1968 Zenith 20Y1C50 chassis - advice needed

[joe111671]

Hello again everyone. I have this all metal Zenith from an estate sale. When I got it over a year ago, I powered it up at the sale and it made some dim fuzzy snow, but was making quite a bit of HV snapping sounds. After taking the cover off, I could see why. It had years of dust in it, thankfully it was not in a heavy smoker's home. I cleaned most of it out, and that took care of the snapping.

When I power it up, and the picture just starts coming up, it has good focus for a split second, then goes out of focus and is out of range of the adjustment. It also has a rainbow colored field moving slowly up the screen, it's almost like the degausser is stuck on, or it needs the main filter caps.

[joe111671]

So I replaced the caps in the power supply. C1A, C2A, C2B, and C2C, as shown in the attached shot of the schematic. That didn't change anything.

What I found is that the 375V on C1A is 389V, 365V source on C2A is 381V, the 235V source on C2B is 256V, and the 95V source on C2C is 178V. That's telling me something on the 95V source is not right. I don't know what to look at next. According to the SAMS, there are two arrow connections from the 95V source. I will post those sections of the schematic if someone thinks that is where I need to look.

Zeno mentioned this when I brought it up on a thread regarding a different set:

Originally Posted by zeno

On that set look at the main choke in the power supply. See if it has a
non polarized electrolytic in series with a resistor. This is in parallel
with the choke. Per Zenith factory its not needed & you can just
snip the cap out.

Is that main choke L53?

[zeno]

Yes it would be L53 but this one dont have the cap. That was a few yrs
later.

If think the DGS is stuck on just unplug the 2 red wires & put a jumper on the chassis pins. Thats usually just a broken thermistor.

For focus take the rear cover & door of the FBT cage off. There is a focus
stick & high ohm resistors & odds and ends back there. Stick is most likely.
then resistor. Sometimes the board will carbon up & arc. If any resistors are burned change the stick also.

The 95 V problem makes no sense. If the stage it feeds is dead it could go up
but besides the focus the sets running pretty good. Could be a mistake in Sams or a its mis wired.

73 Zeno
LFOD !

[JohnCT]

In addition to the focus rect stick and carbon tracking on the flyback phenolic that Zeno mentioned, you might have an internal arc gap carbonized inside the CRT socket. If you have good hearing, remove the socket and put your ear *close* to the socket and see if you can hear any internal arcing.

John

[joe111671]

Originally Posted by zeno

If think the DGS is stuck on just unplug the 2 red wires & put a jumper on the chassis pins. Thats usually just a broken thermistor.

73 Zeno
LFOD !

You nailed it Zeno. The thermistor was not only broken, it was gone. For now I twisted the leads together, gave it some solder and cured that.

Originally Posted by JohnCT

In addition to the focus rect stick and carbon tracking on the flyback phenolic that Zeno mentioned, you might have an internal arc gap carbonized inside the CRT socket. If you have good hearing, remove the socket and put your ear *close* to the socket and see if you can hear any internal arcing.

John

Thanks for the input on the focus, guys. I'll tackle that as soon as I can figure out where to mount the replacement caps. They're still jumpered in, and the set is sitting on its side till they're put in properly. I haven't figured out a good place to mount them without removing the chassis.

[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.

[Electronic M]

Mouser and Digikey stock R5000F diodes. Their modern diodes with 5KV PIV rating and 200ma forward current rating one or 2 in series is my usual replacement for focus rectifiers....I've even put 4-6 in series and used that as a solid state HV rectifier before.
https://www.mouser.com/ProductDetail...NU2tEYAg%3D%3D

If focus is low I usually put one in parallel with old diode and see it focus voltage improves. The old selenium focus rectifiers tend to fail by increasing their internal forward resistance.

[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

[joe111671]

Thanks for clarifying John. I'll try and get to the bottom of it and will share what I find.

[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...

[joe111671]

Here's what I found so far. The focus voltage comes up to 4KV, and does not drop at all as the set goes out of focus. If it does, it's not enough to register on my probe.

There's a bit of hissing in the HV cage that quiets down after a bit but never goes away, but I couldn't see any arcing anywhere with the room completely dark. I'll try to figure out some "stethoscope" way of pinpointing where that hissing is coming from.

What I did notice is that the shunt regulator tube has a bit of blue glowing, it can barely be seen at the top right of the picture. I put a picture of the same area with the lights on for context. I don't know what the shunt tube does, but wanted to mention it since I saw it. I recall seeing something about gassy tubes glowing blue.