Meck XB-702 restoration

[Yamamaya42]

Originally Posted by bandersen

I disconnected the oscillators. The only thing connected to the plate is 220K to B+.

The waveform changes start once the set warms up. It never stops

What does the noise / signal look like on the bias side of R45? And the cathode for that matter? Is it there, or much different ?

[bandersen]

The other side of R45 is B+. No ripple or noise. No ripple or noise on the cathode either.

[Yamamaya42]

Quite strange

[bandersen]

Scratch all that. The 60Hz can be traced back to the detector. I also discovered that the video amp peaking coil and sound pickoff coil do not match the schematic. Also, one of the wires to the sound pickoff coil is longer than necessary. That makes me think it may be an old repair or modification. Now I'm really working in the dark.

[Kevin Kuehn]

Originally Posted by bandersen

I'm curious to try feeding a signal directly into the video amp.

Did you get a chance to try feeding video directly?

[bandersen]

No. The only thing I have that will put out a video signal with enough amplitude is the B&K 1077B. It's not the cleanest image and vsync is derived from the AC line. So it will be difficult to see if it makes a difference.

One thing I did try is setting my TPG to external modulation. So it's just putting out a carrier. Got a strong 60Hz sinewave coming out of the sync tube plate

[Kevin Kuehn]

Originally Posted by bandersen

No. The only thing I have that will put out a video signal with enough amplitude is the B&K 1077B. It's not the cleanest image and vsync is derived from the AC line. So it will be difficult to see if it makes a difference.

Sencore VA62 should do it from the Drive Output.

[bandersen]

I no longer have one. Just a Sencore VG91

[Kevin Kuehn]

Originally Posted by bandersen

I no longer have one. Just a Sencore VG91

I thought I'd heard you were getting out of TV and more into Ham Radio.

[bandersen]

Oh no, not at all. The VA62 was getting flaky and I parted it out years ago. The VG91 is a much nicer pattern generator but it doesn't have any drive signals. I need to get the companion TVA92 for all the bells and whistles.

Just ordered one off eBay. Been meaning to for a while and this gave me an excuse I'll just keep throwing test equipment at it.

Seriously though, I've been working on this set in our guest bedroom / backup workshop while the basement is worked on. That means I can leave it indefinitely while I work on other stuff.
I thought it would be a simple, relaxing project.

[bandersen]

I tried running the 6AL5 detector lean by adding a 4.7 ohm resistor in parallel with the filament. The results were interesting.

Here's the output of the detector feeding the grid of the video amp. A small amount of ripple is present.


Now here's the signal at the plate. It's amplified (range increase on scope), but the 60Hz was amplified much more than the video.


Now here's the output of the sync separator. Seems the 60Hz is being amplified more than the video at each stage.
Perhaps it's just that the amps have more gain at lower frequencies and even a small amount of ripple out of the detector is too much.

[Tom9589]

Have you considered a solid state replacement for the 6AL5?

[Kevin Kuehn]

I've got a dumb question. If you disconnect the sync from one or both sweep oscillators can stabilize the picture enough to tell if the 60 cycle distortion is reduced?

[bandersen]

Originally Posted by Tom9589

Have you considered a solid state replacement for the 6AL5?

Yes, but haven't tried yet.

Originally Posted by Kevin Kuehn

I've got a dumb question. If you disconnect the sync from one or both sweep oscillators can stabilize the picture enough to tell if the 60 cycle distortion is reduced?

Not dumb at all. I did try but couldn't tell.

[Kevin Kuehn]

How about disable the vert or horizontal oscillators alternately and see what the remaining horz or vert line does. It's hard to imagine the 60 cycle AC is leaking in from multiple directions, although it appears to be.