HISENSE F42K20E Blown Main Cap and Fuse

[1beertap]

Hey all,

I found this tv at a local recycler and took it home. Upon trying it out, I noticed that there was no power at all. I took the panel apart and I checked the fuse, which was blown. Next, I then checked the rectifier to be good. I noticed there was a bulge in the main cap and tested it to be bad. I replaced the main cap and fuse. Now, I turned on the tv and yes the tv came on, but briefly. I tried it again and heard a "sizzle" type sound coming from the area the new main cap. Next, I took a voltage reading on the main cap and observed that the voltage, when on, was exceeding the 450vdc that the cap is rated. Also, I took a reading on the plus and minus legs of the rectifier and observed the same results. At one time I saw almost 800vdc on my meter!!! I really don't want to blow a cap. My question is what could cause the overvoltage?

[budm]

Re: HISENSE F42K20E Blown Main Cap and Fuse

"At one time I saw almost 800vdc on my meter!!!" Impossible! What did you use for the ground pointy for the meter? Did you verify the meter that it is OK? You will see big explosion if that cap is exposed to 800V.

[1beertap]

Re: HISENSE F42K20E Blown Main Cap and Fuse

budm, It is randomly going all over the place. My meter is okay. I saw 753vdc to be exact. The voltage seems to be hovering around 540vdc, but again it's not stable. I'm too afraid to leave it on for any length of time because of the risk of an exploding cap. When the tv is cool, the voltage on the cap seems good at around 380vdc and holds steady for a few seconds, but it doesn't take long after startup and the voltage increases and goes all over the place. I know that there is a part that is thermally failing.

[budm]

Re: HISENSE F42K20E Blown Main Cap and Fuse

What voltage do you get if you have the power supply board by itself without having the cable connected to the main board? it should be around 160~170VDC.

[1beertap]

Re: HISENSE F42K20E Blown Main Cap and Fuse

I get 165vdc on main cap with the main board disconnected.

[ben7]

Re: HISENSE F42K20E Blown Main Cap and Fuse

It's not as impossible as you think!

Try disconnecting the feedback sense on a poorly designed smps.... this is the result. You might even be able to get thousands of volts out, but I think that cap would blow quickly.

In fact, I am surprised your new cap didn't vent. Did it get hot? Check to see if it is bulged, it might be damaged now.

[tom66]

Re: HISENSE F42K20E Blown Main Cap and Fuse

I've had a PSU PFC circuit go crazy. Result was an explosive venting of the 450V capacitor. Voltage exceeded 650V for some time.

[Th3_uN1Qu3]

Re: HISENSE F42K20E Blown Main Cap and Fuse

Yes this does sound like a bad PFC circuit. I would try replacing the PFC controller IC. It could also be from the passive components in the feedback loop, but an IC is more likely to blow than a few resistors.

[budm]

Re: HISENSE F42K20E Blown Main Cap and Fuse

I have seen when PFC lost its feedback and the Voltage on the cap went up to around 600V but the cap blew up badly. I can see the cap goes up as high as 800V if it is operating from 230VAC power line.

[tom66]

Re: HISENSE F42K20E Blown Main Cap and Fuse

I know the voltage exceeded 650V on my set as there was a 650V CoolMOS in that area of the circuit which failed after the cap vented. Drain-gate short, drain-source ok, blew the arse-end out of the driver IC. Rare fault.

PFC circuits can pump huge amounts of energy into a circuit when they go bad - they are essentially wide-range boost converters...