Revox A76 User manual

Service Manual

Service SVSanya
R^OX A7
18 174 572 Printed in Switzerland

Contents Page
1GENERAL 4
2. DISMANTLING 4
2. 1.Removal of front panel 4
2.2. Removal from cabinet 4
2.3. Lacing of dial cord 4
3. CIRCUIT DESCRIPTION 5
3. 1.Operating controls 5
3.2. Schematics 5
3.3. RF input section (front end) 5
3.4. IF-section 6
3.5. Demodulator 6
3.6. Subcarrier re-generation 6
3.7. Multiplex decoder 7
3.8. Indicator circuits 7
3.9. Power supply 8
4. ALIGNMENT INSTRUCTIONS 9
4.1. Test equipment 9
4.2. Additional tools and filters 9
4.3. Checking of supply voltages 10
4.4. Tuning of IF filters 10
4.5. Alignment of center tuning discriminator 1
1
4.6. Checking the selectivity 1
1
4.7. Alignment of RF input section 12
4.8. Alignment of IF amplifier 13
4.9. Checking symmetry of the center tuning meter (44) 13
4.10. Checking for correct limiting 13
2

Page
4.1 1. Sensitivity check on the center tuning (44) and the
signal strength meters (43) 13
4.12. Alignment of FM demodulator 14
4.13. Alignment of multipath indicator 14
4.14. Alignment of subcarrier oscillator (76 kHz) 15
4.15. Alignment of 19 kHz circuit 15
4.16. Alignment of 38 kHz circuit 16
4.17. Tuning of the 19 kHz band pass filter 16
4.18. Alignment of multiplex filter 17
4.19. Adjustment for minimum stereo crosstalk 17
4.20. Checking of automatic mono-stereo changeover 18
4.21 .Checking range of output potmeters 18
5.
INSTRUCTIONS FOR MEASURINGTHE MOST
IMPORTANT PERFORMANCE DATA 19
5.1. Input sensitivity 19
5.2. Static selectivity 19
5.3. Image response 20
5.4. Spurious response 20
5.5. Distortion 21
5.6. Signal to noise ratio 21
5.7. Stereo separation (crosstalk) 21
5.8. Frequency response 22
6.
TECHNICAL DATA 23
7.
SCHEMATICS 25
3

The REVOX A76 FM-Stereo Tuner is apiece of high quality
equipment which passes through aseries of scrupulous tests
during production as well as avery careful final inspection.
Therefore, when servicing this tuner, it is absolutely ne -
cessary to check all vital data in order to ensure that the
original characteristics will be maintained.
Important
When soldering Field Effect (FET )and Radio Frequency
Transistors the following precautions must be observed:
Tuner must be turned off.
Work with earthed soldering iron only.
Provide electrical contact between tip of iron and tuner
chassis.
2. DISMANTLING
2J.
Removal of front panel
—pull rotary knobs from their shafts
—carefully pull off front panel which is held by six snap-on
fasteners
ZZ
Removal from cabinet
—place top side of cabinet onto asoft pad
—undo four chassis mounting screws
—pull chassis from the cabinet
—remove spacerplates (with rubber strips)
—remove upper and lower shielding panel
Z3.1.
Stranded steel-wire
—Take off pulley (1)and orient It so that the hole between
two angled eyelets is in 12 o'clock position.
—Lace large loop of steel wire through left hand eyelet and
wind 11/2 turns of wire onto pulley. Slide pulley onto
shaft and secure it by pushing asuitable rod (round needle
file) through hole In pulley and hole in panel behind.
—Lace wire over guide rollers, along pointer slide and hook
onto removed drive-pulley (2)
—Turn tuning capacitor ccw to stop, slide drive pulley with
slot pointing downwards onto capacitor shaft.
—Turn drive pulley cw to stop. (Allow 1to rotate). In this
position there must be exactly 2turns of steel cord on the
drive pulley.
2.3.2.
Nylon cord
—Find approx, centre of cord and wind 2turns onto drive
shaft (3)
—Take off drive pulley (2) and connect R.H. end of cord.
Put drive pulley back onto cap shaft, slot pointing up-
wards.
—Turn pulley (1)cw to stop and attach nylon cord via
spring (4) to the R.H. eyelet.
—Run dial drive several times back and forth and check for
proper winding of dial cords.
4

39 52 51
46 47 48 49 50
1111/
11111
1i?anr
II |o|aa“DD II coxa lol 1FM 1. 1 1 1 1 i,
LX::ri
/
45 44 43 42 41 40
Fig. 3.1.-1
2.3.3.
Positioning of dial pointer
—Turn drive shaft (3)ccw until it stops with the tuning
capacitor fully closed
—attach pointer slider to steel wire
—with front panel and dial installed, slide pointer to the
left hand end of the horizontal line on the dial (5) .
58 59 57
3. CIRCUIT DESCRIPTION
3.1.
Operating controls
The index numbers on all operating controls are Identical to
those used in the Operating Instructions (see fig. 3. 1.-1 ).
3.2.
Schematics
The following schematics are contained in section 7of this
manual:
3.2.1
Functionally grouped Block diagram
One group function corresponds to one actual circuit board.
3.2.2.
Interconnecting diagram showing all power supply and signal
connections.
Please note:
The circuit of the push-button assembly (G)1.076.291 is
shown on this interconnecting diagram only.
3.2.3.
Complete circuit diagram with signal connections. On this
diagram the switching elements of the push-button assembly
are shown as peripheral circuit elements.
3.3.
RF input section (front end) 1.076.150
(schematic A)
The signal arriving on the 60 ohms or 240 ohms antenna
sockets is fed via abalancing transformer (Baiun )to the tuned
RF-input stage with its FET Q101. Coupling to the mixer
Q102 is achieved through the band-pass formed by LI 04/
LI 05.
The RF signal from the stabilized oscillator Q104 passes
through the buffer stage Q103 and reaches (via the oscillator
5

ARF input section
BIFsection
CDemodulator
DSubcarrier re-generation
EMultiplex decoder
FIndicator circuits
GPushbutton board
HPower supply
transformer L108 )GATE 2of the mixer Q102. The buffer-
stage Q103 serves to prevent interactions between mixer and
oscillator. The IF signal is taken from the balanced output
of L106.
For silent tuning and muting of weak stations (button
"MUTING ON" (49 )depressed )the last IF stage may be
biased into cutoff by the variable trigger (indicating circuits)
whereby the triggering point is adjustable to the desired
signal strength.
3.4.
IF-section 1.076.170
(schematic B)
in order to achieve constant transmission characteristics,
IF-filters and IF-amplification are completely separate
entities. Effective signal selection is concentrated in the
passive filter-section which lies ahead of the IF amplifier and
is therefore entirely independent of signal amplitude or
limiting action.
The filter consists of eight individually tuned parallel re-
sonant L/C circuits and has abandpass characteristic which
approximates the Gaussian probability distribution over
the range of ±120 kHz (fig. 3.4.- 1).
In that range, the envelope delay remains virtually constant
(fig. 3.4.-2 )resulting in excellent performance with regard
to keeping any modulation distortion at aminimum.
The IF-amplifier which follows is made up of five Integrated
Circuits (IC's )each one being aseparate amplifier with a
differential input stage. Limiting in the last IFstage is already
effective at alevel equalling the tuners input noise, there-
fore the audio output signal will remain constant from the
lowest antenna input (1juV )on upward. Figure 3.4.-3
illustrates the audio signal to noise ratios for stereo and
mono operation, starting at the point from where limiting
becomes effective.
Level dependent DC currents are branched off from the
diodes D201, D202 and D203 for signal strength Indication.
By adding those currents apseudologarlthmic Indication
characteristic is achieved which allows evaluation of signal-
strengths ranging from afew microvolts up to approx. 10mV.
For centre tuning indication asignal is taken from the fourth
IF stage and fed to aseparate narrow-band detector
(indicator circuits).
3.5.
Demodulator 1.076. 190
(schematic C)
Demodulation of the frequency modulated signal Is achieved
in adelay-line demodulator. This demodulator Is made up
of adriver stage formed by the differential amplifier with
transistors Q301, Q302 and Q303, two coaxial delay-lines
(with an electrical length equalling X/8 of the intermediate
frequency )as the load impedance and D301/D302 as the
rectifying circuit, in this arrangement FM is being changed
into AM by the frequency dependent impedance of the two
coax line sections, one of which is open whereas the other is
short circuited at the end by capacitor C308.
This delay-line demodulator has abandwidth of 5MHz. This
in connection with the broadband characteristics of the IF-
amplifier (5MHz) results in acapture ratio of 1dB thereby
ensuring excellent common frequency suppression. The
operating principle of the delay-line demodulator makes for
asimple circuit which is free of any critical components and
tuning elements (AM suppression depends entirely on the
limiting characteristics of the IF amplifier).
The multiplex signal is taken off at trimpot P301 (DC-
Balance )and is tripped of its IFcomponents by an RC-net-
work. The output voltage of the demodulator has avalue of
15 mV approx. (at 75 kHz deviation) and is then amplified
to 1Vapprox, by the multiplex amplifier Q304, Q305 and
Q306.
3.6.
Subcarrier re-generation 1.076.210
(schematic D)
The 38 kHz subcarrier is re-generated in aphase locked loop
oscillator. The oscillator together with the differential
6

T(fis)
Fig. 3.4-1
amplifier Q405 and Q406 produces a76 kHz frequency
which, after pulse shaping, passes IC401 operated as a fre-
quency divider at the ratio of 1:2:4. The resulting 38 kHz
(subcarrier) is then used to operate the switching demodula-
tor. The 19 kHz signal from the frequency divider is fed to
the phase comparator Q403/Q404 where it is compared with
the pilot-frequency arriving from the bandpass L401/
L402.
The 15 Hz low-pass filter formed by R409, C408, R410
prevents any interference components in the pilot signal
from reaching the varicap diodes D402 in the 76 kHz oscilla-
tor. This arrangement operates in the manner of anarrow
pilot frequency bandpass with a±15 Hz bandwidth. The bias
voltage for the varicap-diodes is obtained from the Zener-
diode D401.
3.7.
Mulitplex decoder 1.076.220
(schematic E)
The Multiplex-Decoder operates on the switching principle.
Separation of the multiplexed signal takes place in the low-
pass network R508, C504, R509 for the main channel and
in the 38 kHz bandpass L501, C501, R502 for the sub-carrier
or difference channel. Amplification of the sub-carrier takes
place in Q501 whereas Q503 and Q504 operate as the
switching demodulator.
The switching principle was selected because in contrast to
envelope-detection it transposes only those frequencies in
the audible range which are uneven multiples of the sub-
carrier. Thus the lowest critical interference frequencies
occur around 114 kHz and they are effectively suppressed
by the 114 kHz filter L505 -C518 and the 38 kHz band-
pass in the difference channel.
Across the load resistors R518/R519 the signals L-R and
—(L-R )are being formed, with the main signal appearing
in phase across those load resistors. Any residual compo-
nents are suppressed by the in-phase operation of the differen-
tial amplifier Q505 and Q506, which works at the same
time as the matrix by forming the Land Rsignals from the
difference channel and from the main channel, which arrives
from the Q502 amplifier.
8
7
6X
/5\
200 100 kHz UK) 2CK)
1pV 2510 20 50 100 2(K} 5(K) 1mV 25 10
Vanteon
Fig. 3.4-3
Alow pass 15 kHz filter to suppress multiplex residuals is
situated between the impedance matching stages Q507/Q508
and the matrix output. The audio level at the low impedance
outputs is adjustable from 50 mV to 1V, referenced to an
FM deviation of 75 kHz.
3.8.
Indicator circuits 1.076.200
(schematic F)
This portion of the tuner contains the auxiliary circuits for
tuning indication and for various automatic functions.
The IF-signal arriving from the fourth IF stage is amplified
by Q601 and then passed to anarrow band detector whose
small band-width produces the sensitive response of the
"CENTER TUNING" (44) meter.
As already mentioned in aprevious chapter, alevel depen-
dent current for signal strength indication is derived from
the first three IFstages and fed to the "SIGNALSTRENGTH"
meter after amplification in the Q602 amplifier.
The level dependent triggering circuit formed by Q604 and
Q605 delivers agating signal to the base of Q510 in the
stereo decoder for mono-stereo changeover and also biases
the last IF stage for noise squelch and silent tuning. By de-
pressing the button "TRIGGER LEVEL VARIATION" (48)
R702 in the collector leg of Q602, which normally provides
maximum sensitivity, is replaced by the potentiometer
"TRIGGER LEVEL" (39 ). This allows awide variation of
the triggering point with consequent muting even of rather
powerful stations. With the button "MUTING ON" (49
)
released, the triggering circuit will respond for automatic
mono-stereo changeover only.
In the multiplex decoder the circuit formed by 0510/051
1
and 0512 analyses whether or not the following conditions
are met for automatic changeover to stereo operation:
1. signal from the level dependent trigger (input base 0510)
2. presence of apilot signal. (This signal is derived from the
band-pass in the sub-carrier regenerating module. It is
amplified by 0509 and then rectified by the diode D502).
7

3. mono-stereo switch "STEREO AUTOMATIC" (47) de-
pressed (thus allowing asignal to reach the base of 051 1).
With all three conditions satisfied, 0512 becomes conductive
and the difference channel amplifier 0501 begins to operate.
With the switching of 0512 the stereo indicator "STEREO"
(output EG 1)will also receive current.
For multipath indication "MULTIPATH" (42) the multiplex
signal is being fed to the monostable trigger 0608/0607 and
signal is fed to the monostable trigger 0608/0607 and
receiving direct and reflected signals, will trigger that cir-
cuit Into the conductive mode, thereby causing the light
bulb "MULTIPATH" to become Illuminated (button "MUL-
TIPATH INDICATOR" (50) depressed).
3.a
Power supply 1.076.130
(schematic H) 1.076.140
The power supply consists of two electronically regulated
sections for —23 Vand +23 V. Zener diodes stabilize the
supply voltages —6.2 Vand +6.2 Vwhich feed the integrated
circuits in the IF amplifier auxiliary circuits.
*Modifications MK II *
Tuners of the MK II version have the function MULTIPATH
INDICATOR replaced by the button labeled STEREO FIL-
TER (50 ). Therefore the circuit of the indicator light
MULTIPATH can no longer be switched off.
With button STEREO FILTER, the signal to noise ratio of
week or distant stereo stations can be improved. This is
being achieved by reducing the bandwidth of the 38 kHz re-
sonant circuit L501/C501 (raising the Qby disconnecting
the 8.2 kohm resistor). The narrow bandwidth of the 38 kHz
filter results in areduced stereo base; stereo crosstalk drops
to 10 dB at 1kHz.
8

tFmatching network
]Test generator |(for IF filter measurements)
“1coaxial Push-on terminal
BNC cable RG 58 54.02.0327
^Q_ O™
I
lOn max. 350 mm
4,7 k
alligator clip
—23 V(blu) from distribution board
(not required with the RF input section 1,076.150)
Fig. 4Z-1 38 kHz bandpass
(for subcarrier minimum adjustment)
15 kHz lowpass filter
(for AF measureme ts
f=15 kHz: attenuation ma .0.2 dB
f>19 kHz: attenuat'on >45 dB
Input 4,7 k53mH 34,4 mH 37,7 mH Output
0348 n
Fig. 4.2.-3
1,41 n<
,<
<
<
>4,7 k
>
1,77 n
Tuner 60 H
Fig. 4.Z-2 Fig. 4.Z-4
4, ALIGNMENT INSTRUCTIONS
4.1.
Test equipment
For correct alignment the following test equipment (or
equivalents) will be required:
Stereo test generator Rohde &Schwarz type SMSF BN
41410, range 87 to 108 MHz and 10.2 to 11.2 MHz
Stereo modulator Philips PM 6450
Audio generator low distortion type (THD less than 0.05 %)
Digital counter (for 38 kHz)
Oscilloscope (with internal and external triggering) including
RF probe 10:1
DC transistor or vacuum tube voltmeter (VTVM )including
RF probe
Multimeter for supply voltages
Distortion analyser (or audio millivoltmeter with suitable
filters)
Note:
The signal voltage indicated for the stereo test generator is
an open circuit voltage (o.c.v. ). If the internal impedance of
the test generator equals 60 ohms, avoltage of exactly 1/2 of
the test generator's open circuit voltage will result accross the
60 ohm input of the tuner (see fig. 4.2.-4).
When working with test generators whose output calibration
is already taking into account the nominal load Impedance
of the equipment under test, the generator's output is to be
set to one half of the Indicated open circuit voltage.
The predominantly used test frequency of 94 MHz is to be
taken as aguide only. Prior to starting any alignment proce-
dures, It should be checked whether or not that frequency
does not produce an interference with aneighbouring FM
transmitter. (This should be checked with the test generator
connected to the tuner, but with the signal turned off ). If
an FM broadcast can still be received, the test frequency
should be altered slightly.
The stereo test generator is to be connected to the 60 ohm
antenna input for all tests (except for the tuning steps 4.4
to 4.6 which require the test frequency to be fed via the IF
matching network).
4.Z
Additional tools and filters All readings are to be taken against chassis.
One set of cables (coaxial cable RG 58)
One set of tuning tools
The following filters and matching networks are required for
IFmeasurements, subcarrier minimum adjustment and audio
frequency measurements. The construction of these circuits
is not critical.
IF matching network fig. 4.2.-1 (for measurements on the
IF filters)
38 kHz bandpass fig. 4.2.-2 (for subcarrier minimum ad-
justment)
15 kHz lowpass filter fig. 4.Z-3 (for audiofrequency mea-
surements)
9