Meck XB-702 restoration

[Kevin Kuehn]

Originally Posted by old_tv_nut

This is interesting, but I don't understand the DVD player case, because a DVD player should not be locked to the line.

This was about 12 years ago so I may be mistaken about the DVD, but that was my recollection. Is it possible the Agile modulator was reestablishing the sync to the AC line? I don't exactly understand the mechanism that allows this sync swim to happen, is it strictly related to the vertical sync beating against any AC ripple riding on the set's B+? When it affects the vertical lines I'd have to assume it's also messing with the horizontal lock? What we need is a good book that focuses on this issue.

[Kevin Kuehn]

Originally Posted by bandersen

OK, I injected to the grid if the 2nd IF. Now, the image is stable.

So this is really interesting.

[bandersen]

Here are some waveforms in the sync circuit.

First, the output of the sync eparator.


That signal goes through a 0.05uF cap then through a long wire to the deflection circuit. The red pointer is this cap.
The scope is hooked up to the other end of that long wire.
Could it be picking up hum?
Also, note that the sync separator is right next to the power supply. No shielding on the tubes.


Here's the other side of that cap


Here's the resulting sync signal on the grid of the horizontal multivibrator.

[bandersen]

There is hum mixed in with the horizontal sync pulses. Seems likely that could affect the lock.

[Kevin Kuehn]

Mr Carlson would probably tell us it's because we didn't pay any attention to the outside foil.

[vortalexfan]

Originally Posted by Kevin Kuehn

Mr Carlson would probably tell us it's because we didn't pay any attention to the outside foil.

Yes he would, and I think in some cases that could make a difference but in something like this (and in most of the recap jobs I've done) it didn't really seem to make much of a difference functionality wise if the caps were installed one way or the other.
I think its because the more modern mylar caps have better shielding than the old paper caps of 50+ years ago did, because otherwise why would they go to the trouble of not marking the "outside foil" on modern caps, if it mattered as much as Mr. Carlson said it does?

Don't get me wrong, I love Mr. Carlson's Videos, and how thorough he is and his great explanations of why they did things the way they did back then, but sometimes I think he's a little too much of a "salesman" when it comes to his inventions and his electronics crash courses.

[bandersen]

Exactly. Also, any circuit that incredibly sensitive to noise should be in a shielded box like a tuner is.

[old_tv_nut]

Hum in the sync separator should make the whole picture move, but what you are seeing is the sweeps changing in amplitude. For example, the vertical has a squeezed area and a stretched area that drift upward, and the horizontal moves much more on the right than the left, which means the width is changing.
So, I am at a loss as to why this isn't reduced by filtering the supply voltages. Something is not adding up for me.

[old_tv_nut]

A further thought - does the width change when you turn the horizontal sync control (not enough to lose lock, but from one end of locked to the other end of locked)? If so, that would indicate that the hum on the sync signal *could* have a similar effect.

[old_tv_nut]

Originally Posted by Kevin Kuehn

... Is it possible the Agile modulator was reestablishing the sync to the AC line? ...

No, that would require a frame store to resync the video frame. The modulator only takes whatever video it's given and puts in on the video RF carrier.

[bandersen]

Originally Posted by old_tv_nut

A further thought - does the width change when you turn the horizontal sync control (not enough to lose lock, but from one end of locked to the other end of locked)? If so, that would indicate that the hum on the sync signal *could* have a similar effect.

No, the width does not change but it does shift the image a bit.

[old_tv_nut]

Originally Posted by bandersen

No, the width does not change but it does shift the image a bit.

Well, that does seem to be a big part of the horizontal problem then, since the horizontal motion is mainly a shift (and I could be imagining I saw a size change in that short clip).

[Kevin Kuehn]

Is it possible the vertical is modulating the horizontal through the power supplies? Could be interesting to try placing a filter cap between the various supply voltages instead of to ground.

[Notimetolooz]

Originally Posted by Kevin Kuehn

This was about 12 years ago so I may be mistaken about the DVD, but that was my recollection. Is it possible the Agile modulator was reestablishing the sync to the AC line? I don't exactly understand the mechanism that allows this sync swim to happen, is it strictly related to the vertical sync beating against any AC ripple riding on the set's B+? When it affects the vertical lines I'd have to assume it's also messing with the horizontal lock? What we need is a good book that focuses on this issue.

I believe the problem is mostly effecting the horizontal sweep, probably at the horizontal oscillator. The hum effects the exact time that the oscillator output changes direction. The hum voltage effects when the horizontal osc. triggers in other words. The change in timing produces a kink in the sides of the picture. It appears to be a vertical problem because the location of the peak of the hum relative to the vertical waveform slowly changes (unless the vertical is synced to the hum), the kink moves.
There may be a similar problem in the vertical circuits but I think that the problem is less severe.

[bandersen]

The image is definitely being distorted in both directions.
Here's another clip showing several test patterns.
https://youtu.be/iN_NH-AnysQ

Originally Posted by Kevin Kuehn

Is it possible the vertical is modulating the horizontal through the power supplies? Could be interesting to try placing a filter cap between the various supply voltages instead of to ground.

I can try, but the voltages are getting a bit high if go between the 300/350 volt and -120 volt rails. I guess I'll put a couple caps in series.