Bad composite video signal

[wasedadoc]

I have noticed something else strange in the three waveforms in Reply #5. The first one, composit video and the second one, luminance (no chroma) show the 600 kHz signal increasing with higher video levels and almost completely absent at sync tip.

But the third waveform, the chroma one, which also seems to be displayed at the horizontal rate and locked to horizontal sync (note below), shows the 600 kHz signal all the way across the display. There is no place where it goes nearly to zero at the tip of sync.

It is not there (at sync tip) in the composit signal, which is the combination of luminance and chroma but it is there in the chroma signal. That does not make sense. If it is present in the chroma and not in the luminance, then the sum of those two should show it present. You can't cancel the signal present in the one with the lack of signal in the other.

Accurate DC levels are very important to the luma component of an analog TV signal.
If such a signal passes through an ac coupled amplifier or network, it loses the reference DC levels like black level, blanking, & sync tips.
These need to be restored by a process called "DC restoration".

Historically, this was done in various ways, but over time, the other methods have fallen into disuse, leaving the almost universal use of "keyed clampers" which produce a keying pulse synched to the line syncs, & either delay it a bit over 5us to be coincident with the "back porch", (the most common method) or somewhat less so it still falls within the sync pulse duration.

The keying pulses will cause the keyed clampers to hard switch either of those two video levels to a selected DC reference value during the duration of the clamp pulse, & would normally remove any signals riding on that DC level, however, keyed clampers used on the "back porch" of  composite colour signals  incorporate "traps" parallel resonant at the colour subcarrier (soft clamping), so as to not "mangle" the colour burst.
Sync tip clamping, which, for a standard 1V p-p video signal, clamps the sync tip to a DC level of around -300mv w.r.t zero volts (exactly that for PAL) does not require such traps,

Perhaps Sony decided on the use of sync tip clamping to reduce the complexity of having to provide LC "subcarrier traps", & further decided to clamp the luma component both before & after it was combined with the chroma signal.

The Chroma signal doesn't have a DC level to be clamped, so the interfering signal isn't "clamped out".

While that treatise on clamps is true it is irrelevant here.  All the signals into that buffer IC (which is the last active stage before the output sockets) are AC coupled into it and there is no clamping pulse input to that chip.  (Datasheet at https://www.datasheets360.com/pdf/-2973768386555007626 has block diagram of internals). Not really surprising as there is no need to have clamped signals at the output. No video equipment (home, pro or broadcast) that I have ever worked on assumes or requires that the signals presented to its external input sockets are clamped to any value.

[vk6zgo]

While that treatise on clamps is true it is irrelevant here.  All the signals into that buffer IC (which is the last active stage before the output sockets) are AC coupled into it and there is no clamping pulse input to that chip.  (Datasheet at https://www.datasheets360.com/pdf/-2973768386555007626 has block diagram of internals). Not really surprising as there is no need to have clamped signals at the output. No video equipment (home, pro or broadcast) that I have ever worked on assumes or requires that the signals presented to its external input sockets are clamped to any value.

OK, but in that case, how come the interference signal is not present on the very parts of the signal that are likely to be clamped if clamps were used?
The point was that my "treatise" was one explanation of why this could be so.
Another could be that the syncs are deliberately clipped.

In any cases, it probably was a "red herring" if all the signals go through the same IC.

[timenutgoblin]

2.  Ignore the suggestion of a notch filter.  Even if it reduces the interference it will have a very bad effect on picture quality.

Thank you. I guess I can notch that up as a poor suggestion to the O.P.

[Gominalo]

1. All the analogue video outputs go through the video buffer chip IC101. Page 4-36 of the Service manual. There is also a DC-DC converter (not the main power supply) at the bottom of the schematic on that page.  Is that running at the same frequency as the interference?

(I once had a problem with a Hi-8 video deck giving distorted audio playback.  DC to DC converter for the vacuum fluorescent display was working very hard because the electrolytic on its output rectifier had dried up and gone really low capacitance.  The extra strong switching frequency from that converter was getting into the audio subcarrier retrieval and demod circuits.)

2.  Ignore the suggestion of a notch filter.  Even if it reduces the interference it will have a very bad effect on picture quality.

You got it,
I checked every voltage regulator i found and IC503 that is used for providing IOAVDD supply for the video decoder chip (IC502) and also for the VREF manually adjustable with a little pot had that 555KHz noise coming out of it, so y desoldered the electrolitic cap and it was bad (see picture).

Now the next problem is to fix the drive because it cannot read CD or DVD and when i try to auto adjust it fails, also i try manual adjustment but trying to power up all controls and motors the sled doesn't, CLVA and focus search it does but drive spins like crazy and hole system powers off.

If anyone have any hint i will be glad to hear all your suggestions.

Thank you.

[wasedadoc]

Those SMD electrolytics are notorious.

[EPAIII]

I have seen video clamping problems many times. None of them looked like this problem.

I have noticed something else strange in the three waveforms in Reply #5. The first one, composit video and the second one, luminance (no chroma) show the 600 kHz signal increasing with higher video levels and almost completely absent at sync tip.

But the third waveform, the chroma one, which also seems to be displayed at the horizontal rate and locked to horizontal sync (note below), shows the 600 kHz signal all the way across the display. There is no place where it goes nearly to zero at the tip of sync.

It is not there (at sync tip) in the composit signal, which is the combination of luminance and chroma but it is there in the chroma signal. That does not make sense. If it is present in the chroma and not in the luminance, then the sum of those two should show it present. You can't cancel the signal present in the one with the lack of signal in the other.

Accurate DC levels are very important to the luma component of an analog TV signal.
If such a signal passes through an ac coupled amplifier or network, it loses the reference DC levels like black level, blanking, & sync tips.
These need to be restored by a process called "DC restoration".

Historically, this was done in various ways, but over time, the other methods have fallen into disuse, leaving the almost universal use of "keyed clampers" which produce a keying pulse synched to the line syncs, & either delay it a bit over 5us to be coincident with the "back porch", (the most common method) or somewhat less so it still falls within the sync pulse duration.

The keying pulses will cause the keyed clampers to hard switch either of those two video levels to a selected DC reference value during the duration of the clamp pulse, & would normally remove any signals riding on that DC level, however, keyed clampers used on the "back porch" of  composite colour signals  incorporate "traps" parallel resonant at the colour subcarrier (soft clamping), so as to not "mangle" the colour burst.
Sync tip clamping, which, for a standard 1V p-p video signal, clamps the sync tip to a DC level of around -300mv w.r.t zero volts (exactly that for PAL) does not require such traps,

Perhaps Sony decided on the use of sync tip clamping to reduce the complexity of having to provide LC "subcarrier traps", & further decided to clamp the luma component both before & after it was combined with the chroma signal.

The Chroma signal doesn't have a DC level to be clamped, so the interfering signal isn't "clamped out".

[EPAIII]

The problem is not absent at sync tip or the level of the back porch, it is just lower there than at the white levels.

What I see in the problem "signal" is that it is demonstrating what is called differential gain. Differential gain in a circuit occurs when the gain is a function of the DC level. It is a frequent problem with the chroma component of composit video, but can occur in the luminance signal too. There are test waveforms to detect just this problem.

Your clamping may, actually probably does occur inside that chip. It is almost a requirement of any composit video processing. Often the clamping signal is derived from the luminance signal after the composit video is separated into the two components (luminance and chroma). But I will say it again, this does not look like any clamping problem I have ever seen.

While that treatise on clamps is true it is irrelevant here.  All the signals into that buffer IC (which is the last active stage before the output sockets) are AC coupled into it and there is no clamping pulse input to that chip.  (Datasheet at https://www.datasheets360.com/pdf/-2973768386555007626 has block diagram of internals). Not really surprising as there is no need to have clamped signals at the output. No video equipment (home, pro or broadcast) that I have ever worked on assumes or requires that the signals presented to its external input sockets are clamped to any value.

OK, but in that case, how come the interference signal is not present on the very parts of the signal that are likely to be clamped if clamps were used?
The point was that my "treatise" was one explanation of why this could be so.
Another could be that the syncs are deliberately clipped.

In any cases, it probably was a "red herring" if all the signals go through the same IC.

[EPAIII]

Yup! I am at the point where, when repairing consumer electronics, I just take a "shotgun" approach to the electrolytics. I replace them all FIRST, before wasting my time on real troubleshooting.

Glad to see that problem fixed.

Those SMD electrolytics are notorious.

[vk6zgo]

The problem is not absent at sync tip or the level of the back porch, it is just lower there than at the white levels.

What I see in the problem "signal" is that it is demonstrating what is called differential gain. Differential gain in a circuit occurs when the gain is a function of the DC level. It is a frequent problem with the chroma component of composit video, but can occur in the luminance signal too. There are test waveforms to detect just this problem.

Your clamping may, actually probably does occur inside that chip. It is almost a requirement of any composit video processing. Often the clamping signal is derived from the luminance signal after the composit video is separated into the two components (luminance and chroma). But I will say it again, this does not look like any clamping problem I have ever seen.

Did I ever say it was a "clamping problem"?

My point was that, far from a "fault" the clamps (if present) were working as they should.
I have seen very noisy signals where clamping had been applied, with just as in this case, some residual noise remaining.

In video stabilising amplifiers, this can be removed by stretching the syncs & clipping them, but usually if the signal is that noisy, it is unuseable, even though functional syncs & blanking can be restored.

In this case, however, it seems the noise was present on the DC supply, so was still present to some extent no matter what is done within  the chip.

"Differential Gain'?-----Yes, I am familiar with this concept, having tested video equipment & TV transmitters for this many times over decades.

Interestingly, in Australia, a distinction is made between (1)"Differential Gain" which is related to, & tested with a stairstep luma signal with a constant level of chroma carrier superimposed on it, & (2)"Line Time Luma Non Linearity" which can use a 1MHz signal superimposed upon a stairstep, or more commonly, a plain stairstep where the video output of the DUT is differentiated & the height of the resultant "spikes" compared to each other on a special graticule.

The signal for (1) is also used to measure "Differential Phase", where colours in the displayed image may be displayed incorrectly at various luma levels.

[Gominalo]

Hello,
I am still checking the optical issue.
The laser has 24h of play time for CD amd 244h for CD. And that's too low for a laser aging problem right?.

I am changing caps like crazy, all of them have an ESR abobe 10ohms. The only problem is that i only have through hole, but good quality ones.

Thank you for all you messages.