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  #31  
Old 07-14-2014, 09:49 AM
old_coot88 old_coot88 is offline
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The plate load resistor of the V.osc is a very common failure. It drifts high due to constant hammering by high level pulse.
Common problem not just in Zeniths but all designs using B Boost to feed V.osc.
B&W as well as color sets.

Last edited by old_coot88; 07-14-2014 at 09:52 AM.
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  #32  
Old 07-14-2014, 01:50 PM
DaveWM DaveWM is offline
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may have been mentioned but look for any caps in the boost filter supply to the vert osc plate load cap. think its a .1 leakage here could drop the supply voltage. IIRC there are two one right off the boost the other is upstream a bit, closer to the vert circuit. Again voltage measurement would prob point this out. I use a vtvm for checking.
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  #33  
Old 11-04-2014, 09:05 PM
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Kamakiri Kamakiri is offline
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Quote:
Originally Posted by old_coot88 View Post
The plate load resistor of the V.osc is a very common failure. It drifts high due to constant hammering by high level pulse.
Common problem not just in Zeniths but all designs using B Boost to feed V.osc.
B&W as well as color sets.
And drift high it did. Supposed to be 473K, tested at 567K. Picture is much improved, but still has foldover on the bottom. After about 5 minutes of folling around with the adjustments, the picture went somewhat out of focus, and I stopped there.

One interesting thing to note....every time the screen changed scenes drastically (dark to light), I'd lose vertical lock.
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  #34  
Old 11-04-2014, 09:12 PM
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After letting it cool down for about 15 minutes, the focus came back. Here's where we're at.....
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  #35  
Old 11-05-2014, 03:03 AM
consoleguy67 consoleguy67 is offline
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Have you checked to make sure that the vertical output transformer is working properly? I had a foldover issue with my Zenith 24NC31Z.
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  #36  
Old 11-05-2014, 06:40 AM
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I did the resistor check and solder job upside down with the chassis still in the set

Think from here I'm going to pull the chassis and start checking all the resistors in the vertical section and ohm out the VOT. I had some time to spare last night, but I didn't want to go yanking the chassis until I get my 730TV2 buttoned up.
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  #37  
Old 11-05-2014, 10:57 AM
old_coot88 old_coot88 is offline
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Re. the plate load resistor of the vert. oscillator, Kamakiri wrote,
Quote:
...And drift high it did. Supposed to be 473K, tested at 567K.
Tolerance-wise, that's really not a big drift. It will often increase by a factor of 10 or more, playing havoc with the height and lin.
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  #38  
Old 11-05-2014, 12:13 PM
zeno zeno is offline
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If you havnt change the integrators. Transformers went but IIRC
you got a very small pix & a resistor burned. 'Lytic in cathode
of vert out also a high fail on any TV, B&W or color. After all
the tests start with caps in feedback loop.

73 Zeno
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  #39  
Old 11-06-2014, 08:50 AM
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The Vertical integrators here I remember having to replace on my grandmother's Zenith of this chassis. One of them twice. Vertical output transformer shorted out too. That and the caps in the vert section and it ran 9 years, the CRT kicked the bucket. It became my bedroom set. A donor CRT from another Zenith roundie saved it's life. Lasted me until 1990 when I moved. Didn't have room for it.
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  #40  
Old 11-06-2014, 09:51 AM
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I was thinking about that too, as I've replaced integrators before. But only with integrators from parts from other sets.

This site shows how they map out, but not wattage of resistor or working voltages....

http://www.boxcarcabin.com/integrator.html
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  #41  
Old 11-06-2014, 10:15 AM
zeno zeno is offline
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Quote:
Originally Posted by Kamakiri View Post
I was thinking about that too, as I've replaced integrators before. But only with integrators from parts from other sets.

This site shows how they map out, but not wattage of resistor or working voltages....

http://www.boxcarcabin.com/integrator.html
1/2 W resistors & 1 KV disc caps more than enuf.......
They caused so much trouble we automaticly changed them
on a "major" job even if no vert problem.

73 Zeno
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  #42  
Old 01-02-2015, 09:40 AM
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Well looky here!

As I hopscotch my way back through my projects, I pulled the chassis and started checking resistors. Replaced some half watters that were somewhat out of tolerance, then I hit this.

The third color band on the top resistor could be red, could be brown, could be black....but in any event it's obviously been overheated. Resistance on that one reads 4K, so either way it's wildly out of tolerance.

The other resistor appears to be a 22 ohm resistor reading 208 ohms, which is leaving me , but in any event both have obviously been overheated and aren't what they should be. I bet these are what got toasted when the yoke failed.

I ordered 5 watt replacements from Mouser, which are en route. Hopefully this is on track to curing the wild linearity issue.
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File Type: jpg 100_5228.jpg (87.6 KB, 53 views)
File Type: jpg schematic.JPG (86.5 KB, 146 views)
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  #43  
Old 01-02-2015, 12:43 PM
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Quote:
Originally Posted by Kamakiri View Post
Well looky here!

As I hopscotch my way back through my projects, I pulled the chassis and started checking resistors. Replaced some half watters that were somewhat out of tolerance, then I hit this.

The third color band on the top resistor could be red, could be brown, could be black....but in any event it's obviously been overheated. Resistance on that one reads 4K, so either way it's wildly out of tolerance.

The other resistor appears to be a 22 ohm resistor reading 208 ohms, which is leaving me , but in any event both have obviously been overheated and aren't what they should be. I bet these are what got toasted when the yoke failed.

I ordered 5 watt replacements from Mouser, which are en route. Hopefully this is on track to curing the wild linearity issue.
Those are the B+ resistors for the vertical stages. They get burned up when the vertical section develops an overload. They definitely need to be replaced.
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  #44  
Old 01-02-2015, 02:08 PM
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Zenith26kc20 Zenith26kc20 is offline
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On my "M" and "N" Zeniths, the vertical output transformer burned them up. My 25MC33 started with insufficient height and loss of sweep momentarily on bright scenes. It then went to what looked like severe interference and vertical instability and non linearity. That's when the resistors started to overheat. The NC33 just shorted and failed with no warning.
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  #45  
Old 01-02-2015, 02:25 PM
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The way those resistors are wired makes it look like you could replace them with a single 1100 ohm 4 or 5W resistor.
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