Zanussi 22ZT907 Factory adjustment.TELETEXT-DECODER ADJUSTMENT
The teletext p.c.b. requires three alignment operations which are all relevant to the video input processor SAA 5030.
These alignment operations eliminate influence of tolerances both Of video input processor and of its peripherical components.
The adjustment procedure need not be repeated any more until no component substitutions are carried out and until the same video input processor is used.
1. Adjustment of the chronological position (phase) of synchro. pulses Cronological position (phase) of vertical sinchro. pulses supplied by the video input processor SAA 5030 is adjuted through trimmer PlT.
The video signal phase (TP8), and in particular the portion including the vertical synchro. signal must be compared on the oscilloscope
against the output signal taken from contact 13 of IC SAA 5030 (TP 10).
The trimmer is adjusted so as to delay the leading edge of the video signal pulse (on contact 13) 48 2 5 )H with reference to the start of the vertical synchro. signal (wide pulses).
This position is shown in Fig. 1.
The leading edge of the vertical synchro. pulse must thus be at 3/4 of line on line 1 and at a l/4 of line on line 314 of video signal.
2. ADJUSTMENT OF QUARTZ CONTROLLED FREQUENCY
The free running frequency of the oscillator is adjusted by means of a capacitive trimmer C26T connected in series with the quartz.
The oscillator is free running when pin 1 of SAA 5030 circuit is directly connected to +12 V (jumper between TPl and TP2) and pin 7 is connected to +12 V via a 5.6 M Ohm resistance (jumper between TP3 and TP4).
Video signal (TP8) on oscillator is compared against sand-castle pulse present on Pin 5 of SAA 5030 (TP 7).
Capacitive trimmer is adjusted so as to maintain ratio between the two signals unchanged (i.e. find the position in which relative shift between the two waves is less emphasized).
Remove again shortcircuiting jumper and 5.6 M Ohm resistance.
Oscillator free running frequency is encreased by means of this resistance from approx. 0.5 kHz to over 6 MHz, this assuring that signal quality detector provided in circuit SAA 5030 operates in the
For capacitive trimmer adjustment it is also possible to use the same teletext reproduction.
Once again it is necessary to connect the short circuiting jumper and the 5.6 MHz resistance as described above.
By means of remote control the teletext decoder is switched to mixer operation.
Page memory does not necessarily have to be occupied with a teletext page, but it is possible to use randomly selected characters contained in memory which are sufficient for adjustment by means of
reproduction on the screen.
Capacitive trimmer is adjusted so that characters occupy a fixed position on the screen.
Remove once again the shortcircuiting jumper and the 5.6 MHz resistance.
If, by means of capacitive trimmer adjustment, characters do not reach a condition of rest, it is sufficient to adjust for minimum character shift.
Photo credit : monotreal.com
3. ADJUSTMENT OF OSCILLATING CIRCUIT COIL FOR DATA CLOCK
Tune to a TV station having teletext signals; the signal must be sufficiently strong (>lmV) and free of reflections and interferences of a certain intensity.
Select a teletext page with continuous data renewal and damp signal until a few errors appear.
Adjust L4T so as to make character errors disappear or for the least number of them.
Use a teletext generator having a pseudo-random sequence.
Connect the oscilloscope's probe to pin 18 of SAA 5030 (CLOCK output) and synchronize it externally with clock signal of teletext generator.
Adjust L4T for a stable clock signal.
Select page III; check for completeness and exactness of the heading line and of page characters.
Check functions of TELETEXT transmitter.
ADJUSTMENT OF RGB LEVELS ON TELETEXT P.C.B
Adjustment must be carried out before "white adjustment".
1) Adjust P4T and P3T on R and G so that characters reach the 100 V DC level.
2) Adjust P2T on B so that the character reaches the 110 V DC level.
After white adjustment visually inspect that white characters have correct tint, otherwise, retouch one or two of the potentiometers P2T, P3T and P4T (reducing amplitude of dominant colours) so as to obtain characters having correct white.