Thorn Studio 500 Series User manual

Boat Service Manual
Thorn Studio 500 and 600 series
Models 501 502 503
004 600 601
602 603 604
TECHNICAL SPECIFICATIONS
Power Output
(Measured with 1kHz sinewave,
both channels driven).
501 and 502: 15 or 20 watts per
channel.
503, 504, 600, 601, 602, 603,
604: 20 watts. per channel.
Frequency Response
30 Hz— 20 kHz+ 1dB using tape
input.
Power Bandwidth
30Hz —20 kHz unclipped at —2dB
points
Distortion
Less than 0.1% up to 1dB below
maximum power output.
Sensitivity
(Measured at 1kHz for 15w output).
Magnetic pickup 4mV
Tape input 200 mV
Auxiliary input 85 mV
Power Consumption
75 watts from nominal 230V
(215-245V) AC 50 Hz. Supply.
Semi-conductor Complement
26 Transistors
10 Diodes
Tone Controls
(Reference 0dB at 1kHz)
Bass =12 dB at 40 Hz
Treble +12 dB at 10 kHz
Contour
(at 30 dB below full volume setting).
+9dB at 40 Hz
+7dB at 10 kHz
Low Filter
—20 dB at 20 Hz
High Filter
—10 dB at 10 kHz
Crosstalk
Better than 40 dB at 1kHz for any
input.
Tape Output
100 mV with 4mV into gram at
1kHz. Source impedance 100 k
Auxiliary Output
100 mV with 4mV into gram at
1kHz. Source impedance 100 k
The right is reserved to vary specifications or use alternative materials as may be deemed necessary or desirable at any time.
RADIO
THORN OUaRES AMEMBER OF THE THORN GROUP OF COMPANIES IN 18 NATIONS

SERVICING 500 AND 600 SERIES AMBIOPHONIC UNITS
The mechanical dismantling procedures to be followed prior to servicing are as follows.
To dismantle the 501, 503, 601 and 603, remove the 6screws indicated. Do not remove
any other screws.
Carefully release the front extrusion whilst sliding the wooden cabinet backwards. Lift
the cabinet directly upwards ensuring that the metal chassis does not damage the inside
faces of the cabinet —particularly at the front.
Most servicing can be done with the record deck directly supported by awooden stay
and still connected to the amplifier. To remove the deck completely, disconnect the two
mains conductors secured in the terminal block on the chassis and the three push on
cartridge leads (at the chassis end). The record deck can then be removed. There are
slight changes in this procedure for the 502, 504, 602 and 604. The 4screws (2 in the
front rail and two at the back of the unit) are first removed. Slide the chassis forward
about 12”. Lower the rear of the chassis to clear the turntable mechanism. Draw the
chassis almost out of the cabinet. On some units it may also be necessary to lift the rear
edge of the turntable during this operation. Disconnect the two mains conductors and
three cartridge leads (at the chassis end). Withdraw the chassis completely ensuring
that the inside faces of the cabinet are not damaged by the edges of the metal chassis.
502 and 504.
602 and 604. Remove the four screws
(arrowed.) Na
Remove the six screws arrowed
Slide the chassis forward about 1/2”, and
lower the rear of chassis to clear turntable
mechanism. *
Most servicing can be done with
the record deck supported by a
wooden stay (below). To remove
the deck completely disconnect the
two mains conductors secured in
the terminal block on the chassis
and the three push on cartridge
leads at the chassis end.
Carefully release the front extrusion (above)
whilst sliding the wooden cabinet backwards
(below)
The chassis is then almost withdrawn
from the cabinet. The two mains
conductors and the three cartridge leads
can now be disconnected prior to
complete removal of the chassis. Take
care that the inside faces of the cabinet
are not damaged by the edges of the
metal chassis. =
Model 600 Tuner Amplifier.
To remove the cabinet from this model remove the four screws from the bottom outer
edges of the chassis and the two screws from the top back edge of the chassis. Slide the
cabinet carefully back so that the dial escutcheon clears the slot in the front of the
cabinet and then lift clear.

WITH DIAL PULLEY WHEEL IN
FULLY CLOCKWISE POSITION AND
CARRIER IN POSITION SHOWN ABOVE
START WITH CORD IN REARMOST GROOVE, AND
WIND CORD ROUND PULLEY 3COMPLETE TURNS
AND LOCK CORD IN PULLEY, AT END OF THIRD
TURN, AS SHOWN.
TOTAL LENGTH OF CORD -1020mm
DIAL CORD STRINGING
ADJUSTING QUIESCENT CURRENT
To set the quiescent current remove fuses F401 and F501 and connect a100 mA meter
across the fuse holder.
Link pins Aand Bon power amplifier printed circuit board to increase the current or
Link pins Band
C
to decrease the current.
The quiescent current should read between 20 and 60 mA under no signal conditions.
After adjusting both channels replace the fuses F401 and F501. If the quiescent current
cannot be adjusted using the above procedure the amplifier should be checked for a
fault. See Service Hints.
TUNER ALIGNMENT
1. IF Stage
Set dial pointer to the datum mark at the right hand end of the scale with the tuning
gang fully open (that is fully clockwise).
See that the aerial coupling coil L1 is in the centre of the rod and seal with wax.
Connect an AC millivoltmeter between the junction of R114 and R112 and ground.
Connect an AM, RFsignal generator to the aerial terminal and inject a455 kHz
signal. Adjust T101, T102 and T103 for maximum reading on the millivoltmeter.
Ensure that the output from the signal generatoris kept at alevel just sufficient to
operate the meter. Do not inject too strong asignal and over-load the receiver as a
false reading may be obtained.

2. Calibration and sensitivity RF Alignment
Set the signal generator to 600 kHz and tune the receiverto this frequency on the dial.
(a) Adjust T100 (oscillator coil) and L2 (aerial coil) for maximum meter reading at this
frequency.
(b) Tune the signal generator and receiver to 1400 kHz and adjust C106 (oscillator
trimmer) and C102 (aerial trimmer) for maximum meter reading.
Repeat Aand Buntil no further improvement can be obtained.
Seal aerial coil L2 to rod with wax.
SERVICE HINTS
Power Amplifier board
1. By removing fuses F401 and F501 from the two power amplifiers the power supply
can be checked off load.
If the fuse of one of the power amplifiers has blown it may be removed and replaced
witha 100 ©10 watt resistor sol dered across the blown fuse. By using this resistor the
current to the amplifier is limited. The quiescent current can be ascertained by
measuring the voltage drop across the 100 resistor and using the conversion factor
IV=10mA.
If the current drawn by the amplifier under no signal conditions is excessive then the
fault is probably leakage in the output pair or driver transistors. The symmetry of the
two amplifiers allows easy checking for leakage with minimum removal of devices.
Also check for the correct operation of the biasing transistors Q403 and Q503.
To break the DC feedback loop:-
Disconnect speaker 4/2 for RH channel and unsolder the base lead of Q402. If the
amplifier is now switched on, the centre rail should be between 42V and 44V. Ifit is not
then Q402 is probably leaking.
If Q402 is not leaking connect a4k7 resistor between the collector and emitter and under
this condition the centre rail should be approximately 22V. The quiescent current
should also be correct. This then allows Q402, Q404, Q405, Q406, and Q407 to be
checked for correct operation.
When thecentre railis at the correct voltage (22V) the operation of Q401 can be checked
by connecting avoltmeter across R408 and adjusting the preset pot R402. When fully
clockwise the voltage across R408 is zero and when fully anti-clockwise approximately
2v.
+200B
+15dB +15dB
+10dB +10dB
+5dB +5dB
—5dB -SdB
1008 —10dB
-15dB -15dB
-20d8
020 20 50 100 200 500 5 10 20
—200B
125120 1
125
kHz kHz «Hz kHz kHz Hz Hz Hz Hz Hz kHz kHz kHz kHz kHz
70 50 100 200 500
iz Az Hz Hz Hz
Tone Control Range Filter &Contour Responses

Description of complementary power output stage.
The collector of transistor Q402 is directly coupled to the bases ofa pair of complemen-
tary transistors Q404 and Q405 acting as drivers for the push-pull class Boutput pair,
Q406 and Q407. Direct coupling is used between the drivers and their corresponding
output transistors. The driver Q404 is effectively in the common collector mode and the
Q405 also in the common collector mode. Since the drive transistors Q404 and Q405
comprise an NPN and aPNP, they conduct on alternate half-cycles of signal, thereby
driving the output transistors likewise. There is no voltage gain, but power gain occurs
due to impedance changes, being the emitter-follower action of Q404/Q406 and Q405/
Q407 alternately. In the absence of signals, asteady current flows through Q404/Q406
and Q405/Q407 and their emitter load resistors. Asteady current also flowsin the driver
base resistors R406/R409 making Q406 base potential slightly positive with respect to
its emitter. Current through Q402 causes Q405 to beslightly negative with respectto the
centre rail. Thus Q404 and Q405 havea small forward bias which aids in the removal of
cross-over distortion in the output.
Transistor Q403 provides bias stabilisation for the driver stage output. This is achieved
by having the Ic of Q402 flow through Q403 and making biasing of Q406 and Q407 less
dependent on theIc of Q402. Effective thermal tracking is achieved by placing Q403 on
the same heat sink as the output devices (Q406 and Q407). Q403 acts as alow impedance
path for both signal and changes in d.c.
D.C. coupling is used in one feed-back loop to set up the d.c. working point of the output
transistors. This feed-back loop also keeps the distortion level low. In this regard, the
potential at the junction of R418/R419 is critical. If it is upset, severe clipping will result
in one half-cycle only at high signal levels. The junction of R418/R419 is returned via
R407 to the emitter of Q401. In addition to acting as an amplifier, Q401 compares the
voltage at the junction of R418/R419 with that at its emitter. For example, if the
mid-point starts to rise, Q401 emitter is driven more positive. Q401 collector voltage
rises causing acorresponding rise at Q402 base. Q402 collector falls, lowering the base
voltages on Q404 and Q405. As Q404 is an NPN transistor it will conduct less, with the
PNP transistor Q405 conducting more. This causes the centre rail voltage to fall
counter-acting the original drift upwards. Pre-set control R402 in avoltage divider
chain between the +44V rail and earth, sets the centre-rail (working point).
R420 and C409 in parallel across the ouput provide the necessary high frequency load to
the ouput transistors required due to the rising impedance ofthe loud speaker at high
frequencies. They also prevent destruction of Q406 and Q407 if speakers are discon-
nected.
R412 and R408 form an A.C. feed-back loop.
C402 acts as an RF bypass, as RF can present asubstantial signal level at this point.
C405 is the high frequency roll-off capacitor, ensuring high frequency stability.
C406 and C407 determine the maximum slewing rate possible at the output and hence
the upper frequency of the power band width.
R411 prevents high loop currents flowing in the ground line to which Aclass stages
Q401 and Q402 are connected.
C404 acts as abootstrapping capacitor and alsofilter for line hum.

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AMBIOPHONIC DESCRIPTION
4+
RIGHT AMP
RIGHT FRONT RIGHT REAR
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LEFT FRONT LEFT REAR
LEFT AMP
Fey AMBIOPHONIC CIRCUIT
FIG. 1
It will be seen in Fig. 1that the rear speakers are connected in abridge configuration
across the outputs of the right and left amplifiers. If the right and left amplifiers are
driven in phase then the voltage at “A” will always equal the voltage at “B” ie. VAB =O
and no output will appear in the rear speakers. Fig. 2.
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FIG 2MONO OR STEREO (Left -Right) (Left -Right)
AMBIOPHONIC
FIG 3
If there is adifference between the left and right channels this appears in the rear
speakers (Fig. 3) and gives an ambiance to the music. All music is acombination of these
two situations. The resistor R(Fig. 1) is added to obtain separation between the rear
speakers and to give some output to the rear channels when amono signal source is
used.

iORN in New Zealand by
HO RADIO
INDUSTRIES
N.LZ. LIMITED
P.O. Box 59051, Mangere Bridge, Auckland.
36-38 Hastie Ave, Mangere, Auckland.
Specifications may be subject to change without notice. PN 935
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