|
#1
|
|||
|
|||
Zenith 25MC33 Set
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.
Troubleshooting so far:
Here is what I get: Highlighted parts replaced: |
#2
|
||||
|
||||
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.
__________________
Tom C. Zenith: The quality stays in EVEN after the name falls off! What I want. --> http://www.videokarma.org/showpost.p...62&postcount=4 |
#3
|
|||
|
|||
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.
__________________
Erich Loepke |
#4
|
|||
|
|||
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. |
#5
|
|||
|
|||
Where did you manage to find a replacement horz lin coil?
|
Audiokarma |
#6
|
|||
|
|||
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. |
#7
|
||||
|
||||
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.
__________________
Tom C. Zenith: The quality stays in EVEN after the name falls off! What I want. --> http://www.videokarma.org/showpost.p...62&postcount=4 |
#8
|
|||
|
|||
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. |
#9
|
|||
|
|||
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. |
#10
|
||||
|
||||
Quote:
__________________
=^-^= Yasashii yoru ni hitori utau uta. Asu wa kimi to utaou. Yume no tsubasa ni notte. いとおしい人のために |
Audiokarma |
#11
|
||||
|
||||
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.
__________________
Tom C. Zenith: The quality stays in EVEN after the name falls off! What I want. --> http://www.videokarma.org/showpost.p...62&postcount=4 |
#12
|
|||
|
|||
Quote:
|
#13
|
||||
|
||||
Quote:
I like Yamamayas solution of 2 18M 2W parts... Can't get the math wrong if you are not doing any math.
__________________
Tom C. Zenith: The quality stays in EVEN after the name falls off! What I want. --> http://www.videokarma.org/showpost.p...62&postcount=4 |
#14
|
||||
|
||||
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.
__________________
=^-^= Yasashii yoru ni hitori utau uta. Asu wa kimi to utaou. Yume no tsubasa ni notte. いとおしい人のために |
#15
|
||||
|
||||
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.
__________________
=^-^= Yasashii yoru ni hitori utau uta. Asu wa kimi to utaou. Yume no tsubasa ni notte. いとおしい人のために |
Audiokarma |
|
|