos. "0"

Unwe ighted * S/N ratios

relative to peak level

73dB

OUTPUT

RADIO

0 Tolerance includes differences between channels and differences in amplification
factors.

LEVEL max.

LEVEL max.

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6.5.7. Wow and flutter

The wow and flutter values as specified in the technical data section are measured
with test equipment which conforms to the German standard DIN 45507 (consistent
with IEEE standard 193-1971).

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