TV keeps blowing fuses !

[Pauldesign]

Hi Folks,

I've this Telefunken TV with power board labeled F-3445 Sun6132 that just keep blowing fuses instantly.

I've done the following to no avail:

- Check for shorts between live and neutral as well as ground.
- Check for shorts across the main rectifier / filter caps.
- Check for short in the bridge rectifier.
- Check for shorts in the MOSFETs
- Check for unintentional shorts in the ICs
- Check for shorts in the feedback circuitry.
- Check for shorts between the primary and secondary sides
- Check shorts in the diodes
- Clean the board and VIP it
- Replace the fuse with the same ratings (F5AH)

Any other hints besides ripping the PSU apart, as this is becoming a mystery now.

[charis]

Re: TV keeps blowing fuses !

Put out the brige rectifier.
Now with a multimeter put the com to + and V/Ω to -.
You see a resistance of 1.5KΩ.
Now put the com to + and VΩ to one of ac input.Do this again with the other ac input.
You see a resistance of 650 Ω.
If you put the V/Ω to + and the com to - or to ac you have infinity.

[CapLeaker]

Re: TV keeps blowing fuses !

Only thing you didn't check is if the mosfet driver and the circuitry that drives the gate of the mosfet. Mayby it is "stuck on".

Next thing... depending on how the PSU is made, maybe you are barking up the wrong tree. Some have a second "high efficiency" portion in the PSU with a separate bridge rectifier, PWM, transformer and rectifier diode for the STBY voltage.

[Caleb]

Re: TV keeps blowing fuses !

I place a 100 watt light bulb across fuse connection and see what gets hot, you have a major short somewhere. I had one last year that ended up being the primary filter cap but would only fail short under voltage.

[budm]

Re: TV keeps blowing fuses !

The main filter cap may have Voltage break down under applied Voltage, I believe reeceyburger123 ran into this problem before.

https://www.badcaps.net/forum/showth...t=blowing+fuse

[dick_barton]

Re: TV keeps blowing fuses !

Is you track open circuit or nearly oc?

Edit,
Ignore my comment. Looking again it appears to be some sort of support stuck over the track.

[Pauldesign]

Re: TV keeps blowing fuses !

Thanks gents for all the hints.

What I realised is when the primary MOSFET is removed, it doesn't pop fuse (in essence, when the secondary (about 700V) of the primary inductor is disconnected from the rest of the circuit) , however, when the primary inductor output is connected back (withour the MOSFET) and only to the diode which is normally in series with the drain of the primary MOSFET, which are together disconnected from the rest of the circuit, it pops the fuse.

Running out of fuses, I'll try all the hints at once and see the outcome; otherwise, I'll start ripping the inductors apart -- although they're tricky to convincely troubleshoot esp with limited resources.

[budm]

Re: TV keeps blowing fuses !

Are you talking about the PFC inductor or the main SMPS transformer.
Can you please point out those parts?

[Pauldesign]

Re: TV keeps blowing fuses !

@Budm, without schematic is difficult to refer to such parts. But I think, I found the cause of the severe short and I'll finally confirm on Monday after more conclusive tests. I only hope other components didn't blew as well.

[Nicegut]

Re: TV keeps blowing fuses !

This may seem very obvious, but I had a Dynex 37" doing the same thing. I had an issue with it blowing fuse after fuse. I was frustrated, and tried a different power cord. My set has been working flawlessly ever since. It had an visibly undetectable short in the wires.

[budm]

Re: TV keeps blowing fuses !

@Budm, without schematic is difficult to refer to such parts. But I think, I found the cause of the severe short and I'll finally confirm on Monday after more conclusive tests. I only hope other components didn't blew as well.

So which parts?

[Pauldesign]

Re: TV keeps blowing fuses !

Well, after more testings, the instant short was due to a faulty PFC rectifier diode, that had it's cathode shorted to its metal tab and since the tab was connected to the heatsink and subsequently to the AC hot-side ground; the output from the PFC transformer which is connected to the drain of the PFC MOSFET as well as to the anode of the PFC diode, was instantly shorted to ground once the PFC diode conducts on switch-ON, hence the big bang !

The reason i couldn't find the issue until after applying intuitive troubleshooting by disconnecting the rest of the circuit and narrowing it to the AC input source / de-solder/ test and working my way upwards was:

- The PFC diode (TO-220 package) although faulty (shorted from tab to cathode), tested Ok from anode to cathode and in reverse and this was the biggest mask; until i've to remove and test it properly out of circuit and guess what, the famous "Eureka" -- after blowing 10 fuses and i was left with just 1 fuse

- The heatsink of the PFC diode is coated with black paint as well as insulated /isolated from the conductive heatsink using plastic spacer behind the diode as well behind the screw, which at times tested OK/opened and masked the fault.

- The PFC MOSFET survived the ordeals and tested OK in and out of circuit.


Now to Part 2, unfortunately, the PSU ins't still working, as from further probings, the PWM IC (part # mirrored: like ST 128Z3 D29597) that drives the PFC MOSFET as well as the PWM IC (KA3846) that drives the rectifiers to provide the needed DC voltages seems dead as well as the likely SMT TO-252 diodes / transistors in between (difficult to properly test without schematic / part numbers / indications). There are no reasonable DC voltages at the Vcc pin of both PWM ICs -- the inverter / DC output PWM IC (KA3846) Vcc reads about 3V and fluctuates while the Vcc pin of the PFC PWM IC reads about 0.1V. It seems to get no voltage at all, which can be traced to no avail to the voltage divider resistors at the transformer output.

The output 24V line reads 0V, the 12V and 5V read 0V at the output / connector but read 12 and 5V respectively at their respective rectifiers outputs or input to their drive MOSFETs but 0V after their drive MOSFETs -- which tested OK, i guess the 12 and 5V outputs drive MOSFETs gates is not getting the required level / turn ON signal. The 5V USB is OK but 5V STB is 0V.

I must say, i'm working on the PSU without the TV/main board, so i'm not so sure if there must be an enable button on the TV panel that turns on the TV if pressed, although the isn't any Enable marking / point in the PSU that can be used to quickly check this.

As for the replacement PFC rectifier diode (ER806), i'm using equivalent with part marking 10UP60S (600V 10A), borrowed from my faulty Samsung LCD monitor PSU.

It was a relief and satisfaction to solve the supposed mystery although the PSU is still broken but l might investigate further why the PSU has no DC outputs, besides the cause being likely faulty PWM ICs -- which are scarce to find here / huge price tags, not to even talk of difficulty in reading the correct part markings off one IC, even with magnifying glass! -- (something mirrored like ST 128Z3 D29597) !

The PSU has marking sun 6132 rev 2:1 and F-3445A which turns-out meaningless from internet search.

Phew !

[Pauldesign]

Re: TV keeps blowing fuses !

Sorry Budm -- i just saw your post now with the labelled PSU after cross posting (due to different time zones) my findings.

Nevertheless, thanks for the follow-up and i hope my explanations suffice your request.