I have read many posts here regarding the use of modulators to enable vintage televisions to work in the absence of suitable broadcast signals. It seems to me there is little point in going to the trouble of modulating the signal so it just has to be demodulated again .There is another way.

The composite video signal can be injected directly into the grid of the video amplifier valve. The advantage of this is that there is no need for a modulator, and no need to maintain the tuning or IF circuits. The disadvantage is that composite video signals on modern equipment usually operate at 1v peak to peak whereas 2 or 3 volts are typically required for the video amplifier stage of vintage televisions. Therefore it will be found that contrast is severely lacking giving rise to a washed out looking picture.
The solution to this problem is the construction of a simple supplementary video amplifier circuit.

R1 = 150 1/4W
R2 = 2200 1/4W
R3 = 4700 1/4W
R4 = 4700 1/4W
R5 = 68 1/4W
R6 = 330 1/4W

P1 = 22000 lin (A)
P2 = 2200 lin (A)

C1 = 100 à 220 µF/25V
C2 = 100 à 220 µF/25V
C3 = 47 à 220 nF/63V
C4 = 47 à 220 nF/63V

T1 = BC182
T2 = 2N2907A

D1 = 1N4003
DZ2 = BZX55C10V

E = Entrée (input) S = Sortie (output)
P1 = output voltage level (lightness)
P2 = gain (contrast)


The schematic shown was taken from the French website

http://www.cfp-radio.com/.

Further details regarding its construction can be found there, by clicking on Realasitations on left, scrolling down to the television section and clicking on “Preamplicicateur video pour anciens teliviseurs monochromes”

It can be made as a standalone unit or incorporated into a TV set. A composite input source is connected to the input from freeview box or DVD player etc and the output to the grid of the video amplifier valve, without a capacitor.

This circuit has been built by the writer and installed in an HMV F5.After a bit of fiddling with the controls it gave a beautiful clear picture. The gain control was wired to the front panel to act as a contrast control and the lightness pre-set to an appropriate level which was found by trial and error by adjusting the gain (contrast) and the original brightness control which was left intact. A separate mains transformer was used to supply power however depending on the wiring layout of the valve heater circuit it may be possible to derive adequate voltage from that.

Though there is a capacitor on the input, it appears that the DC component of the video signal is effectively restored by the amplifier giving black blacks and white whites.

The sound is quite simple - an RCA lead is connected to the input of the sound amplifier, usually just before the volume control. It is advisable to connect two RCA leads together in the case of stereo output from the device concerned.

There’s also a perceivable delay when you modulate and demodulate the signal for nothing.
Enough to get the audio misaligned if it was coming more directly from the source.

Thanks for posting this, I've been looking for something like this for a while.

Is there a way of isolating the ground from a live chassis? I have several transformerless sets I'd like to try this on and don't want to fry my set top box.

Cheers,

Adam

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