Zenith 25MC33 Set - Videokarma.org TV - Video

bhegges · #1 03-04-2023, 08:21 PM

Working on a Zenith 25MC33 color roundie chassis and would like some troubleshooting advice. I have gone through and replaced any resistors over 20% which was only 4, weak tubes were swapped with good tested tubes, replaced the electrolytic capacitors, and like others had to replace the horizontal linearity coil as the plastic form had fallen apart.

The set powered up but I can not get full vertical deflection, it appears to be folding over short from the top. I can fill the screen by adjusting the vertical size and vertical linearity but then can not get the vertical hold to lock.

117v line voltage
adjusted high voltage to 25kv
230ma HOT current (a bit high, 110v line voltage brings it down to 210ma)
4:3 ratio DVD player feeding composite video/audio into RF converter outputting to ch3
quick and dirty first pass on color & convergence

Troubleshooting so far:

I have tried substituting both vertical tubes (V7 & V10) but get the same results.
Knowing that the vertical output transformers are subject to fail I went in and replaced the film capacitors and resistors in the vertical circuit.
I also tried replacing K1 & K2 with a homemade circuit, thought they might be the problem and swapped the originals back in (same performance)
I took pictures and now have rechecked and retested each component to ensure I didn't make a mistake

Here is what I get:

Highlighted parts replaced:

Electronic M · #2 03-04-2023, 09:44 PM

Have you tried adjusting the centering rings (fishing line loops protruding from the yoke are how you adjust them) to move the image up then trying to set the height and linearity?
That may help.
Subbing the vertical tubes also may help.
If you can get it high enough you could adjust it to the edge of lock and try paralleling capacitors across the the existing capacitors to try shifting the operating frequency such that it can stay in sync with full screen deflection.
Voltage checks may also be worth doing...If a socket has carbonized and created a leakage current path or some similar nonsense that changing parts won't fix then voltage checks should catch it... Maybe do this before the parallel cap decade box engineering hack.

trinescope · #3 03-05-2023, 06:20 PM

Foldover at the top is going to be a problem with the start of the scan waveform, check the waveforms with a scope at the grid of the vertical output tube and the yoke winding of the output transformer for nonlinearity at the start of the ramp.

bhegges · #4 03-06-2023, 06:40 PM

Did some more troubleshooting and ultimately found the issue was the vertical output transformer. Installed a replacement transformer and I now get a full raster.

Next up continued testing & monitoring.

old_coot88 · #5 03-06-2023, 08:34 PM

Where did you manage to find a replacement horz lin coil?

bhegges · #6 03-06-2023, 09:46 PM

To my surprise Talon Electronics had a Miller replacement coil in stock. I must have gotten the last one as they now show out of stock: https://talonelectronics.com/shop/item.aspx?itemid=6636. The failed coil was wound on a light blue plastic, the new Miller coil was on white. Part of me still worries that this one will fail in time too; to that point I installed a 250ma fuse to protect the HOT and flyback. The fuse holder also provided an easy access point to connect an amp meter to.

I would think a person could search old parts catalog and look for a generic coil with a matching inductance range. The coil should come with the metal clip that locks into the chassis via drilling a small hole.

Electronic M · #7 03-06-2023, 10:15 PM

Another viable option is if you come up on a scrap without that coil, but with others that use that form is to wind the wire from the original onto a good from from a different Zenith coil.

old_coot88 · #8 03-06-2023, 11:08 PM

FWIW, here is an exerpt from a post of several years ago, although it doesn't mention the light blue form:

It's easy to verify that the coil form is mechanically sound by 'wiggling' it briskly. If it's compromized, it'll be obvious and the coil will act like a Slinky.

There were three types coil forms that were not prone to this problem:

1. Brown phenolic
2. Yellow plastic
3. Solid white (opaque) plastic

The one that's always suspect is a whiteish semi-translucent plastic.

bhegges · #9 03-07-2023, 08:51 PM

Next item:
I am checking over the flyback cage and found the 18M resistors way over tolerance. I was looking to use high voltage rated resistors which are very large or could use a combination of parallel and series resistors to get close on resistance and power.

My question: was the 2 watt power rating for withstanding voltage vs power dissipation? If so than a single 39M 1 watt HV resistor would make for a simple replacement: https://www.digikey.com/en/products/...5JAC00/9693129 they even make a 36M value but are not in stock nor sold per each.

Yamamaya42 · #10 03-07-2023, 09:29 PM

Quote:

Originally Posted by bhegges

Next item:
I am checking over the flyback cage and found the 18M resistors way over tolerance. I was looking to use high voltage rated resistors which are very large or could use a combination of parallel and series resistors to get close on resistance and power.

My question: was the 2 watt power rating for withstanding voltage vs power dissipation? If so than a single 39M 1 watt HV resistor would make for a simple replacement: https://www.digikey.com/en/products/...5JAC00/9693129 they even make a 36M value but are not in stock nor sold per each.

this probably has something to do with High Voltage Creepage and the fact that resistors back when the set was made had a less potential to handle high voltages like the ones made today. IE one can do today where 2 or 3 was needed 50 years ago.

Electronic M · #11 03-08-2023, 12:49 PM

2x 18M 2W in series is 36M 4W. I would be concerned that a 1W part wouldn't be able to handle that.
There's no harm in placing 2-4 1W or higher resistors in series and or parallel to spread out the power. If I'm spending the money to special order unusual values of parts for a set I rather over build then under build.

Yamamaya42 · #12 03-08-2023, 02:09 PM

I just used the same type of resistors in my Hoffman Colorcaster in the 6BK4C area, it did not specifically call for HV type ones, but I chose to use them, as it did say 1W, but these type do not come any larger than 1W(10kv), however.

kvflyer · #13 03-08-2023, 03:21 PM

Quote:

Originally Posted by Electronic M

2x 18M 2W in series is 36M 4W. I would be concerned that a 1W part wouldn't be able to handle that.
There's no harm in placing 2-4 1W or higher resistors in series and or parallel to spread out the power. If I'm spending the money to special order unusual values of parts for a set I rather over build then under build.

I'm thinking that in a series circuit, the current is always the same, not double?

Yamamaya42 · #14 03-09-2023, 10:06 AM

https://www.digikey.com/en/products/...2W618/11651620

These will most likely work for you.
They are not HV rated, but I doubt that's what was in there in the first place anyway, and 2 in series will be what was in there to start with.

Electronic M · #15 03-09-2023, 11:44 AM

Quote:

Originally Posted by kvflyer

I'm thinking that in a series circuit, the current is always the same, not double?

You're not wrong, but power is not current. Power is voltage times current, or current squared times resistance. Replacing 2 resistors of equal value with a single part you're doubling the resistance in the second equation and doubling the voltage drop across the resistor in the first equation so 1 part needs to be 2x the wattage of 2.

I like Yamamayas solution of 2 18M 2W parts... Can't get the math wrong if you are not doing any math.