1968 Zenith 20Y1C50 chassis - advice needed

[JohnCT]

Originally Posted by joe111671

I looked in the cage, and nothing looks out of the ordinary, other than possibly that blob of silicone or whatever on the top right. Also the dust inside has a metallic, shiny glimmer to it. Not sure if that's normal. I'll search for a replacement stick and change it. In the meantime, I'll try and find homes for those new caps.

John, when you said remove the socket, it went right over my head . I was planning on putting my ear close to the socket while it was plugged in and the set was on.

The good news is that the flyback phenolic board looks like it's in excellent shape. But look closely at it, particularly when it's running in a dark room. I remember one I had many years ago that was arcing *between* the layer of phenolic, not on the surface which is where that normally happens. I used a Dremel and cut into the pheloic to remove the underground river of carbon and layered in some RTV. Also check for internal arcing at the focus pot with the lights out.

As far as the CRT socket, what I meant is to pull the CRT socket off the back of the tube. If there's any internal arcing, it might be easier to hear it from the pin side of the socket because of all the holes, although if the arcing is severe, you might be able to hear some sizzling when it's actually connected to the tube.

I don't know if you have a HV probe, but the easiest way to find this is to check the focus voltage at the flyback where the black lead attaches to the 4.7M resistor (should be 4-6KV or so). If it starts high when the TV is in focus and drops off when the pic gets blurry, pull the socket off the back of the tube and see if the voltage comes back up. If it does, there's either a problem inside the tube itself or the plastic pin guide may have carbon tracking on it or *under* it. If the voltage stays down with the socket removed, unsolder the black lead on the 4.7M resistor and see if the voltage stays up. If it does, the socket is bad.

If the voltage only drops when the socket is actually connected to the tube, you might try *carefull* removing the guide pin and looking between the guide and the tube glass. Carefully rock the guide using a thin blade to slide between the guide and glass to cut the RTV that's used to locate the guide to the tube. Fortunately, arcing under the guide pin is very rare. I've only seen four or five in 40 years.

John

[zeno]

CRT sockets did go especially on flat chassis solid state sets. Usually
caused by excessive focus voltage but can happen with normal F.
The problem is with the spark gap part of the socket. Another test is
to pull the ground straps off the DGS shield & measure them with a
HV probe. Should be zero volts. As John said noise, smell, & physical
damage works as well. If we had a junker repair we would cut up the socket
& eliminate the SG to get it out the door. It was better to do that, make a bucks & give the customer a little more time. Otherwise its the dumpster
& zero cash flow.

73 Zeno
LFOD !

[zeno]

Oh BTW
With delta gun Zeniths ALL CRT sockets that are the heavy white type
are electrically the same. Only difference is lead length & any plugs
on the ends. We stocked just one for the flat SS chassis. Wires always
long enuf & you could just solder on any with plugs. SO keep that
in mind, subs are fine !

[joe111671]

Originally Posted by JohnCT

I don't know if you have a HV probe, but the easiest way to find this is to check the focus voltage at the flyback where the black lead attaches to the 4.7M resistor (should be 4-6KV or so). If it starts high when the TV is in focus and drops off when the pic gets blurry, pull the socket off the back of the tube and see if the voltage comes back up. If it does, there's either a problem inside the tube itself or the plastic pin guide may have carbon tracking on it or *under* it. If the voltage stays down with the socket removed, unsolder the black lead on the 4.7M resistor and see if the voltage stays up. If it does, the socket is bad.
John

I think I understand, and I do have a HV probe. Correct me if I got it wrong, but what I'm checking is the focus voltage to the CRT on the black wire. If it drops when the picture goes out of focus, it's either the CRT itself, or the socket, and I'm going to be finding whether it's one or the other by these tests.

But if it's the rectifier stick - will the voltage just come up to a certain point and stay there without dropping? That's the only thing I'm unsure of right now, could a bad rectifier stick produce the higher initial voltage and then drop?

[JohnCT]

Originally Posted by joe111671

I think I understand, and I do have a HV probe. Correct me if I got it wrong, but what I'm checking is the focus voltage to the CRT on the black wire. If it drops when the picture goes out of focus, it's either the CRT itself, or the socket, and I'm going to be finding whether it's one or the other by these tests.

Yes.

Originally Posted by joe111671

But if it's the rectifier stick - will the voltage just come up to a certain point and stay there without dropping? That's the only thing I'm unsure of right now, could a bad rectifier stick produce the higher initial voltage and then drop?

If the stick is bad, the voltage will be low. I have to admit that the initial higher voltage followed by a drop off isn't the main symptom of the stick being bad, but it's still a real possibility, hence the tests. Most sticks just gave bad focus, but I do remember a few drifting as the TV ran, yes.

There were some chassis designs (including Zenith) where the focus voltage was sourced from a divider off the second anode that could cause excessive (high) focus voltage, but yours is not like that. For your situation, we're assuming that when the TV goes out of focus, the voltage is dropping.

So, if you remove the black wire and the voltage still drops, the stick or the focus pot itself is bad (internal leakage - often visible as small purple arc spots in the dark). We changed tons of those sticks back when those TVs were in every day use, and it's a good probability here. By disconnecting the black lead, we're making sure the focus voltage isn't being pulled down by the CRT socket (fairly common), some corrosion under the CRT guide pin (really uncommon unless some liquid got spilled on it), or internal problems in the CRT socket (really really uncommon).

But us old guys aren't parts tossers, which is why we're giving you specific tests. When we had to get five to ten of these out the door a day, we had to be reasonably sure what part was the problem.

John

[Electronic M]

Originally Posted by JohnCT

Yes.


There were some chassis designs (including Zenith) where the focus voltage was sourced from a divider off the second anode that could cause excessive (high) focus voltage, but yours is not like that. For your situation, we're assuming that when the TV goes out of focus, the voltage is dropping.

John

Those HV divider resistor could also cause focus voltage to drop...That was the case on my 1971 Zenith 12B13C52 chassis. The special HV divider resistor on the HV end of the focus pot was increasing in value, and I couldn't source a spare so I ended up increasing the value of a 1W carbon comp on the ground end of the pot...