RCA 621ts Restoration

[old_coot88]

Originally Posted by vts1134

One last thought.
If the problem is coming from the video section would it still manifest itself with no video signal present?

Yes it would, assuming the raster has still got that heavy white bar in it. That bar's still present, right? If it is, there's another trick you could use that doesn't involve lifting any connections or injecting a signal (the bar is the "signal"). Get that trusty .1mf cap again* and hook one end to ground, and the other end to a clip lead. Starting with G1 of the CRT (this set injects video via the grid instead of the cathode), and connect the cap to it. This shorts the signal and should produce a blank or near-blank raster. Then take the clip lead and progressively work back, shorting the plate of the 2nd video stage, then the grid, then the plate of the 1st video, then the grid, and on back through to the plate of the 6H6 and then the cathode. If at any point the bar remains unaffected, you'll know it's originating in the stage you just left, 'downstream' of the shorting cap.

Be sure and discharge the cap to ground after taking it off the plate of a tube!
-------------------------------------
*If the single cap is not killing the bar sufficiently, and if you have more than one of those .1 caps, put as many as you can in parallel to get higher capacitance for better shorting effect.

[vts1134]

When you say blank raster, do you mean just a normal raster minus my white bar problem?
Shorting (through a cap) G1 to ground makes no change in raster. .1 didn't do it so I moved to .25 which didn't do it so I jumped all the way up to 10mfd which also had no change, aside from the fact that it was a bit more dramatic to discharge. Assuming I didn't do this test wrong would that put the problem in the CRT?
Fearing I did the test wrong I injected my pattern generator on the video amp to ensure that when I shorted G1 to ground that the video went away, which it indeed did.

[old_coot88]

Da-yum. You're sayin' that even with the CRT's G1 clamped to ground with a 10mf cap, the white bar remains unchanged? That means the bar is definitely not originating in the video chain (see how one quick test can eliminate or confirm a whole signal chain?). Does the bar roll if you turn the vert hold control or not? I'm gonna take a SWAG that the bar is locked to the vert sweep, in which case it will not roll. And another guess that the bar is actually appearing on the CRT's G2.. and that since G2 has boost voltage on it, something's allowing vertical pulse to get into the boost source.

Try this- do the ripple test thingy again with the .1cap, on the CRT's G2. I'll betcha there's some signifigant ripple there, causing the white bar.

Of course i could be completely wrong.

The CRT appears to have a partial aquadag coating on it. Is that coating securely grounded? An ungrounded or poorly grounded 'dag can cause some really weird symptoms mimicking sync and AGC problems (a little factoid worth bookmarking).

[DavGoodlin]

Oldcoot88: "Is that coating securely grounded? An ungrounded or poorly grounded 'dag can cause some really weird symptoms mimicking sync and AGC problems (a little factoid worth bookmarking)."

Good one! A spring arcing in a curb-found 1961 Philco BW, left the room, and the CRT "blowed up". Made a hell of a mess out the back. Cant be too careful grounding.

[old_coot88]

Originally Posted by DavGoodlin

...A spring arcing in a curb-found 1961 Philco BW, left the room, and the CRT "blowed up". Made a hell of a mess out the back. Cant be too careful grounding.

Dang. First time i heard of that happening (other than those SS color Zeniths with the 4-legged Safety Cap which would fail, letting HV soar to over 50KV, cutting the neck off the picture tube).

[vts1134]

Originally Posted by old_coot88

Da-yum. You're sayin' that even with the CRT's G1 clamped to ground with a 10mf cap, the white bar remains unchanged? That means the bar is definitely not originating in the video chain (see how one quick test can eliminate or confirm a whole signal chain?). Does the bar roll if you turn the vert hold control or not? I'm gonna take a SWAG that the bar is locked to the vert sweep, in which case it will not roll. And another guess that the bar is actually appearing on the CRT's G2.. and that since G2 has boost voltage on it, something's allowing vertical pulse to get into the boost source.

Try this- do the ripple test thingy again with the .1cap, on the CRT's G2. I'll betcha there's some signifigant ripple there, causing the white bar.

Of course i could be completely wrong.

The CRT appears to have a partial aquadag coating on it. Is that coating securely grounded? An ungrounded or poorly grounded 'dag can cause some really weird symptoms mimicking sync and AGC problems (a little factoid worth bookmarking).

The white bars roll right along as I adjust the vertical hold out of lock, and back in place when I adjust the vertical into lock.
I'm not sure how much ripple would be needed to cause a symptom like that but G2 has in the neighborhood of 3VAC present on it without any signal injection.
The aquadag still looks intact and making connection to ground, looks can be decieving though I'm sure. Is there a good way to test it's connection to ground to rule it out?
What a mystery we have. Maybe it was the butler in the library with a pipe wrench?

[Electronic M]

LOL. Here is a clue on that aqua dag check. If you use the grounded screw driver CRT technique to discharge the HV, then use the same tools set up the same and with the set on touch the blade of the screwdriver to the aquadag coating in a few spots and if you get an arc or spark then the CRT's dag ain't properly grounded. Might also be able to use a HV meeter instead of a screwdriver.

Awhile back I accidently confirmed I had this issue on my CTC4 with my hand.....it shook me up and made me swear like a pirate. 25KV can certainly alter ones vocabulary and mood.

[old_coot88]

Originally Posted by vts1134

The white bars roll right along as I adjust the vertical hold out of lock, and back in place when I adjust the vertical into lock.
I'm not sure how much ripple would be needed to cause a symptom like that but G2 has in the neighborhood of 3VAC present on it without any signal injection.

Well that rules out G2 and leaves only the cathode of the CRT. Try this for the heck of it- hook that 10mf electrolytic from ground to the cathode (observing polarity of course) and see if the bar remains.

One other thought- After doing the cathode shunt, try shunting G2 with the 10mf cap. Hook its neg. end to top of C116B (which carries smoothest B+) and the positive end to G2. This will effectively kill all ripple on G2, just in case the 3VAC is sufficient to cause the bar.

Maybe it was the butler in the library with a pipe wrench?

Maybe he was in the library with a wench.

[vts1134]

Originally Posted by old_coot88

Well that rules out G2 and leaves only the cathode of the CRT. Try this for the heck of it- hook that 10mf electrolytic from ground to the cathode (observing polarity of course) and see if the bar remains.

One other thought- After doing the cathode shunt, try shunting G2 with the 10mf cap. Hook its neg. end to top of C116B (which carries smoothest B+) and the positive end to G2. This will effectively kill all ripple on G2, just in case the 3VAC is sufficient to cause the bar.

Negative on both counts .
I also did the grounded screwdriver trick to spots on the dag with the set on to check grounding of the crt and it was ok.
Could it be the tube itself?

[Electronic M]

I'd try running the filament of the CRT on it's own transformer to rule out hum leakage from a heater-cathode short.

[vts1134]

Interesting suggestion. What kind of amperidge would that need? The schematic calls for 8A on the heaters for everything else, but no rating on the CRT filament.

[Electronic M]

I don't know the exact specs, but probably around 800mA should be enough. As long as the filament is not running more than 1 volt high or low it should be safe for a quick test.

[old_coot88]

The CRT heater already has its own dedicated 'floating' winding in the power xfmr, designated 'Y and YZ', going to 'KIN' (for kinescope, the archaic term for CRT).

Try this- unplug the CRT from the socket and check for heater to cathode short with the ohmeter on highest range. It should read infinity (although a short might occur when the heater warms up). Also check DC resistance form the floating winding to ground. It should also read infinity, but double check the schematic to verify this, and that it's not connected to B+.

With the set on (and the CRT reconnected), measure the DC voltage between heater and ground, and see if it reads the same as the cathode (or not). Ideally, it should not if it's truly 'floating'. Then try shunting the heater to ground with the 10mf cap, first from one side of the heater and then the other. And see if the bar remains.

[vts1134]

Originally Posted by old_coot88

The CRT heater already has its own dedicated 'floating' winding in the power xfmr, designated 'Y and YZ', going to 'KIN' (for kinescope, the archaic term for CRT).

Try this- unplug the CRT from the socket and check for heater to cathode short with the ohmeter on highest range. It should read infinity (although a short might occur when the heater warms up).

It reads infinity on the Megohm scale.

Originally Posted by old_coot88

Also check DC resistance form the floating winding to ground. It should also read infinity, but double check the schematic to verify this, and that it's not connected to B+.

It has a path to ground, but it's connected to B+

Originally Posted by old_coot88

With the set on (and the CRT reconnected), measure the DC voltage between heater and ground, and see if it reads the same as the cathode (or not). Ideally, it should not if it's truly 'floating'.

It reads a different voltage than the cathode. Heater reads +225V and cathode reads +195V, which I see is a little low so I took a measurement where it should read +425V and it only got +350V. It's right on the 20% mark but it looks like I'll be going resistor hunting at some point to address that.

Originally Posted by old_coot88

Then try shunting the heater to ground with the 10mf cap, first from one side of the heater and then the other. And see if the bar remains.

The bar wins this round.

[Reece]

Why is the heater connected to HV? Is that to reduce the potential and the arcing possibility between it and the cathode?