Working on my RCA 21D645 KCS97 again

[rca21d645]

Originally Posted by old_tv_nut

Is this the first time you've looked at the VCR signal on this set? It could be the recovery from the head switching transient that many pre-VCR sets suffered from.

Is it steady, or does it wave from side to side? Waving = head transient. Perfectly steady = some coupling from vertical sweep into horizontal, possibly a bad power supply decoupling cap.

Its wavy, and after trying the tv set with some dvds the picture looks fine. I never knew that some of these sets just couldn't handle video tapes.

[old_tv_nut]

This is because the horizontal AFC was designed to be very slow (low bandwidth) to reduce effects of random noise, but then it couldn't respond quickly to the large timing transient after head switching. It is possible to change the time constants in the horizontal AFC to speed it up, which is what TV makers did after the introduction of VCRs. It makes the horizontal a bit more jittery under fringe reception conditions.

[rca21d645]

Originally Posted by old_tv_nut

This is because the horizontal AFC was designed to be very slow (low bandwidth) to reduce effects of random noise, but then it couldn't respond quickly to the large timing transient after head switching. It is possible to change the time constants in the horizontal AFC to speed it up, which is what TV makers did after the introduction of VCRs. It makes the horizontal a bit more jittery under fringe reception conditions.

Just out of curiosity, how do I go about modifying the tv this way?

[old_tv_nut]

The best thing is to know the theory of AFC loops, measure what's in yours in terms of the open loop detector gain and oscillator control gain, and calculate the changes required to get quicker response and still have good damping.

Without knowing the theory, experimental determination is possible, but you have to be good at discerning what the changes you make are doing so that you can converge on a solution rather than making things worse.

The result will mostly involve reducing the size some of the filter capacitors between the phase detector and the oscillator, and perhaps changing some resistor values there also.

If you measure and calculate correctly, you will get the desired result without fail, but I have seen engineers struggle with it, generally due to botching the open loop measurements.