1968 Zenith 20Y1C50 chassis - advice needed

[joe111671]

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.

Originally Posted by nasadowsk

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

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.

[joe111671]

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.

[zeno]

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
LFOD !

[joe111671]

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.

[zeno]

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
LFOD !

[Jeffhs]

Originally Posted by zeno

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
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.

[joe111671]

I got the parts today. I replaced the burnt 1000 ohm resistor off pin 3 of the HV regulator, and the new MOV to replace the VDR that I was hoping was causing the excessive HV. I can't tell if it made any difference. The picture looks the same, the HV is 27-28kv with the bright/cont all the way up, and still buries the needle (32+ kv) on my probe with bright/cont all the way down.

I got the HV adjustment pot to turn, and it only varies by 1kv from one end to the other with bright/cont all the way up. No change when the probe is maxed out at 32kv.

When I first powered it up, as soon as the HV came up, there was a SNAP and a flash of light from behind, but I didn't see where it came from since I was looking at the screen. I shut it off, got the camera rolling and turned it back on but it didn't do it again. Maybe something was crabby in the HV cage, the cover is still off.

So I'm stuck again.

[nasadowsk]

I'm gonna ask the dumb question:

Have you tried replacing the pulse regulator?

[joe111671]

Originally Posted by nasadowsk

I'm gonna ask the dumb question:

Have you tried replacing the pulse regulator?

Is that the 6HS5 tube listed as the shunt regulator? I haven't replaced it, but I will if need be.

[joe111671]

Got back at it today and I think the HV control is bad. I studied the schematic for a while and decided to check all the resistors in the regulator circuit, then check voltages. All the resistances came out fine except the HV adjust pot.

Schematic shows that it's a 1 meg. It measures 1.1 meg end to end, so I think that if I center the control, it should be half that from the center to either end, but it measures 1.1 meg from center to either end. If I vary the control, resistance starts rising to 1.3 meg, then drops to about 130 ohms from center to either end.

I have a good feeling replacing this will finally take care of the excessive HV. Now to try and find a replacement.

[JohnCT]

If you don't want to unsolder the pot to check it, put a jumper across R163 and measure the center tap of the pot to ground. You should be able to read to 50 ohms or less when you rotate the control.

John

[joe111671]

Originally Posted by JohnCT

If you don't want to unsolder the pot to check it, put a jumper across R163 and measure the center tap of the pot to ground. You should be able to read to 50 ohms or less when you rotate the control.

John

Thanks John. It's 168 ohms.

If Chester's in Kenosha has one I'll grab it Monday, otherwise I found a NOS one online. Can that regulator tube make the HV excessive and unadjustable if it's bad? I might as well get one of those too if it's a possibility.

[Jeffhs]

Originally Posted by joe111671

If Chester's in Kenosha has one I'll grab it Monday, otherwise I found a NOS one online. Can that regulator tube make the HV excessive and unadjustable if it's bad? I might as well get one of those too if it's a possibility.

I would replace the HV regulator tube on general principles, especially if the tube now in the TV is the original or hasn't been replaced in some time. It wouldn't surprise me if a new tube cured your problem. Your set dates back to the late 1960s-early '70s; if few or no tubes have been replaced in it since then the originals are all but certainly weak, so it would be a good idea to test all tubes in the set, replacing any showing low or no emission.

Being a Zenith, your TV is one of the best CRT sets made in the analog TV era, so when you get this one working at peak performance you won't be disappointed. I have owned several Zenith televisions and still have one, a 19-inch SMS1917SG table model, which still works as well as it did when it was new (it is now sitting, unused, in my bedroom since being replaced by a flat screen HDTV some years ago).

Good luck. As I said, your Zenith TV is one of the best CRT sets there is, IMHO; you won't be diappointed when you get it working at peak performance. This set should make a darned excellent picture with a cable box or satellite dish (or even with an antenna, if a suitable converter box is used between the set and the antenna). It was a darn shame Zenith went out of business 20+ years ago, as the TVs this company made were, again, some of the best analog CRT sets ever manufactured.

BTW, I like the styling of the cabinet on your Zenith TV. I have a coffee table and two end tables in my apartment with the same style of legs (I believe this style is known as cabriole) as your TV has, and I like them quite a bit.

I hope you will enjoy your Zenith TV for many years to come once you get it working. As I said, these were some of the best televisions ever made; it was a shame Zenith left Chicago and moved to Korea in the 1980s-'90s. We will, unfortunately, never again see this level of quality in TVs or in anything else.

[old_coot88]

Originally Posted by joe111671

If Chester's in Kenosha has one I'll grab it Monday, otherwise I found a NOS one online. Can that regulator tube make the HV excessive and unadjustable if it's bad?

Here's a re-posting from post# 19:

[QUOTE]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.

[JohnCT]

Originally Posted by joe111671

Thanks John. It's 168 ohms.

If Chester's in Kenosha has one I'll grab it Monday, otherwise I found a NOS one online. Can that regulator tube make the HV excessive and unadjustable if it's bad? I might as well get one of those too if it's a possibility.

I think the control is OK. In a control that high in value, there's a bit of resistance from where the wiper finishes on the carbon track and the terminal crimp is.

To be sure, unsolder the other end and sweep the control through it's range to make sure there's not an open spot in the middle.

John