hitachi P50TO1u psu problem.

[buncey]

hi got a hitachi P50TO1u plasma in. set is dead suspect psu problem.

on inspection i noticed 1 trace was burned on underside of psu board.

also 2 resistors had burnt out.

repaired the trace and replaced both burned out resistors.

tested tv on low voltage on the variac.

i have noticed the 2 new resistors i replaced on the psu are overheating and getting red hot to the touch..

anyone have any ideas or come across this set or psu.

i have attached a picture of board.

thanks

[ozzietech]

Hi buncey, sounds like a short in the primary, possibly one or more of the FETs have gone down. Those plug in boards on primary side should be checked also for dry joints or shorted components.

[buncey]

hi ozzietech checked the fets while onboard and they seems ok.

also removed the plug in board done a reflow incase of dry joints but the  resistors still heating up? .

[Turnip]

Hi Buncey,

Your pic was great but didn't show resistors mentioned. Could you do it from the other end, as it seems a mains input problem  ?  - Chris.

[buncey]

hi chris.

i have took a picture from the other end mate.

you can see the 2 new ceramic resistors in place where the glue i used to hold firmly in place has remelted and made a mess due to the 2 new resistors getting red hot again..

[Turnip]

Hi Buncey,

Nice piccy, but haven't yet met that chassis - hope other Quuqers might - Chris.

[daz]

very strange,
it looks like you have used hot melt glue?, the resistors are 7watt  wire wound resistors and are designed to run very hot, if you’ve got no shorts on the primary side , turn the power on and see what happens.
it may work or at least something will go pop narrowing down the fault. but there aren’t too many things in that part of the circuit that will pull out both those resistor, a leaky bridge perhaps?
got to be worth a go, whats the worst that can happen. either way if that is hot melt glue you might want to remove it.
good luck
daz

[buncey]

hi daz, i only used the hot glue to keep them in place while i tested the psu as i had to make them fit and did not want them oving while i tested psu.

i have not tryed the psu on full power on variac, i noticed the old 5w resistors had burnt out and i noticed when i put the new resistors in place they were getting red hot and my variac was only pushing out 100v so i think there must still be a problem someware as i know they do run hot but think they should not be running hot to the touch when psu is only being fed only 100v?.

[daz]

aahhh that makes more sense, thought you had run mad for a second.
ive had these 5w 10r resistors fail many times for no reason on the 42inch sets, must admit to not seeing this ver yet. but really strange there’s no obvious short,
apart from the bridge being leaky you could check the smoothing blocks arnt open or short and the isolator pad behind the fets are ok (no tracking to earth)
but had faults many times when cold testing everything checks out only for the components to fail under load. no easy answer to this one im afraid, apart from having someone else flick the switch while you cower in the office , head between knees.
keep plugging away
good luck
daz