Wg 25k7501 please help

fredxd9

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I just bought a Midway Off Road Challenge that has a Wells Gardner 25K7501 monitor that is dead , All I get is the ticking noise of the high voltage protection CKT . I checked the fuse it's good, I checked the input power and it's 120V ac . So my next step was to get the overhaul kit from Bob Roberts ( new caps, new HOT , and a new Flyback ) . While board was out I re-flowed several solder points that looked questionable and checked for broken traces and found none. Reinstalled board and made all proper connections and it's still dead and still ticking. I am by no means a electronics guru, I do however understand the basics and take pride in tackling most repairs myself, This one however is kicking my butt and I could really stand some good advice on where to look next. Thank you in advance
 
the HOT is most likely bad. Pull it and test it. Probably not the flyback but if you want to order one so you have it "just in case", then okay. I usually order more than one HOT in case it blow again upon power up. Make sure you have an insulator installed between it and the heat sink, too...
 
the HOT is most likely bad. Pull it and test it. Probably not the flyback but if you want to order one so you have it "just in case", then okay. I usually order more than one HOT in case it blow again upon power up. Make sure you have an insulator installed between it and the heat sink, too...
Have already replaced flyback, HOT and done cap kit to the board also checked board over for cold solder and broken traces. I'm stumped even pulled critical diodes off board and tested out of ckt.
 
i guess you made sure the HOT was insulated, right?

Check the parts around the width coil...
 
You can try isolating your power supply from the monitor using a 60W light bulb. Do this by unsoldering R728 and R100(jumper) at C121. This isolates the power supply. Hook up the bulb between the pad where the resistors were hooked and ground. If the light bulb lights, you know it's in the secondary. Measure your B+. Should be about 118vdc. If it does not light, then you have a problem in the power supply of your monitor. Also make sure you checked and/or changed C117 in your supply section. It's a 22uF @ 63V electrolytic but I usually up the voltage rating to a 100V.
 
Just double checked all, Mica insulator is in place even put another HOT in just to be sure. B+ still at zero. tried adjusting B+ down then back up and still nothing. there is no activity at the tube neck. I discharge flyback each time I remove chassis and It appears no voltage is ever getting to the flyback, never even a small spark. It appears the board is never fully powering up. I left it run for a few minutes and the R121 in the power supply section of the board gets warm, according to schematic it feeds degausing coil. I have to be overlooking something obvious ( can't see the forest thru the trees sort of thing ).
 
will try light bulb test tomorrow, I double checked C117 and it did get changed (22uf @ 100v) Thanks for the tip.
 
Ok preformed the "Light Bulb Test" Bulb DOES Light up. What would be the next logical test, Or next most common failure since the HOT, Flyback and caps have been done? BTW the voltage was at 114vdc .
 
Well, if you replaced every cap, start testing every single diode and transistor on the board....
 
Well, if you replaced every cap, start testing every single diode and transistor on the board....
Seems to me there would be a more logical way to determine the problem that lies beyond the power supply circuit, I mean I'm getting a little cross eyed starring at this board a couple hours an evening. If I know that the power supply section is functional, The HOT is new and tests good, Flyback and electrolytic caps are new, yet the board is still in voltage shutdown (ticking) and there is never any discharge from the anode when I discharge it after powering it up. How could it be in over voltage shut down when no voltage ever got there to charge the flyback? I am overlooking something here and am curious if anybody could point me to the next step to properly diagnose this board short of pulling 50 different components to test out of circuit. What would be the result if the tube where bad? would I still get neck glow? Would the flyback internal cap still charge? Or would it just sit there and click.
 
The flyback doesn't get charged, it's just a transfomer - winds of copper coils around a ferrite core.

Have you measured your B+?

The 7500 (and related monitors) can be a real bear to work on.
 
The flyback doesn't get charged, it's just a transfomer - winds of copper coils around a ferrite core.

Have you measured your B+?

The 7500 (and related monitors) can be a real bear to work on.
I understand the coils being windings and ferrite but the internal cap in the Flyback normally retains voltage that requires discharge after system has been powered up , Right?? why does mine not discharge any voltage. I mean if the internal CAP did not ever get charged, how can the voltage protection circuit shut down ( voltage never got there ) maybe I don't fully understand what is being protected by the shut down
 
When the OCP (over current protection) circuit detects a rise, it disables the entire HV, so there is nothing there to discharge. You have a short or open somewhere. Unless you know how to power up the chassis SAFELY while it is connected to the tube, but still accessible for you to do voltage measurements without shorting anything out, you'll need to just start checking the parts that tend to cause this. It doesn't take long to just find all the diodes and transistors and do quick checks of them with a DMM.

If all appear to be okay, then you check the big ceramic caps near the flyback, as well as the big power resistors in that area. If all still looks good, then you need to trace the power through and find where it starts going bad.

Here's what is normally recommended to check when you get a clicking K7500, but it doesn't cover everything:

K7500click.jpg
 
Are you reading 12V at pin 10 of U701? I also recommend checking Q703, Q704, and R728.
 
When the OCP (over current protection) circuit detects a rise, it disables the entire HV, so there is nothing there to discharge. You have a short or open somewhere. Unless you know how to power up the chassis SAFELY while it is connected to the tube, but still accessible for you to do voltage measurements without shorting anything out, you'll need to just start checking the parts that tend to cause this. It doesn't take long to just find all the diodes and transistors and do quick checks of them with a DMM.

If all appear to be okay, then you check the big ceramic caps near the flyback, as well as the big power resistors in that area. If all still looks good, then you need to trace the power through and find where it starts going bad.

Here's what is normally recommended to check when you get a clicking K7500, but it doesn't cover everything:

K7500click.jpg


"It doesn't take long to just find all the diodes and transistors and do quick checks of them with a DMM."

Funny. Testing diodes in circuit have rarely been of much value. Testing out of circuit would be tedious.
 
I do it like this:

Test in circuit - Funny reading? Then lift one leg and test again.....
 
OK after much grief chasing diodes and transistors down for 2 hours and finding nothing, I went to Wells Gardner website and found the description of operation for the power supply circuit and after a bit of reading decided to make a list of components that could cause my problem, It seems U101 plays a big roll in over current protection There were many things I know it could not be after doing the light bulb test (I thought ). So I'm running down my list and come to R104 a 10k 7 watt resistor which I have never touched with a meter because it looked perfect, and my previous experience with power resistors is when there bad they look bad ( burned, cracked and so on ) . Well mine reads open circuit, which makes me think "OK how did this thing light up that light bulb?" If this is a component that can put the board in over current shutdown, Why did the bulb light??? Why with the bulb in place did it not trigger OCP?? I am at this point just guessing this is my problem because out of 10 other game monitors and 3 old computer monitors there is not a 10k 7 watt resistor in the shop. I PRAY that Monday the only electronics supplier in the area has one ( really I will buy 2 ) . Then I'll see what happens or what doesn't
 
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