AC In My B-

[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

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.

[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

[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 jr_tech

How are you measuring the AC on the cathodes ?

jr

Measuring between cathode to ground...

[jr_tech]

Originally Posted by Jon1967us

Measuring between cathode to ground...

I should have been clearer... are you using an oscilloscope or a meter that measures frequency as well as voltage? In other words, how do you know that you are measuring 60 hz AC rather than a legitimate signal?

jr

[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?)

[Jon1967us]

Originally Posted by jr_tech

I should have been clearer... are you using an oscilloscope or a meter that measures frequency as well as voltage? In other words, how do you know that you are measuring 60 hz AC rather than a legitimate signal?

jr

I have a Hz function on my meter that I have measured 60Hz on a couple tubes, but I haven't checked it on all - that's a good point. On pins 2,3 off the CRT, the AC does jump around a little, and that's probably video signal added to the AC. I'll go back and verify on the sync and CRT what the freqs are. I find that easier than a scope.

[dtvmcdonald]

I have a Pilot TV-37. Its got stray 60 Hz everywhere. I've verified that its there even if there is no B+ or B- because the rectifiers AC feeds are disconnected (but heaters are connected). Thus its heater line pickup. In most cases I verified that its not heater-cathode problems by moving tubes of the same type around.

I've also verified that a lot of the problem is modulation
of the RF in the tuner.

I managed to get OK pictures by careful adjustments. Its vitally important to get
the RF level in a good range: if the gain is too high or low it gets worse.
Thus, I need an external attenuator.

You can't really win. The set was designed for pictures that had a constant
average gray, and a constant contrast, no black or white screens. Give it such
a program, and its much happier!

[Jon1967us]

Originally Posted by dtvmcdonald

I have a Pilot TV-37. Its got stray 60 Hz everywhere. I've verified that its there even if there is no B+ or B- because the rectifiers AC feeds are disconnected (but heaters are connected). Thus its heater line pickup. In most cases I verified that its not heater-cathode problems by moving tubes of the same type around.

I've also verified that a lot of the problem is modulation
of the RF in the tuner.

I managed to get OK pictures by careful adjustments. Its vitally important to get
the RF level in a good range: if the gain is too high or low it gets worse.
Thus, I need an external attenuator.

You can't really win. The set was designed for pictures that had a constant
average gray, and a constant contrast, no black or white screens. Give it such
a program, and its much happier!

Yes, but does your do the Hula?

[Yamamaya42]

Originally Posted by Jon1967us

I have a Hz function on my meter that I have measured 60Hz on a couple tubes, but I haven't checked it on all - that's a good point. On pins 2,3 off the CRT, the AC does jump around a little, and that's probably video signal added to the AC. I'll go back and verify on the sync and CRT what the freqs are. I find that easier than a scope.

SO, w/o an oscilloscope, its hard to know what you are really looking at, other than what this "Hz function" on your meter is saying, you have no real tell what wave form it is...

[Kevin Kuehn]

Yep, I keep assuming we all love our scopes. It's anyone's guess what AC you're picking up on without visually seeing the signal. Quite likely you're chasing a red herring.

[Jon1967us]

It definitely seems like I am chasing something misleading around. I have taken quite a few readings around the set with the scope and other than the video signals I'm seeing the same thing as the multimeter-AC current at unexpected points. Going to run this and the better running set at the same time and go through them with meter and scope and try again to spot any suspicious differences.

[Jon1967us]

One question about the HV deflection caps coming off the horizontal output. Schematics call for two 300uuF and I have two 470s instead. Could this potentially be creating a problem?