Remove Teletext Lines & VCR Problems
With the arrival of Teletext ( Ceefax) and Video cassette recorders in the early 70’s this was a frequent bug many sought a solution for. This article will provide the various mods you need to perform to remove the text lines and VCR related problems from Vintage CTV’s.
This one document will list the various sets and the modifications to be made. As and when I come across mods in the tele mags, I will add them. If in the meantime any readers have others, please leave them in the comments section below.
Up to 6 line Teletext interference – 3000/3500 series CTV.
- Replace R425 with an 82 Ohm (was 120)
- Replace R441 with a 5.6 kOhm (was 2k2)
- Replace R442 with 39 Ohm (was 58)
- Replace C429 with 10uF (was 16uF)
- Replace VT422 with BC107A (was BC107B)
- Replace VT421 with an SGS device (was Texas).
No satisfactory modification is considered possible if the number of Teletext lines increases beyond six.
To make a G8 VCR compatible the panels have the flywheel sync time-constant altered as follows:-
This modification can be carried out on all panels.
- R4495 changed from 33R to 47R
- C4496 from 47uF to 10uF
- C4498 deleted.
- Finally on the I.F. panel connect a 10k resistor between pins 3 and 5 of IC2001.
Removing Teletext lines on the G8 chassis :
Teletext shadow is the term that has been applied to indicate the effects of secondary emission created by the presence of lines of teletext data at the top of the CRT screen. These effects can be described in many ways, the two most common being a dark band across the top portion of the picture, and two areas of brighter picture also towards the top.
To remedy the effect, which is aggravated by CRT and associated circuit manufacturing tolerances together with the picture position and height adjustment, it is necessary to extend the field blanking by adding a series resistor into the base of T4488 (type BC148) on the timebase panel. This may be achieved by cutting the print which joins the base of T4488 to the junction of R4486/C4515, then connecting the resistor across these points.
The value of this resistor must be selected to provide adequate blanking of the teletext data yet avoiding the clipping of the start of the picture. The value will generally be between 220 Ohms and 470 Ohms 0.5 Watt rating, depending on the timebase panel and deflection coil tolerances.”
On the Philips G6 text lines etc are blanked out with the top 1″ of the picture darker than the rest.The problem can be rectified by changing C4053 from 0.027uF to 0.01uF. If the problem persists check X7323 (BA148)
Philips KT3, KT30
Apparently the black -level clamping problem (random horizontal bars) has been encountered with later versions of the Philips KT3 and K30 chassis that use the TDA3560 single-chip decoder. The recommended modification is to increase the value of the clamp reservoir capacitors C44/5/6 from 0.022μF to 10μF. This problem could occur with other chassis using the TDA3560 – one thinks of the Thorn TX10 (clamp capacitors C616/7/8) and later versions of the TX9(C64/5/6) for example, where the capacitors are 0.1μF.
The mod is detailed in “Television”, May 1982, “TV-VCR Compatibility” by Eugene Trundle:
- If it has the A803 Scan Drive Panel, then Change 5C12 to 0.01uF and 5C13 to 680pF.
- If it has either the Z504 or Z968 scan drive panel, remove 5C12. Change 5C10 to 1.5uF and 5R15 to 4K7.
- If jitter ensues on off air reception, change 5C15 to 0.068uF.
- Replace 4R3 with 2 120K resistors.
- Replace 4VT2 with a BF337 or equivalent.
- Disconnect 4VT2 Collector and leave the leg on the component side.
- Solder in the 3 additional components (33K resistor, .03uF 250V capacitor, and BA148 diode )
- Replace 4R4 with another BA148 diode.
PYE 691, 693 & 697
Slightly re adjust RV24, just enough to remove dot pattern but without introducing a field linearity error. If this does not work change the value of R257 to 1K and repeat the adjustment of RV24. Further improvement may be achieved by adding a 100nF capacitor between the anode of D44 and chassis.