1968 Zenith 20Y1C50 chassis - advice needed

[dieseljeep]

Originally Posted by zeno

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

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!

[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]

Originally Posted by zeno

Should be a regular carbon. Its a good idea to use whatever was in there.

73 Zeno
LFOD !

Originally Posted by Jeffhs

I agree with your advice as to using the same component as was originally in the set.

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.

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.

[zeno]

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
LFOD !

[Electronic M]

Originally Posted by joe111671

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.

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.

[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]

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.

[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.

[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