PHILIPS 14TX1004 /08Z CHASSIS TX INTERNAL VIEW.

On a monocarrier are deployied all parts of the receiver inclusive the power supply with mains transformer.




Power Supply: The examples chosen are taken from manufacturers' circuit diagrams and are usually simplified to emphasise the fundamental nature of the circuit. For each example the particular transistor properties that are exploited to achieve the desired performance are made clear. As a rough and ready classification the circuits are arranged in order of frequency: this part is devoted to circuits used at zero frequency, field frequency and audio frequencies. Series Regulator Circuit Portable television receivers are designed to operate from batteries (usually 12V car batteries) and from the a.c. mains. The receiver usually has an 11V supply line, and circuitry is required to ensure that the supply line is at this voltage whether the power source is a battery or the mains. The supply line also needs to have good regulation, i.e. a low output resistance, to ensure that the voltage remains constant in spite of variations in the mean current taken by some of the stages in the receiver. Fig. 1 shows a typical circuit of the power -supply arrangements. The mains transformer and bridge rectifier are designed to deliver about 16V. The battery can be assumed to give just over 12V. Both feed the regulator circuit Trl, Tr2, Tr3, which gives an 11V output and can be regarded as a three -stage direct -coupled amplifier. The first stage Tr 1 is required to give an output current proportional to the difference between two voltages, one being a constant voltage derived from the voltage reference diode D I (which is biased via R3 from the stabilised supply). The second voltage is obtained from a preset potential divider connected across the output of the unit, and is therefore a sample of the output voltage. In effect therefore Tr 1 compares the output voltage of the unit with a fixed voltage and gives an output current proportional to the difference between them. Clearly a field-effect transistor could do this, but the low input resistance of a bipolar transistor is no disadvantage and it can give a current output many times that of a field-effect transistor and is generally preferred therefore. The output current of the first stage is amplified by the two subsequent stages and then becomes the output current of the unit. Clearly therefore Tr2 and Tr3 should be current amplifiers and they normally take the form of emitter followers or common emitter stages (which have the same current gain). By adjusting the preset control we can alter the fraction of the output voltage' applied to the first stage and can thus set the output voltage of the unit at any desired value within a certain range. By making assumptions about the current gain of the transistors we can calculate the degree of regulation obtainable. For example, suppose the gain of Tr2 and Tr3 in cascade is 1,000, and that the current output demanded from the unit changes by 0.1A (for example due to the disconnection of part of the load). The corresponding change in Tr l's collector current is 0.1mA and, if the standing collector current of Tr 1 is 1mA, then its mutual conductance is approximately 4OmA/V and the base voltage must change by 2.5mV to bring about the required change in collector current. If the preset potential divider feeds one half of the output voltage to Tr l's base, then the change in output voltage must be 5mV. Thus an 0.1A change in output current brings about only 5mV change in output voltage: this represents an output resistance of only 0.0552.


TDA2540 IF AMPLIFIER WITH DEMODULATOR AND AFC

DESCRIPTION
The TDA2540 and 2541 are IF amplifier and A.M.
demodulator circuits for colour and black and white
televisionreceiversusingPNPorNPNtuners. They
are intended for reception of negative or positive
modulation CCIR standard.
They incorporate the following functions : .Gain controlled amplifier .Synchronous demodulator .White spot inverter .Video preamplifier with noise protection .Switchable AFC .AGC with noise gating .Tuner AGC output (NPN tuner for 2540)-(PNP
tuner for 2541) .VCR switch for video output inhibition (VCR
play back)

.SUPPLYVOLTAGE : 12V TYP
.SUPPLYCURRENT : 50mATYP
.I.F. INPUT VOLTAGE SENSITIVITY ATF = 38.9MHz : 85mVRMS TYP
.VIDEO OUTPUT VOLTAGE (white at 10% oftop synchro) : 2.7VPP TYP
.I.F. VOLTAGE GAIN CONTROL RANGE :
64dB TYP .SIGNAL TO NOISE RATIO AT VI = 10mV :
58dB TYP .A.F.C. OUTPUT VOLTAGE SWING FOR
Df = 100kHz : 10V TYP



 The Philips TX monochrome portable chassis has been in production for several years and large numbers have been sold in the Philips  model ranges. several versions, with 12 and 14in. tubes, and with/without remote control. There have also been a number of modifications - most of these are of little significance from the servicing point of view, though it's worth noting that a simplified field generator stage is used in later production.

Power Supply Circuit:
 As with any set, the power supply is the key to what goes on. The circuit of the TX's power supply, which consists basically of a transformer -fed mains bridge rectifier followed by a series regulator, is shown in Fig. 5. This is conventional though there are one or two points worth noting. First, one of the diodes in the bridge rectifier circuit, D110, also serves as the reverse polarity protection diode on battery operation. Switch SK2 is part of the battery input socket. This can cause problems, as we shall see. Secondly the error detector/amplifier transistor TS112 is operated from the line output stage derived 26V boost rail. This provides protection against excessive voltages in the line output stage, since excessive boost voltage will cut off TS112 and in turn TS111 and TS110. In the event of line output stage failure, TS112, TS111 and TS110 will again be cut off. The result could be excess voltage on the 10.8V line which will also be unstabilised, i.e. fed via RHO only. This would damage the tube, whose heater is connected across the 10.8V rail. To avoid this situation, diodes D115 and D116 conduct when the line output stage is not working, thus reducing the voltage on the 10.8V line. These diodes were not fitted in early production sets. The fourth transistor TS113 provides the tuner with a stabilized 11.3V supply. The tuning voltage is stabilized by a TAA550 in the usual way.

Line Timebase:
The line generator circuit (Fig. 6) is rather unusual. The first transistor TS380 provides the flywheel sync action: a line -frequency sawtooth is applied to its emitter while the line sync pulses are applied to its base. Following the flywheel sync filter, TS392 sets the voltage conditions in the line hold control network. The oscillator itself consists of TS390 and TS391 which are connected in an emitter coupled astable multivibrator configuration.






The driver and output stages (Fig. 7) follow normal practice. D450 is the efficiency diode, D451 the boost diode, C451 the boost reservoir capacitor and C450 the flyback tuning capacitor. The output stage provides 9.5kV e.h.t. for the tube, a 350V supply for the tube's first anode, a 95V supply for the video output stage and the tuning system, and the 26V boost line.










 No Sound or Raster:
If there's no sound or raster, check the voltage at the emitter of TS110. If there's no voltage here, check the fuses - VL100 (on the mains transformer), VL110 and VL111. If VL100 or VL110 is open -circuit, check the bridge rectifier diodes D110/111/113/114 and the protection capacitors C116-9 for shorts and if necessary the mains transformer T110 for shorted turns. If VL111 is open -circuit, the 1.t. reservoir capacitor C112 could be leaky. Alternatively there could be a short-circuit in the line or sound output stage. Check the output transistor TS450, then D450, C450 and the scan coupling capacitor C455 in the line output stage. Check the smoothing capacitor C314 (47μF) and the output coupling capacitor C311 (100μF) in the audio output stage. If the fuses are o.k., check the voltage at the collector of TS110. If this is low at 2-8V, check TS110, TS111 and TS112 as necessary. If the voltage at the collector of TS110 is more than 8V, check the boost voltage - at pin 6 of the line output transformer. If the voltage here is less than 15V, check the line output transistor, check whether C455 is leaky, then check the line output transformer by substitution. If the voltage at pin 6 is in excess of 15V, check whether R451 is open -circuit, thus removing the supply to the line oscillator. In the event of R451 being open -circuit, check for shorts in the field generator circuit. If R451 is o.k., check the voltage at the base of the line driver transistor TS410. The reading should be about -0.1V. If this is present, check TS410 and TS450. If the reading is absent, check whether R401 is open -circuit, thus removing the supply to the line oscillator stage. Finally check TS390, TS391 and TS410 by replacement.

Normal Sound, No Raster:
For the sound normal, no raster condition, first check whether the tube's heater is alight. If not, check the continuity of the heater winding. Next remove the aerial plug. If there's insufficient brightness, check the a.g.c. amplifier transistor TS351 (BC548) by replacement. If there's still no brightness, turn the contrast to minimum, the brightness to maximum, and make voltage checks at the c.r.t. base. The cathode voltage (pin 2) should be 67V. If this is incorrect, check the video output transistor TS560 (BF422) and if necessary the field flyback blanking transistor TS565 (BC548C). If the voltage at pin 2 is correct, check the grid voltage (pin 5) which should be about 57V. If this voltage is missing, check whether the grid decoupling capacitor C572 (0.1μF) is short-circuit, then check whether the 95V supply is being developed across C452. If not, check R450 and D453 for being open -circuit. Next check the first anode voltage (pin 6) which should be 160V. If not, check R570 (820kOhm), R452 and D455. Finally check the e.h.t. circuit if necessary - from pin 8 of the line output transformer through the rectifier to the final anode of the c.r.t. Normal Sound, Weak or No Picture In the event of normal sound with a weak picture or no picture, check the voltage at the emitter of the video output transistor TS560. This should be 3.3V. If incorrect, check TS560; if correct, check the video driver transistor TS350 (BC558).

Field Collapse:
In the event of field collapse, check whether the field output stage feed resistor R529 (33n) is open -circuit. If so replace it and check the output transistors TS521/2 (BC338/BC328). Next check the field output stage midpoint voltage - 10.1V at the emitter of TS521. If this is incorrect, check the output transistors, the field driver transistor TS523 (BC548) and the preamplifier transistor TS520 (BC559B). If necessary check the scan coupling capacitor C527 (100μF) and the earth return resistor R527 (4.7Ohm), then suspect the field generator circuit. Check the transistors TS505/TS509/TS515 and the values of resistors R515 (470kf1), R517 (3901(n) and R516 (height control - 220kn). The original field generator circuit is shown in Fig. 1. The charging capacitor C503 charges from the 26V rail via R503, R507 and the constant -current transistor TS505. When the ramp at the emitter of TS505 reaches the voltage at its base, set by R502/4, TS505 switches off. The positive -going voltage at its collector then switches TS509 on, and in turn TS515 to discharge C503.

 The later simplified circuit is shown in Fig. 2. This time C503 charges from the 95V line via R503 and R507. When the voltage at the junction of R503/7 exceeds the voltage at the base of TS509, both transistors switch on as before. In normal operation the positive -going field sync pulses fed to the emitter of TS509 drive this transistor on just ahead of the free -running switch -on -point.

Loss of Line Sync:
In the event of loss of line sync, first remove the aerial input and check that the 1.t. line is correctly set for 10.8V. If the correct voltage cannot be obtained by adjusting R113, check TS110, TS111, TS112 and make sure that R114 is 3.9Ohm (in some sets it's 4.7MOhm). If the supplies are correct, check the voltage at the positive side of the a.g.c. smoothing capacitor C351 (47μF). With the aerial discon- nected the reading should be 4.3V. With the aerial connected a reading of 6-8V should be obtained. If the voltage conditions are incorrect, suspect the a.g.c. amplifier transistor TS351 (BC548). If necessary, try adjusting the line hold control R394 with the emitter of the flywheel sync transistor TS380 shorted to chassis and the aerial connected. If line lock cannot be obtained, replace the line oscillator transistors TS390 and TS391. If line lock can be obtained but the sync floats on removing the shorting link, suspect TS380 and TS392.

Miscellaneous Faults and Modifications:
Sound buzz with unstable picture, possibly intermittent: Suspect the battery socket - the switch can become tarnished. Replacement cures. Uncontrollable sound: Suspect the d.c. volume control R302 (4.710 or the TBA120AS intercarrier sound chip (IC310). Intermittent line collapse, with vertical line: Change C393 to 0.0015μF. Philips advise that the value of C393 in all sets bearing factory code HU on the chassis or serial plate is checked and changed to 0.0015μF if necessary. Bright vertical line at left-hand side: If a replacement line output transformer does not cure this, change C412 to 0.006814F and TS410 to a BC637 (note that the base connections differ). Distortion at low volume: Change R300 to 18kOhm, R311 to 56f1, R312 to 3.3kOhm/ and R315 to 120kOhm. Brightness range: Where the tube is type 12VCUP4, R576 should be 470kOhm Where the tube is type 12BJP4 it should be 820kOhm.