Leak stereofetic fm tuner service manual

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STEREOFETIC
F. M. TUNER

SERVICING INSTRUCTIONS

W - manna - mum-in

For Service Manuals Contact
The STEREOFETIC FM Tuner is best considered as four separate units: mfllfi?§;§g§fl%§"cfinlmmfi
I. The Power Supplies Oxon OX9 4QY
Tel? 01844-351694 Fax:- 01844-35254
The Front End EtttIiI:-enquities@mautitton.m.flt

7
3. The LF. Strip
4. The Decoder
Items 1. 3 and 4 are on primed circuit boards which can be swung up from the chassis for case of servicing.
Item 2 is located on the chassis itself.

I. THE POWER SUPPLIES (Diagrams Page: 4 8t 5)

The Tuner requires +9 volts at about 90 milliamps. and -5 volts at about 5 milliamps. Both these
supplies are critical with regard to voltage level and ripple. Any variation in level of the voltage lines will
cause mistuning of the oscillator in the front end. and may cause limiting of the output audio signal. Any
increase in ripple will cause an increase in hum level at the output of the decoder.

The stabllisation circuit for each supply consists of a series regulator driven by a constant voltage source.
Each constant voltage source consists of a zener diode which controls its own current by means of a constant
current transistor feeding base current back into its own transrstor feeder (e.g. VTl is a constant source feeding
base current to VTZ. VTZ feeds a constant current to zener diode D3. which in turn sets up to constant
current in VI]. R5 acts as a starter resistor).

The output voltage from each regulator can be adjusted by about i f volt by varing a potentiometer which
is across each zener (VRl and VRZ).

Both-supplies can accept short circuits in their outputs for a few seconds without any adverse efiects.
Continuous short circuits will overheat the series regulators.

The voltages given on the circuit diagram refer to a 235 volt rms input voltage to the 200-250 volt tap. Other
mains voltage will give different voltages for all except the + 9 and - 5 volt outputs. At 100 and 200 volt mains
input the ripple may increase by about 30%.

DO NOT RUN THE TUNER FOR LONG PERIODS WITHOUT A FESTOON LAMP as this may
cause failure of one or both of the 1.000 microfarad reservoir capacitators. This is because the festoon lamp
forms a substantial part of the load on the mains transformer. and when this load is removed the voltage across
the LOOOmF capacitors may rise above their maximum rating if the mains voltage is high.

2. THE FRONT END (Diagrams Pages 10, 11 8t 12)

The basic front-end circuit consists of an RF amplifier. mixer and separate oscillator system. It is
unconventional in that a dual-gate Field Efiect Transistor is used for the mixer. and the oscillator is of the
grounded collector variety. THE POSITIONING OF COMPONENTS IN THIS PART OF THE TUNER
IS CRITICAL. MOVEMENT OF ANY COMPONENT MAY CAUSE MISTUNING OR UNWANTED
OSCILLATION.

The 75 ohm coaxial socket is isolated from chassis by two ZOOpf capacitors to stop any chassis currents
which might otherwise cause noise or interference. The RF transistor is a single gate F_E.T. to which AGC is
applied when the input signal exceeds I millivolt. This stage can accept several hundred millivolts without
overloading. Both input and output are tuned by simple LC networks. and the tuned output is fed into gate 1
of the mixer. The oscillator input is fed into gate 2 (L3 & C18 form a lO-7MHz trap to Increase the mixer
efficiency). -

The mixer output is transformed through a double tuned IO-7MH2 transformer and fed to a capacitance
divider and thence to the IF Strip.

The oscillator feeds out about 500 millivolts rms from its capacitor at 100MHz This decreases to about
400 millivolts at IZOMHZ.

The AFC rate is 27 millivolts per kHz change in tuning. and gives an overall AFC of 21}: l.

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3. THE LF. STE? (Diagrams $235 8 & 9) For Service Manuals Contact

MAURITRON TECHNICAL SERVICES

The LP. Strip can be considered in two sections: GChegZol'lregxgtégcinmr
.. Tel:-01844«351694F :- 1844-6152554

la) The |U-7MHz amplilier and dectcctor Emjgenqummfiagmmwm

(bl The muting and indicator system

The 10-7MHz Signal Path and Detector

Two integrated circuits and two ceramic filters form the main [(l~7MHz signal path. and as these are very
reliable components. the [.F. strip should give many years of trouble-free service. The ceramic filters are factory-
tuned and non-adjustable. the exact centre frcuuency being indicated by a coloured dot on the top of each filter.
If one of these devices requires replacement it is essential that the new filter is of the same colour so that its
centre frequency matches the remaining filter in the strip. This frequency will probably not be exactly lU-7
MHz: it can be determined h} feeding about 25 micromlts of unmodulated R.F. into the LE t ip \ia C35
and watching the tuning meter as the signal generator frequency is manually swept through the 10'7MHZ region.
Maximum meter reading coincides with the filter centre frequency. lfthe tuning meter is not functioning. then
a minimum voltage reading at the collector of VT13 also coincides with the filter centre frequency.

The input transistor VTlt) acts as an impedance match from the output of the front-end" to the first filter
(both filters require 300 ohm matching in and out]. The overall gain from LF. strip input to lst LC. input is
about OdB. as the transistor guilt of IGdB compensates for the filter insertion loss of about - lOdB. The AGC
amplifier should give about - 1 volt dc. out for 3 millivolts of |0-7MHz led into the LF. strip input.

The overall circuit gain from input to audio output should be such that about 4 microvolts of fully modu.
lated |0-7MH2 at the input will produce about [20 ittillivolts of audio output with a signal/noise ratio of better
than 30:18. The primary of the ratio dectector is tuned for minimum audio distortion. and the secondary for
zero d.c. volts at the AFC take-off point. It should be noted that this primary tuning does not necessarily
coincide with maximum d.c. volts across C63 : unless the proper distortion measuring apparatus is available the
ratio detector cores should not be touched. Likewise. as the AFC volts are so low compared with most other
ratio detectors. and are fed from a high impedance. the secondary tuning should be adjusted only if a high
input impedance digital voltmeter is available.

The Mating and Indicator System

The mute operates in two ways-it feeds a wideband inhibit signal to pin 3 of the second LC. and it feeds
a narrow band inhibit signal to the audio amplifier in the decoder. The wideband inhibit signal is derived
directly from the 10-7MH2 signal which appears at the output of the second I.F. filter. This signal is ampli-
fied by VTlZ and rectified by D8 and D9. The rectified output is fed onto the base of VT13 such that if the
IO-7MHz input to the LP. strip exceeds about 40 microvolts VT13 switches on. Thus. D10 ceases to conduct
current7into {1:3 3. and I.C.2 switches on too. The actual level at which VTlJ switches on can be controlled
by VR .

The narrow band signal is derived from the AFC signal and helps to reduce the edge-of-the-station "plop"
which is common in a lot of muting systems. The dc. output from the ratio detector is fed via VTI7 into one
side of the dilfercntial pair VTlli and VTI). The output voltage from this pair is rectified by DH and D12
so that the voltage to R50 is unidirectional each side of the ratio dectector centre-tuning point. Thus. a peak
voltage is produced by the differential pair. the actual peak coinciding with true centre tuning. (The position
of this peak is adjusted by VR4 to balance the offset caused by the Vbe of VT17). The wideband signal from
VTl3. and the peak signal from VTlS and VTl9 are summed into VTl4. l5 and 16. so that the output voltage
from VT16 drops from +8 volts to +0.8 volts at centre-tuning. and the output voltage from VTlS changes from
+l volt to +26 volts. (This voltage swing is set by adjusting VRSl.

The point at which the mute is removed is dependent upon the output from VTIG. and the ratio between
the wideband and narrow band voltage levels at this point is set by the ratio between R48 and R50, and
between R61 and R60. To ensure that the plop" is kept to a low value. while at the same time keeping a
reasonably wide tuning range. the volts at MUTE 3" drop to about 55 at the edge of the band (when the meter
jumps to halfway up the black band). and down to 3% before the audio circuits can pass signals.

The indicator system is fed directly from VTIS via a 5-6K ohm resistor (R58). It therefore reads a true
peak when the detector is set to the exact centre of the tuning range. This coincides with the minimum distor-
tion point. However. because this centre peak is a relatively narrow part of the ratio dectector pass band. for
ordinary domestic listening it is sufficient to tune only into the wide band range such that the demute operates
satisfactorily. Over this range the audio distortion will be less than 1% for most average music programmes.