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Old 13th Oct 2017, 6:41 am   #49
Argus25
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Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
Posts: 2,679
Default Re: My first antique TV-Olympic Model 755-need help.

Quote:
Originally Posted by ct92404 View Post
I noticed on the diagram something called a "sound trap." I read a little online and is it supposed to help to separate the sound out of the video signal?
Chris, once the sound IF carrier signal is split off after V8 (in the resonant circuits L5) it is better that any remaining sound IF carrier signal (at 21.25 MHz) is not amplified by the video IF amplifier, otherwise it will or could appear as sound modulation in the picture.

So you will notice a cathode trap L7 in the cathode of V10. This raises the impedance in the cathode of V10 at the sound IF frequency of 21.25MHz, crippling V10'S ability to amplify it to any great extent.

In addition, there is another trap to help eliminate sound interference, it may seem odd that it is 4.5MHz, L11 resonat circuit in the anode of V12 pin1. This is because, at that point in the circuit, the video and sound IF carrier frequencies, which in the USA system, are 4.5MHz apart, just as they are for the transmitted signals from the TV station, can beat together at 4.5MHz.

You will also notice that once the sound IF carrier frequency is split off by L5, and feeds the sound IF amplifier, in that amplifier the resonant circuits L1 and L2 are tuned (peaked on) the sound IF frequency of 21.25 and they can be just peaked on that for alignment purposes. (The FM discriminator coil L3 is a bit more tricky to set up and really needs a sweep generator/scope to get it perfect, but you don't absolutely need that and you could probably get away with it by ear).

One other thing you might notice; the video IF coils are not peaked on the same frequency, for example L4 is peaked on 23MHz, L6 on 26.5Mhz and L8 on 25MHz. This is because, for a video IF amplifier, it needs a wide bandwidth to accommodate the vision signal frequency range which for practical purposes needs to be at least 4 MHz or more wide, for a clear, crisp picture. So the video IF resonant circuits are therefore "stagger tuned" even though the actual video IF carrier frequency they are being fed with(that comes out of the tuner) is 25.75MHz I think.

This is one reason why, when trying to fix this set, it is easier to get the sound channel working first, because you don't have to worry initially at least about the stagger tuning in the video IF, if say it happened to be miles out of adjustment. The sound system can be tuned up much more easily.
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