K4900 blowing fuse.

HunterStephens1

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Im stumped on this and have been chasing it around like crazy. The fuse on my 4900 keeps blowing so I followed the flowchart and checked the HOT, seemed to be fine. Continued to replace diodes 501-504. blew the fuse. removed the HOT from the circuit and continuity tested from the fuse to ground. shorted directly. removed the VR from the circuit. same result. continued on testing things on the board and I come across t351, the horizontal drive transformer. one side of the transformer reads at 52 ohms, looks fine, the other side is at 7 ohms and sets off the meter as a short. Im super lost here and I hope someone with more knowledge than me knows whats going on.
 
Have you checked the diodes in the power supply area? You can Isolate the power supply section from the deflection circuits by lifting one of the jumpers at the back of the board.

connect a 120volt 40-60watt incandescent bulb to the power supply side of the jumper and the metal frame of the chassis. If the supply is good the fuse wont blow and the bulb will light.

If it still blows your power supply is the problem. If not its something within the deflection board. Something large, polypropylene cap, large diode, something thats large enough to handle that b+ load and not blow up.
 

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ok i disconnected j16, the jumper that is the HV line, and r601, the first place power goes out of the rectifier. also replaced d501-d506 again with tested good diodes. the fuse still blew so that must mean that the issue is somewhere in the rectifier circuit if my assumption is correct?
 
do you have the degaussing coil unplugged? I've seen those wear and short to frame.
 
how likely is it that C501, a mylar cap, or c508, a cap not listed in the manual but labeled on the board, are bad?

Its got to be something of good size to blow that and not go itself. There are not that many parts in the power supply. Check all of them.

The VR has the insulator on it, right?
 
yes vr has its paper insulator. hard to determine if parts are good or bad in circuit. should I pull them to check?

replace the fuse and ohm out the AC input. I'm curious to see if theres something so shorted that its making its way back to the input power connector.
 
upon more investigation something is causing d501 to fail. pulled it and it has now failed. replaced it and still shorted between ac in lines
 
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