Meck XB-702 restoration - Page 9 - Videokarma.org TV - Video

bandersen · #**121** 02-14-2022, 09:59 PM

Thanks for the offer, but luckily I have a few spares. I'm also considering adding a separate filament transformer for the CRT. Then I can keep using the shorted one.

Meanwhile, here it is with a 7VP1 scope CRT.

There is still some 60 cycle leaking into the horizontal. Pretty much looks like it did before.

The vertical is much better though. I was getting seasick trying to watch it before. I'll poke around a bit more, but I think I could live with this.

old_coot88 · #**122** 02-14-2022, 10:40 PM

Quote:

Originally Posted by bandersen

...I'm also considering adding a separate filament transformer for the CRT. Then I can keep using the shorted one.

A transformer will add some parasitic capacitance-to-ground, which might cause smearing of fine detail in the pic. To minimize any capacitance effect, try to find a xfmr that's physically small as possible consistent with required heater current.

bandersen · #**123** 02-15-2022, 12:58 AM

Sigh. Forget all that. I installed a replacement and it has exactly the same problems! So I reconnected the original and placed it as far away from the chassis as possible.
Bingo, the vertical distortion went away. So it is hum being picked up from the surrounding circuitry.
The short must be mild enough it's not causing problems

Yamamaya42 · #**124** 02-15-2022, 08:13 AM

You are defiantly in The Twilight Zone with this one...

I can't the the chassis layout from the Riders info alone, would have to re-watch your u-tube videos.
By any chance, could this be magnetic interference from the tuner filament transformer perhaps having some effect on the 7JP4? Be it bad/ overloaded or whatnot?

As mentioned, just a guess, cause w/o seeing your videos again, I have no idea where it is in relation to the CRT, but I can not think of much in this set that can cause enough EMI to distort the picture, very strange.

Notimetolooz · #**125** 02-15-2022, 08:50 AM

I guess that set does not have a magnetic shield around the CRT.
Looks like it needs one.

Interesting that the audio also had a problem with hum.

bandersen · #**126** 02-15-2022, 09:05 AM

Here's a top view of the chassis. The tuner filament transformer is on the right just in front of the filter caps. It's completely encased in grounded metal.

Perhaps hum in the audio may have been an indicator of too much 60Hz being radiated by something?

Yamamaya42 · #**127** 02-15-2022, 09:38 AM

At this point, it's just major weirdsville... :O

Perhaps you can find an old coil someplace with a ferrite core, hook it to an amp with a magnetic cartridge input, provided the impedance is close enough, and use it as some kind of sniffer / locator of the source of the problem?

Kevin Kuehn · #**128** 02-15-2022, 10:36 AM

That one choke in the power supply could induce hum, more so if it has shorted turns. As well as that 12vpp ripple on B++ is worth investigating. Just seems excessive.

Yamamaya42 · #**129** 02-15-2022, 11:50 AM

However, he also mentioned that having the CRT away from the chassis made the problem less visible than mounted in the normal location, which would somehow point to magnetic interference, odd that it is, if it was some kind of power contamination, should it not have shown the same problems when tried with P1 tube that was a few inches away from the normal location?
It's all very strange.

Perhaps an exorcist is needed! :P

old_coot88 · #**130** 02-15-2022, 12:27 PM

That tube tester's veracity is questionable regarding an HK short in the CRT. If there were a heater-to-cathode short, it would kill the video signal going into the CRT as well as show a heavy hum bar (since the heater is 6 VAC away from ground, thru the 6X5's heater). In a case like this, the TV itself should be the best "tester" of the CRT.

bandersen · #**131** 02-15-2022, 01:45 PM

Quote:

Originally Posted by old_coot88

That tube tester's veracity is questionable regarding an HK short in the CRT. If there were a heater-to-cathode short, it would kill the video signal going into the CRT as well as show a heavy hum bar (since the heater is 6 VAC away from ground, thru the 6X5's heater). In a case like this, the TV itself should be the best "tester" of the CRT.

You are correct. I had both F2 and K set to pin 2

So back to the drawing board. The tuner filament transformer and filter choke seem likely candidates. I could power the tuner with a separate 6 volt source.

bandersen · #**132** 02-15-2022, 01:49 PM

Quote:

Originally Posted by Kevin Kuehn

That one choke in the power supply could induce hum, more so if it has shorted turns. As well as that 12vpp ripple on B++ is worth investigating. Just seems excessive.

Adding another 47uF got it down to 2vpp ripple but made no difference in the display.

Kevin Kuehn · #**133** 02-15-2022, 01:59 PM

Quote:

Originally Posted by bandersen

Adding another 47uF got it down to 2vpp ripple but made no difference in the display.

Possibly you can move the CRT around enough to hone in on the source?

Kevin Kuehn · #**134** 02-15-2022, 02:22 PM

Provided your signal source is isolated can you try powering the set without the isolation transformer? I know grasping at straws ...

old_coot88 · #**135** 02-15-2022, 02:37 PM

On regular program material, that wiggle is so subtle it'd probably scarcely be noticeable. If noticed, some might consider it part of the 'charm' of an early TV. Sort of analogous to how a vintage radio always has bit of background hum even when the volume's off.

Others' opinions may vary.