WGK7400 Power Failure?

[ToplessZ]

Ive got a WG K7400 (I think, the chassis is a P763) in a MK4 machine.  The monitor was working great and then suddenly today it went black.  Its making a clicking sound from the chassis and doesnt seem to be powering up.  I also smelled a little burn but I dont see anything burnt.  Anyone know what failed?

edit.  The fuse is still good.  Everything is still connected correctly.  So something failed.  Flyback?(no cracks on it)  Is there some kind of voltage regulator?

[Ken Layton]

it's possible you have some bad solder joints particularly around the power input connector pins and the flyback terminals.

[ToplessZ]

no luck, I resoldered the input pins and several other legs that where in the power section.  It seems to be getting power, but something is makeing a fast clicking sound onthe board?

[sarge]

C117 can cause this kind of symptom among other things. 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.

[qrz]

the h output xistor (2sc3686) is likely shorted ......

and do check for poor solder connections - and check c724,d708

qrz

[ToplessZ]

I cant find anything shorted,, The flowchart from wells gardner seems to be pointing at the flyback.  I went ahead and ordered one from Bob Roberts, so hopefully I didnt just spend 30 bucks for no reason.  Ill update thursday since thats when it should be here.

[Ken Layton]

Make sure you're ordering the correct flyback. This is NOT a k7000.

[ToplessZ]

bobs got them for 19 bucks plus shipping , so i got the right one.  I hope it fixes it.

[Ken Layton]

The k7400 flyback sells for $25 while the k7000 series flyback is selling for $19. These have different part numbers and are NOT interchangeable.

K7400/K7500 flyback part number is 053X0651-001.

K7000/K7191/K7600/K7900 flyback part number is 053X0528-001.

[ToplessZ]

I actaully had Witchboard order it with his account since Bob drop ships for him already; and he lives down the street from me.  So I hope he ordered the right one(He knew this was a K7400 so Im sure he ordered it correctly)   We'll find out tommorow, and hopefuly with good news after the install...

[ToplessZ]

Bad news,,, I got the right flyback, but it didnt fix the problem.. Im gonna try and replace the HOT next even though its not shorted.

[SirPeale]

Going WAY back to when I was working on many more of these than I do now, I seem to remember the 7400 being kind of fickle regarding it's B+.  Might want to check that. 

[sarge]

Why keep replacing parts that you know are not bad?  I suggest you try and isolate the problem rather than shotgun the repair.  It will be faster and cheaper.  If your monitor repeatedly clicks, that means your OCP is keeping your high voltage from turning on either because you have a short on your secondary or your power supply itself is faulty.

[Witchboard]

Looks like TZ has thrown in the towel.  I'm going to pick up the troubleshooting on the chassis.

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.

Out of curiosity, is there a reason why I can't use a voltmeter to verify the voltage?  Does the power supply need a load or something?

[sarge]

Yes  you need to check your voltage under load.  All this is doing is isolating the problem to either the power supply or the secondary.  The light bulb just provides a "visual" load.  Your B+ should be around 118VDC.  Post back what you find with this test.

[Witchboard]

Interesting development.  C121 appears to be charred on one side.  Possible point of failure?

[castlesteve]

thats it.  There is another one right next to it of the same value.  Replace both of those caps.  Either one of them can short out.  That should likely do it.  I see this all the time on k7400

[Witchboard]

There is another one right next to it of the same value.  Replace both of those caps.

You mean C119?

[Witchboard]

I need assistance verifying the rating of the cap in question.  Obviously it's 500V, but the monitor manual states that it's a .1 and I'm presuming that's the uF.  However, the pic confuses me, unless the 104 means .1 uf and 04 something else.  Can anyone clarify?

Yes  you need to check your voltage under load.  All this is doing is isolating the problem to either the power supply or the secondary.  The light bulb just provides a "visual" load.  Your B+ should be around 118VDC.  Post back what you find with this test.

Just for clarification, the flyback will still be activated during this test, correct?

[SirPeale]

With SMD resistors, the first two numbers are the value, and the last number is the number of zeroes after that value.  So for 104 it would be 100000.

This is of course different, but the convention may be similar.

[sarge]

That capacitor is a .1 uF capacitor.  The 104 means 10 x 10^4 picofarads which converts to .1 uF.

As for the light bulb test, no the flyback would not be involved.  All it is doing is testing the B+ power supply section of the monitor.

[Witchboard]

I've ordered some replacement caps.  ToplessZ dropped by with another of the same chassis.  Related issue, but different chassis.  C119 is burned off to the legs.  What causes this?  Is there a particular component down the path that's faulty?  I figured I might as well attempt to fix both chassis while I'm at it, but I don't want to replace parts just to have them burn up again.

[Witchboard]

I figured I'd update this and let everybody know that replacing the capacitors at C119 & C121 resolved the issue.  The replacement caps were larger in size, but the correct values.  Worked like a charm.  Thanks all for the help.

I'd still like to repair the other chassis, so if anybody knows of any possible reason these caps would go bad so frequently, please let me know.  I can accept they just may be crappy caps, but I'm not wanting to toast my new parts if another component is the culprit.