A blanking circuit to eliminate retrace lines on old B/W TV sets.
For example, let’s consider the partial schematic of a Philips TV, model TF1446A (introducing in 1953) :
In red area, here is the frame oscillator, at 50Hz in Europe. This is a blocking oscillator synchronized by the pentode on the left, the synchro separator.
At the output of the oscillator, the signal is near a sawtooth, and is applied to grid of the power pentode, via correctors (to obtain parabolic signal). In same time, the output is used to blank CRT, via condensator C : the negative edge of the sawtooth applies a negative voltage on A2 which cuts off electronic beam of CRT.
This circuit did work in 50’s. But now, first lines of the frame (from 1 to 22 and 313 to 334 in 625L standard) are used to carry sevral informations (teletext, clock, DVD anticopy …). Theese informations are logical, then contains numeric bits which are translated in analogic to black or 100% white level.
When bits are 100% white, they are really visibles and they occur during return fly of the spot, so, the picture is polluted by white (or clear grey) lines at the top of the screen.
To eliminate theese lines, none passive circuit is suitable, because, it is necessary to cut the crt beam during exactly the time between frame synch and the start of composite video. In other words, we have to make a circuit which gives a rectangular, going-down voltage triggered by negative edge of frame sawtooth.
This circuit is then a monostable, the duration of which is about 1.2msec.
Consider its schematic :
We can use an AF twin triode, like 12AU7. It is wired in cathod coupling mode, and is synchronized by applying a negative going edge on grid of the left triode. At this moment, the right triode is blocked and this state duration is calculated by C404 and R405 values. On schematic, the noted values give a time delay of 1.2 msec.
It is necessary to use a tube heating current of which is in accordance with general heating of the TV set ! In Europe, we had two methods to heat TV tubes : in parallel, and in this case, voltage is 6.3V, or in series, and then, the current is 0.3A in the chains. So, we had electronic tubes in the « P » serie, like : PCF80, PL82, PY82 … I know that in USA, there were several heating currents : 0.45, 0.6A …not in Europe !
So, in parallel mode, no problem : the twin triode will be heated like the other tubes of the set. In series mode, it is necessary to cut one chain and insert it. This will decrease a little the voltage on each tube, but will be without inconvenience. But, the choice of the twin triode is very important for heating.
The monostable circuit is installed where there is free place on chassis, near the frame oscillator stage of the TV.
It is necessary to add a resistor, R, betwwen brigthness pot and wenhelt grid of crt to apply blanking signal via the capacitor C’.
The monosable must be synchronized on negative going edge of sawtooth. The best way is to take the rectangular signal on anode of the pentode, on the resistor Ra. But it is necessary to eliminate positive going edges which would reset the monostable : it is sufficient to use a diode D.
Here is the modified schematic where you can see the monostable and the added components :
Blanking circuit in the TV set
No adjustement is required, except if you want a different value for delay time. In this case, increasing R405 will increase delay time in the same ratio !