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Hi all,
Please help. I have a Tatung (possibly a VT 1440  :103-) When I remove the deflection yoke, all the voltages are fine, but when I attach the yoke the power supply starts "oscillating" and the voltages drop.

I have measured the yoke and it is around 2.6 ohm on the vertical and 0.6 ohm on the horizontal  :103- :103-
This sounds a bit low to me?? but the yoke looks fine.
Anyone have any info or ideas on this?

The CRT is a Chunghwa A48AGY13X57 and it tests fine with the CRT tester.

You didn't power it up with the yoke unplugged did you?

Yes I did... I know I shouldn't have but it made no difference.
maybe the guy I'm repairing it for has tried it too ???

powering it up with the yoke unplugged there's a good chance the horizontal output is shot. You can check that with a multimeter, center leg of output to each of the outside legs should not show shorted. I don't remember how a yoke will show on a multimeter, but you can't usually tell visually if they are bad. It is HIGHLY unusual for them to go bad though. Probably the toughest part they ever put on ANY tv.

Thanks, The H.O.T is fine, I have HV but it's only about 8 kV and the SMPS is pulsing so I think it's overloaded.
The heater voltage is also very low or missing??

I don't where to search for the error now  :99-

Sounds like it could be a bad voltage regulator or a diode in the power supply or Horz circuits although if it was in the horz I'd expect the HOT to be bad. Flyback is a possible source also, but you'd expect arcing from it if that was the case although not always. I'd start by checking diodes and maybe look for capacitors that appear to be stressed in those circuits. The fact that it's pulsing to 8k volts means it's trying to start and shutting back down. These repeated attepts are probably what's warming up your power supply

Cowboy is on the right track....
The power supply USUALLY makes B+ by taking the AC line and multiplying it. 120 X 1.414 (RMS) and charging caps with that. If any of those dioes shorted (or opened) it will drop.
If the HOT does go bad (Shorted) it will pull the B+ down, But here is one a lot of people will miss.....
There are 2 or 3 diodes in SERIES that serve as a "Clamp" to prevent the high voltage from exceeding whats "Safe". They shunt the overvoltage to ground. When one or more of them fail, this will happen. It's a safety circuit.
* WARNING *  DO NOT substitute other diodes in this clamp! They are chosen to do this job carefully. If you use one too low, you wont get the voltage needed, if you use one too high it wont clamp anymore...
They are USUALLY 600 volt diodes (1N4004) is a common choice.
How to test?  Simply lift one leg of the string and fire it up. If it runs OK REPLACE THEM ALL!!!
Be careful in there, this line will be around 800-1200 volts.... it will kick your ass.....

WHY are they in there?  If you take a CRT and accelerate the voltage it will burn the screen and emit "X-Rays". This diode stack prevents that from happening.


Kirk

Kirk and Cowboy,  K+ to both of you. :3- :3-  :131- :131-


Stat :31-

Thanks all,
I will try this later today. Hope this solves the problem....
I''l get back to you with the results later

Something went wrong my my earlier post so I'll try to sum up....

----------------------------------------------------------
Hi all,
Please help. I have a Tatung (possibly a VT 1440  :103-) When I remove the deflection yoke, all the voltages are fine, but when I attach the yoke the power supply starts "oscillating" and the voltages drop.

I have measured the yoke and it is around 2.6 ohm on the vertical and 0.6 ohm on the horizontal   :103- :103-
This sounds a bit low to me?? but the yoke looks fine.
Anyone have any info or ideas on this?

The CRT is a Chunghwa A48AGY13X57 and it tests fine with the CRT tester.
----------------------------------------------------------

Thanks, The H.O.T is fine, I have HV but it's only about 8 kV and the SMPS is pulsing so I think it's overloaded.
The heater voltage is also very low or missing??

I don't where to search for the error now   :99-

----------------------------------------------------------

Quote from: KirkLasVegas on Yesterday at 01:26:48 PM
Quote from: cowboygames on Yesterday at 01:10:59 PM
Sounds like it could be a bad voltage regulator or a diode in the power supply or Horz circuits although if it was in the horz I'd expect the HOT to be bad. Flyback is a possible source also, but you'd expect arcing from it if that was the case although not always. I'd start by checking diodes and maybe look for capacitors that appear to be stressed in those circuits. The fact that it's pulsing to 8k volts means it's trying to start and shutting back down. These repeated attepts are probably what's warming up your power supply


Cowboy is on the right track....
The power supply USUALLY makes B+ by taking the AC line and multiplying it. 120 X 1.414 (RMS) and charging caps with that. If any of those dioes shorted (or opened) it will drop.
If the HOT does go bad (Shorted) it will pull the B+ down, But here is one a lot of people will miss.....
There are 2 or 3 diodes in SERIES that serve as a "Clamp" to prevent the high voltage from exceeding whats "Safe". They shunt the overvoltage to ground. When one or more of them fail, this will happen. It's a safety circuit.
* WARNING *  DO NOT substitute other diodes in this clamp! They are chosen to do this job carefully. If you use one too low, you wont get the voltage needed, if you use one too high it wont clamp anymore...
They are USUALLY 600 volt diodes (1N4004) is a common choice.
How to test?  Simply lift one leg of the string and fire it up. If it runs OK REPLACE THEM ALL!!!
Be careful in there, this line will be around 800-1200 volts.... it will kick your ass.....

WHY are they in there?  If you take a CRT and accelerate the voltage it will burn the screen and emit "X-Rays". This diode stack prevents that from happening.


Kirk

I tried using your advice, but fell short when I wasn't able to find the diodes you talk about ???
I have attached the schematic I'm using. I found an annotated schematic someone has made, but it not helping me much at the moment either   :47-

I've tried lifting pin 8 of IC701 (Vertical IC) then the supply voltages are fine, I guess this means it's this IC ??? but how can I be sure??

I found a burnt resistor and capacitor (R636 and C870), replaced them, but nothing changed.

Checked both Q807 and Q808, but they are both fine ????

I'm really confused right now.....!!!!
--------------------------------------------------------------

Still confused  :99-

Unable to upload schematic, so here's a link

http://b.imagehost.org/0797/vs14428_annotated_schematic_diagram.jpg (http://b.imagehost.org/0797/vs14428_annotated_schematic_diagram.jpg)

http://d.imagehost.org/0570/Tatung_T-009_VT-1440S_VS-14428.jpg (http://d.imagehost.org/0570/Tatung_T-009_VT-1440S_VS-14428.jpg)

If you can lift the leg of the vertical out put and get the chassis to run allbiet having vertical collapse then it's 99% you've got a bad vertical output. Lift the leg and turn the power on. Look for a horizontal line across the middle of the screen, You may have to turn flyback output up to see it depending on what brightness was set at before it went bad. If it runs ok except for a lack of vvertical output then replace the output.

Thanks, just tried it and I got a horizontal line  :131-

I'll just wait for the LA7838 I've ordered.... thanks a lot I'll get back with the results, when I get them  :89-

Crap, I've probably got a 7838 in my diminishing pile of tv parts somewhere. While you're waiting for it be sure to check all resistors and diodes that feed it. You can check diodes in place by putting the leads of a multimeter across them one way then the other(cathode to anode then anode to cathode). Should check open one way and have .4 or .5 something reading the other. The 7838 was very common failure part, especially on Magnavox piece of crap sets in the middle to late 90s

I was able to delete the offending post which opened up that thread again... I then went ahead and merged the posts together thinking that it may make for better continuity... If not send me a PM with any posts that should be deleted, or pruned or whatever. :103-

There is some funny thing that happens in the software that is a mystery and it deals with quoting of posts, maybe multiple quotes.. It does not happen all of the time. I have had to fix this in the past before. It is usually not too easy to fix. :25- Not sure why it happens, but here are some things people can do:

IF you are typing your post and have added quoted posts, try using the preview feature (I am paranoid about losing alot of typed content, so I try to remember to hilight my entire post, and then right click and select copy just in case! :89- :89- ) before posting... If you see a white blank page, just hit the back button in your browser and you should be back in the editing window.

There is a way to still make it work buy deleting some of the quote header information, I will let Statfreak :31- detail this as he played with that trick. :186-

After that you can preview again... If you see your proposed post, then you are OK to proceed, if you see a white screen again, something is still weired out in the quotes.

Hope that htis helps explain the mystery a little... It is just a bandaid, but better than nothing!

CaptainHappy :95-
:admin-

I tried replacing the vertical output IC, but it still wont start up  :47-

If I de-solder pin 12 (output) on IC701, I get the horizontal line. If I then connect pin 12, while the power in on, I get raster  :103- but it wont start up by it self.....

can this be some sort of protection circuit not working as it should??? I'm really confused be this  :99-

One thing to check that you probably missed...

Many monitors power the vertical output section off of a tap from the flyback transformer. There's a resistor followed by a high speed rectifier diode in there (NOT a slow 1N4001, etc...) and a capacitor to smooth out the voltage.

If the cap and resistor you replaced are next to that diode then replace the diode. When they start leaking the cap will pop and the resistor will act as a fuse and burn.

Replace the diode and try it with the vert output ic in place.

RJ

Thanks,
I'll try that later today....

YES I found the problem.... :96-

It was a 2,7nF capacitor in the primary SMPS circuit. it's used for "measuring" the primary current in the transformer.
Changed it and now it works.....

Thanks for all your input guys :259-