1968 Zenith 20Y1C50 chassis - advice needed
 

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.

Replaced power supply caps
 

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:

Is that main choke L53?

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:smoke:
LFOD !

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

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.

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.

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 :screwy:. I was planning on putting my ear close to the socket while it was plugged in and the set was on.

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.

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

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:smoke:
LFOD !

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 !

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?

Yes.

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

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

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...

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.

Focus voltage is to low. IIRC should be abt 5KV.
Measure at the yellow wire. Should be abt 6 KV IIRC.
If not lift the yellow wire & see if it comes up. If not its the stick or the green
3W resistor.
2) lift the black wire to the CRT socket. At the focus control
center pin you should get 4-6 KV at the center pin. If so suspect the CRT
socket. Voltage will be dependant on the control setting.
Hope I got that straight, been a long day !
Just remember its a simple voltage divider....

73 Zeno:smoke:
LFOD !

Slightly off topic - I see the Horz output right next to the HV box, and the Damper in front of it. What's that power handling tube right next to the damper?

HV regulator. 6HS5 or 6HV5 IIRC. On some Zeniths you have to match it
with the right damper tube. On those it will be noted on the tube
chart or a big warning label on the HV box as to which pairs you can
use.

Zeno

Even back in the day, that regulator tube frequently went bad, causing low HV and bad focus, even tho the tube tested 'Good' on a tester. The only fix was to replace it.
Just a small nit, but it's not a shunt regulator (like 6BK4 etc.) but a pulse regulator for the new-fangled reg system Zenith came up with.

Thanks for the input so far everyone. The hissing in the HV cage is coming from the area around the rectifier tube. I'll ignore that for now. Maybe it's even normal. The HV is only 4KV on the yellow wire.

Here's my plan:

Since I have low focus voltage, and it doesn't drop as the set goes out of focus, I'll hold off on condemning the crt or socket. I'll lift the yellow wire and see if the voltage goes up to 6KV. If not, I'll change the stick and/or green resistor. If it does go up, I'll lift the black wire and see what I get at the center pin of the focus control. If the voltage is between 4-6KV depending on the position of the control, I'll pursue the socket or CRT. If it stays low, I'll change the 6HS5 regulator tube. I hope I got that right, and someone correct me if I'm missing something.

Hissing isn't normal. Clean the HV box by the rect up.

Progress
 

Focus voltage at the yellow wire was 4KV. I lifted it and the voltage went up to just under 5KV.

Hooked that back up then lifted the black wire, and the focus control center pin measured between just under 4KV to just above while adjusting the focus control.

HV at the 2nd anode is 26KV so that should rule out the 6HS5 tube.

So at this point I think I just need the stick.

I can get a NOS ECG118 for $5.50 shipped. Do these have a shelf life, or am I good with NOS? I'm hoping Chester's still has one so I can just drive over and grab one.

Will do.

NOS usually will work, but most vintage parts are selenium rectifiers so reliability isn't guaranteed.

I typically try to keep 5-20 of these modern parts in stock.
https://www.mouser.com/ProductDetail...NU2tEYAg%3D%3D
I usually put 2 in series in focus applications to ensure the PIV rating isn't exceeded.

Chester had an NTE118 in stock so I went over and got one for 3 bucks. You were right that it's selenium, but it worked. The ones you linked to are cheap, so when I need to place an order, I'll grab some just to have.

The focus voltage on the yellow wire went up to a hair under 6KV. That brought it into focus, but the control still needs to be all the way up, so I can't tell if it's as good as it's going to get since I can't dial it in. It still needs a lot of tweaking, and most of the controls need to be cleaned. I'll let it run and at least do a setup on it and see how it goes.

I let it run for a little while. It doesn't look bad but not enough contrast. The brightness control worked normally, just acted like it needed to be cleaned. The contrast control had no effect, other than make a bit of "static". I cleaned them both. The brightness is smooth now, but the contrast does nothing. The schematic shows that it's a 500 ohm pot, and when I measure it, it goes smoothly from 2-480 ohms so it seems good. When the set is running, there's less than a volt across the terminals of the contrast pot. I guess the next thing is to check R85, L41 and change C3 (which I have).

Getting there
 

I set the gray scale and put a good source on it. It's working so much better than at first even with no contrast control. It's still laying on its side and the purity is out of whack with the degausser not working yet.

I'm a little late to the party on this observation but sometimes when focus voltage is good but proper focus is unobtainable and picture is bright (especially if previous owners have been monkeying with it) it can be because the CRT G2 AKA screens are set too high...I had that on a Setchel Carlson monitor and was chasing my tail till I turned G2 down...That brought the HV way up and I had to readjust the regulator.

That looks pretty damn strong. The green may be a little off though.
Contrast looks good, almost as if its at max. Keep in mind these sets
wont show the high contrast of newer sets & keep a good pix.
They are meant to be used in a dimly lit room & not driven to the max.
BTW we used ECG sticks. ECG was a good quality sub line almost all
the time.
BTW#2 Flipping a set on its side will throw off the purity, thats normal.

73 Zeno:smoke:
LFOD !

I set the G2s in service mode for a barely visible line, so I believe they're good. I did have slight blue retrace lines at first, so they may have been set too high. I don't think I touched the focus after that though so maybe it needs a touchup. The good news is that it didn't take much to get a line on all 3 guns, not even half, so the CRT looks to be happy! I should get the HV set when it's straightened out, that's a good reminder. It's supposed to be 25KV, and it was 26 before I even worked out the focus and adjusted the G2s.

I think the picture exaggerates the contrast. I'm almost certain it's too low. In any case, the contrast control has no effect whatsoever, so something is wrong there. I took some voltage measurements around the video output tube, and the values on the schematic vs. the actual readings are below.
All readings vary w/ brightness control, and Contrast control has little to no effect on any reading:

Pin 1 (connected to contrast pot) shows 10.5V. It's 10.7-0.5

Pin 2 shows 5.5v. It's 5.8-(-19v)

Pin 7 shows 300v. It's 280-390

Pin 8 shows 130v. It's 187-273

There are 2 electrolytics in the vicinity that I didn't touch yet. Can they cause an unresponsive contrast pot?

Contrast fixed
 

I decided to try and answer my own question on those electrolytics since I already have them, and the 50uF C3 was directly connected to the contrast pot. I chopped it out, tested and it was open, and changing it made the difference. The contrast works great now.

Now the vertical, which always had a little bit of jumpiness to it for the first half minute or so after power up, is now constantly jumping/changing height/linearity. All this stuff must work together. So now I have a vertical thing to work out. I'm tempted to just change all the electrolytics at this point, but spending time on issues makes it rewarding to find the exact culprit.

Not sure what I'll do yet, but I'm happy to be bringing this thing back to life.

You want to talk about foolishness, the damper and regulator had to be matched. I checked the schematic and noted that those tube heaters were connected in series, running off a center tapped 12 volt heater secondary. All the other tubes were run off one side or the other to ground. I just grounded the center point of the damper and the regulator. Problem solved!

I'm happy with today's progress. I permanently installed the main filter caps, they had been jumpered in and hanging by wires since I connected them. I wasn't going to try and restuff the cans, and to mount them near the cans would mean removing the chassis, which I don't want to do at this point. So I mounted them under the chassis as neatly as I could. I j-hooked a few and was able to make use of a couple empty solder cups for one of them which was my first experience working with them.

I labeled the ones that are nowhere near the cans for future easy identification.

Also got rid of the hissing in the HV cage. I took the tube out, cleaned everything really good, put the tube back in and it's nice and quiet now. Next thing is to adjust the HV.

I still have that vertical jitter. There are three electrolytics in the vertical circuit. I replaced one, that didn't make a difference. Desoldered the next one and jumped in a new one, didn't make a difference so I soldered the original back in. There's one left, but it's tied into an electrolytic in the horiz section, so I left that alone for now.

Wow this thing is a basket case. I was going to adjust the HV to 25KV as it says on the label at the HV adjustment. My probe goes up to 30KV, and the HV buried the meter (with the brightness all the way down). The label says to use a rounded tapered tool such as a wood pencil to adjust it. I sharpened a pencil, tried to turn the pot and it won't budge.

So I was looking closely at both sides of the pot to try and figure out how to get it to turn, and found a charred resistor R44 on pin 3 of the HV regulator. It's supposed to be 1000 ohms and it measured 19 ohms with the tube out. Plus a chunk of it fell off while I was probing it.

I guess I'll replace the resistor, then try to break the HV adjustment pot free if needed. Has anyone run into this situation? I'm guessing this is why the HV is way too high.

Those controls used to get bad spots. Also the VDR was trouble.
All the NOS VDR's are bad now out of the package. Somewhere here
there is a post from Doug ( drh4683 ) he found the modern sub.
Nice work Doug !
Vert jitters may be the integrators, another common problem.

73 Zeno:smoke:
LFOD !

Thanks Zeno, I found the thread, here it is for anyone following along. Nice work by Doug indeed! I'm going to order a couple.

http://www.videokarma.org/showthread.php?t=261780

I'll still need to replace that burnt R44 1000 ohm resistor off the regulator tube. Is there anything special about it or can I just use any 1000 ohm resistor?

I recall seeing a youtube video by shango066 where he rebuilt a vertical integrator on some Zenith set. That didn't even cross my mind, so thanks for mentioning that as well.

Should be a regular carbon. Its a good idea to use whatever was in there.
In some apps using a wire wound will cause trouble because its not only
a resistor but also an inductor. Newer sets also use low value resistors as fuses
so stick to the original when you can.
Modern carbon film flameproofs are a good sub for carbons.
Sams manuals as a rule just list carbons by value. Other special resistors
they give specs & OEM part numbers.

73 Zeno:smoke:
LFOD !

I agree with your advice as to using the same component as was originally in the set. The manufacturer had a good reason for using the values originally installed in the chassis; the use of other parts can cause problems. In fact, it is very important to replace resistors, for example, with identical components; resistors are often used to drop voltages, and some, as you mentioned, are used as fuses, as they are designed to open under severe current overload conditions--not unlike fusible resistors in older, tube-type TVs. Replacing such a resistor with a standard carbon one could cause more damage than was originally done when the original part failed, particularly if the original resistor was meant to open in case of an overload or short.

Thanks for your advice, guys. I added the correct resistor to my order.

I'm trying to make a list of as many components as know I'll be needing, to make the shipping cost lower than the parts cost, so I'm ordering the degaussing thermistor. Since all I do is ask questions, I'll share something I learned, that most people on here probably know but maybe some don't. :sigh:

There are two types of thermistors. NTC and PTC. NTC means the resistance drops as it heats up, and PTC means the resistance increases as it heats up. So in my case, I need NTC since the degausser is in parallel with the thermistor, where I would need PTC if it was in series. I need a 120 ohm, but Mouser doesn't have it. They do have a 130 ohm, so I'll try that. It's only a buck, so if it doesn't work right, I'll jump it out again and go looking for a 120 ohm.

Me & Jeffhs are working at the same bench ! A few examples:

Hallicrafters SX 71 SW radio. I saw a toasted resistor in the front end.
We had a huge collection of Dale precision wire wound resistors thanks
to Raytheon & the American tax payers. I used one of them. The
radio went deaf. It was the new WW resistor acting as a coil.

1963 Zenith 19" tin can B&W. Found at the dump. It was in showroom condition so I fixed it up for the wife to be bedroom. 'lytic in cathode of vert out was opened. A common wham bam repair. Installed a seperate cap about 6 inches away & the set went into vert blink mode. Moved it back near the old can & it was fine.

Early 70's 12" B&W Panasonic. Fixed & was buttoning up. Noticed the best
text book case of jail bars I have ever seen. The cathode lead to the CRT had moved over by the FBT & was picking up the HV spikes.

enuf fer now
73 Zeno:smoke:
LFOD !

Some sets have both PTC and NTC...RCA around the CTC16 would put the degauss in series with the B+ winding of the power transformer...they would put a PTC in series with the degauss and an NTC in parallel with the degauss and PTC to bypass them as the PTC got close to open circuit.