Meck XB-702 restoration

[Kevin Kuehn]

Assuming that's through the tuner, have you tried signal injection at any other points?

[bandersen]

Originally Posted by Kevin Kuehn

Assuming that's through the tuner, have you tried signal injection at any other points?

It is and, no, I haven't. The generator I'm using is for 44 MHz IF. I'd have to dig out my B&K 1077.

Would this be caused by the incoming signal? I was assuming it's an issue with some 60 cycle line voltage getting into the deflection circuit

[Kevin Kuehn]

Originally Posted by bandersen

Would this be caused by the incoming signal? I was assuming it's an issue with some 60 cycle line voltage getting into the deflection circuit

I wouldn't think, but it's always comforting to rule out the unexpected if it's not too difficult.If nothing else inject directly to the video amp. Have you tried adding another large value filter cap in parallel across the various points feeding B+ the horizontal circuits?

[bandersen]

No, not yet. Another clue is that it's affecting the vertical too as can be seen in that video clip. Perhaps it's as simple as more B+ filtering

[bandersen]

Nope, B+, B++ and B- rails are clean. I probed the oscillators and they both have 60Hz sine riding on the sawtooth waveforms. Thinking tube HK or socket leakage

[bandersen]

I switched to a better scope and took some screenshots.

Here's the ripple riding on the horizontal deflection plate sawtooth


I was mistaken, there is about 12 VPP ripple on the 350 B++ rail.


I tried tacking in various filter caps near the deflection amps. Eventually, I got up to 120uF. That brought the ripple down to about 2 volts.

However, that had no impact on the display distortion. Very frustrating.

[bandersen]

Something else I just discovered. The tuner has 14 positions! More evidence that it's not the original.

[old_tv_nut]

Did you also try filtering the +300 V and/or -120V more?

[bandersen]

Yes, no improvement. There's only about 50mV ripple on B- but it does find its way into the cathodes of both horizontal and vertical multivibrators. Then it gets amplified along with the sawtooth. I tried tacking in a couple 22uF caps on B-, but it made no difference.

The only thing I haven't tried at this point is adding more filtering to B+. Since it's not used in the deflection circuits I figured why bother, but I'll give it a try.

[old_coot88]

Just wondering.. isn't the vert sweep from your signal source(s) locked to the AC mains frequency? If so, the bend should be stationary and not undulating. Whereas an over-the-air signal would show a slowly-moving bend. Or maybe I'm having a 'senior moment'.

[bandersen]

No, the vertical of my test pattern generator is not locked to the mains

[Kevin Kuehn]

Time to dig the B&K analyst out. I believe they are synced to the AC line. I had this same weird thing going on several years ago, although not a static deflection set, but on the B&K it was rock solid. We never did come to any conclusion on how to cure it, but it's works fine with a DVD player through an agile modulator. Conclusion was it's an over the air sync issue.

[old_tv_nut]

Originally Posted by Kevin Kuehn

Time to dig the B&K analyst out. I believe they are synced to the AC line. I had this same weird thing going on several years ago, although not a static deflection set, but on the B&K it was rock solid. We never did come to any conclusion on how to cure it, but it's works fine with a DVD player through an agile modulator. Conclusion was it's an over the air sync issue.

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

[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.

[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.