AC In My B-

[Kevin Kuehn]

Originally Posted by Jon1967us

The pulling only occurs when a video signal is present. When there's only raster, like just at start up, the image is stable.

This sounds to me like the AC is coupling into the video signal as apposed to power supply ripple. Could be heater to cathode or grid leakage affecting the video signal.

[old_coot88]

The pulling only occurs when a video signal is present. When there's only raster, like just at start up, the image is stable.

That's a major clue right there (I shoulda caught it in your original post). It it were supply ripple, the raster edges would be hula'ing on a blank (or snowy) raster.

[Jon1967us]

Originally Posted by old_coot88

That's a major clue right there (I shoulda caught it in your original post). It it were supply ripple, the raster edges would be hula'ing on a blank (or snowy) raster.

Well, at the very start the plain raster does sometimes hula, sometimes I'll start it up, turn up the brightness and the raster fills the entire screen and is stable and then the video comes in and hulas.

[Kevin Kuehn]

Originally Posted by Jon1967us

Well, at the very start the plain raster does sometimes hula, sometimes I'll start it up, turn up the brightness and the raster fills the entire screen and is stable and then the video comes in and hulas.

Oh, OK, you just moved the target on us. But if it's really intermittent as you say, I'd still be suspicious of heater leakage.

[Kevin Kuehn]

Swap out the 35W4 while you're at it. It too could have heater to cathode leakage.

[Jon1967us]

Originally Posted by Kevin Kuehn

Swap out the 35W4 while you're at it. It too could have heater to cathode leakage.

Yep, already did this 3 times...even after I shorted it trying to bridge an extra 47uF across the - filters!

[jr_tech]

How are you measuring the AC on the cathodes ?

jr

[Jon1967us]

Originally Posted by Kevin Kuehn

Oh, OK, you just moved the target on us. But if it's really intermittent as you say, I'd still be suspicious of heater leakage.

Yeah, that kinda just started happening...

When we're talking heater leakage, I thought we're concerned about leaking into the Cathode. Could it be leaking into the grids? I've already subbed out all the tubes, and this makes no change.

Not sure what else there is to do other than modding it by adding the ballast resistors from the Sams.

Just about every relevant component has been changed. The remaining original resistors are within tolerance.

[Kevin Kuehn]

Originally Posted by Jon1967us

Yeah, that kinda just started happening...

When we're talking heater leakage, I thought we're concerned about leaking into the Cathode. Could it be leaking into the grids? I've already subbed out all the tubes, and this makes no change.

Not sure what else there is to do other than modding it by adding the ballast resistors from the Sams.

Just about every relevant component has been changed. The remaining original resistors are within tolerance.

It's the .2v AC on the cathode of your sync/DC restorer tube that worries me. It stands to reason that same AC is then present at pin 2(grid) of the CRT? When it's doing it's hula, what if any AC do you measure on pin 3(cathode) of the CRT? As an experiment you could hang a few uf's cap alternately from pin 2 or 3 of the CRT to ground to see if that clears up the hula. Observe polarity of whatever DC you measure on those pins to ground. Of course you won't get any video with the cap on pin 3 of the CRT, but at least you'll find out if there's AC modulating the signal there. Somethings got to give.

[Jon1967us]

Originally Posted by Kevin Kuehn

It's the .2v AC on the cathode of your sync/DC restorer tube that worries me. It stands to reason that same AC is then present at pin 2(grid) of the CRT? When it's doing it's hula, what if any AC do you measure on pin 3(cathode) of the CRT? As an experiment you could hang a few uf's cap alternately from pin 2 or 3 of the CRT to ground to see if that clears up the hula. Observe polarity of whatever DC you measure on those pins to ground. Of course you won't get any video with the cap on pin 3 of the CRT, but at least you'll find out if there's AC modulating the signal there. Somethings got to give.

Yes, that assumption was right. I did measure .7AC /-96DC off the CRT, pin 2.
I also measured 2.4AC/-22DC off pin 3. Both measured to ground. These values jump around a little compared to all the other points I've tested, I guess due to video signal.

That's the highest measurement of AC on a cathode in the entire circuit that I've tested so far.

Raw line current goes straight into the CRT of course (which is why I always bring up on a variac )

Wondering about putting one of those TVS diodes across the CRT heaters to limit current and maybe it will take care of this problem as well.

I feel like I need to take resistance measurements again in these areas between the sync, B- and CRT.

I've looked at all the underside tube pin terminals for any shorts or crud that could be creating a short with a magnifier and wiped the undersides with contact cleaner.

NOTE: many of the metal surfaces were coated with a white powder when I got the set, which I have cleaned off what was visible, but there could be some that got into an area and is conducting - who knows

Haven't tried hanging a cap from the CRT pins 2,3 to ground yet (I assume the - terminal will be on the CRT side, right?)