Basic Fuse Question Philips TV

[edy]

Hi folks,

I have a basic question. My Philips TV died and so I took a crack at it (as Dave Jones would say.... "Don't Turn It On... Take It Apaaaarrrt!"). I suspected a capacitor as I have replaced caps on my Samsung before. Anyways, I opened it up and first thing I noticed was a blown FUSE!  See the attached photos.

So I just want to clarify the type of fuse to replace it with. It is marked 125V 4A, but whether it is fast, slow, time-delay, etc... I'm not exactly sure. The part number on it is <PS>E NSF FBT. From a Google Search I gathered it is made by NSF and it is "Fast Blow Type"? I found this website but it only lists 250V fuses and I have a 125V 4A one:

http://nscable.co.jp/products/fuse/glass/04-3/index.html

If anyone can explain why Fast Blow versus Slow Blow and Time-Delay or Semi-Time Lag and what exactly the applications are typically for these types it will help me understand. My assumption is that fast blow is preferred to avoid damaging the appliance, but that short non-critical current spikes (say at power-up time) may mean more fuse replacements over time.

[Tandy]

Typically a fuse is marked with a T or an F T4A or F4A. F being Fast and T being time delay.

Time delay (slow-blow) fuses are used on equipment where there is an inrush current, in other words it needs moor power to start-up. This is typically inductive loads like motors or resistive loads that change rapidly with temperature like light bulbs.

In your TV it is more than likely a standard fast blow fuse.

BEFORE YOU REPLACE THE FUSE
Fuses rarely blow for no reason, you should first investigate any obvious sources of short circuit.

[Tandy]

PS chances are that the MOSFET on the switch mode PSU has failed and causing a short on the power supply.

[retiredcaps]

I suggest checking the 4 diodes acting as the bridge rectifier to see if they are shorted as well.

[Vito_R]

Yes most likely a fast blow fuse.  Before replacing the fuse make sure you check ALL diodes and transistors for shorts or leaks as you most likely have a bad component that blew the fuse. While you're at it being it is a TV SMPS I would also highly recommend you check all Electrolytic caps and even replace them if they are cheap Chinese capacitors often used in these power supplies and often the cause for repeated failures in these TVs.  Don't just change the bad ones, replace them all with high quality low ESR Japanese Nichicon caps like PW, HE, or HW series. You will just save yourself time opening up the tv again in the future.

[SeanB]

Fast blow fuse, you can use the 250V one to replace it with no problem. Check the diodes and transistor are not shorted ( easy as they are on the top) and check across the mains input for shorts and try it again. As it did not blow too violently it likely went out from a mains spike that operated the transient suppressor on the board.

[wraper]

Time delay (slow-blow) fuses are used on equipment where there is an inrush current, in other words it needs moor power to start-up. This is typically inductive loads like motors or resistive loads that change rapidly with temperature like light bulbs.

In your TV it is more than likely a standard fast blow fuse.

No! SMPS is the device where very high inrush current is present while primary capacitor is charging. Therefore usually slow blow fuses are used in SMPS. Anyway there is T4A of F4A written on the fuse which will solve the dilemma.

[Vito_R]

Time delay (slow-blow) fuses are used on equipment where there is an inrush current, in other words it needs moor power to start-up. This is typically inductive loads like motors or resistive loads that change rapidly with temperature like light bulbs.

In your TV it is more than likely a standard fast blow fuse.

No! SMPS is the device where very high inrush current is present while primary capacitor is charging. Therefore usually slow blow fuses are used in SMPS. Anyway there is T4A of F4A written on the fuse which will solve the dilemma.

No they are Fast acting fuses not slow blow.

[wraper]

Time delay (slow-blow) fuses are used on equipment where there is an inrush current, in other words it needs moor power to start-up. This is typically inductive loads like motors or resistive loads that change rapidly with temperature like light bulbs.

In your TV it is more than likely a standard fast blow fuse.

No! SMPS is the device where very high inrush current is present while primary capacitor is charging. Therefore usually slow blow fuses are used in SMPS. Anyway there is T4A of F4A written on the fuse which will solve the dilemma.

No they are Fast acting fuses not slow blow.

How often do you repair SMPS to say that? In my experience they can be either type.

[Vito_R]

Almost everyday and these TV SMPS always use Fast acting fuses. Slow blow would not provide adequate fire protection and safety in a TV.  From my understanding the fast acting fuses are still designed to provide a certain tolerance against surge currents and shouldn't blow instantly unless there is a short circuit or reach a certain threshold current and time based on the fuse's datasheet.

[edy]

Thanks for all the help. I took out the power board and took some photos. I can't find any obvious problems. I understand I am looking for shorts but I am having issues testing components in board using my multimeter set for continuity. For example, the diode bridge has no continuity, so using resistance I am getting the same values whether I test forward or reverse on each diode, probably because they are in circuit and I am able to get flow through the other. If I had them out of circuit I should get infinite resistance going backwards through the diode, no? As far as caps go, I don't see any damage. If it is that large blue one, how would I go about checking it? Just test resistance across it? I tried another diode in circuit and it tested as continuous in one direction... but then tapping it a few times it stopped beeping... then I tried it in reverse and it beeped a few times, then it stopped... and then forward direction it beeped again and so on. It was like I was charging up something in circuit with it and that was causing the strange behaviour. I guess I will need to de-soldering each component to test?

[edy]

More close-up photos...front and back of the hot side of power supply.

[edy]

Here are some components on the cold side of the board. Probably not an issue, just for those curious to see.

I found this on another thread: http://www.badcaps.net/forum/showthread.php?p=426429

I was doing repair on an Emerson LED TV LE220EM3 and there was a HAI 7209PC chip on board for the LED lighting circuit. I was hoping that this HAI 7209PC was an replacement for HAI 7207PC but it not. The HAI 7207PC is defective straight out the factory.Pin 6 and 7 will short out in time and then your fuse will blow. Your Idea is good (the heat sink or fan) for HAI 7207PC chips that are still functional (MAYBE).The inverter is the only solution for now

[Vito_R]

Thanks for all the help. I took out the power board and took some photos. I can't find any obvious problems. I understand I am looking for shorts but I am having issues testing components in board using my multimeter set for continuity. For example, the diode bridge has no continuity, so using resistance I am getting the same values whether I test forward or reverse on each diode, probably because they are in circuit and I am able to get flow through the other. If I had them out of circuit I should get infinite resistance going backwards through the diode, no? As far as caps go, I don't see any damage. If it is that large blue one, how would I go about checking it? Just test resistance across it? I tried another diode in circuit and it tested as continuous in one direction... but then tapping it a few times it stopped beeping... then I tried it in reverse and it beeped a few times, then it stopped... and then forward direction it beeped again and so on. It was like I was charging up something in circuit with it and that was causing the strange behaviour. I guess I will need to de-soldering each component to test?

Hi Edy,

You need to use the Diode test function on you multimeter to test for forward voltage drop in all diodes and transistors.  Voltage should indicate roughly 0.4-0.7v If you get a voltage indication on one side then reverse your leads and it should show no reading the other direction.  If you get a reading then unsolder one leg of the diode and test again to confirm the diode is not leaky.  If you have a reading in both directions (leaky) or 0V then it indicates a short and your component is defective.  O/L in both directions is an open.  Careful testing the transistors as some could be Mosfet and there is a slightly different method to test them as you have to turn them on.  Just check the part number of that transistor on the heat sink to see if it's a mosfet and then test it. They can sometimes fail.  You would also need to desolder the mosfet to test it. 

[Vito_R]

To test the caps you should use an ESR meter to check for high ESR which would indicate they are bad.  If you don't have one then unless these caps are high quality Nichicon, Rubycon, United Chem, etc.. I would simply replace all Electrolytic caps with low ESR Nichicon PW, HE, or HW series.  You will probably extend the life of the power supply for many years after as most manufacturers used cheap chinese caps in them for some reason. Before you do that confirm your issue with the fuse blowing is resolved and you have no more shorts.  Then if you want to upgrade the caps go ahead.  Make sure you use only LOW ESR caps to replace them.  You can find them at Digi-key or Mouser.

[Vito_R]

Just checked for you and the K5A50D is a Mosfet.  Desolder it and test it by using the diode function but you need to turn on the gate.  Test it using this method:

Connect the 'Source' of the MosFet to the meter's negative (-) lead.

 1) Hold the MosFet by the case or the tab but don't touch the metal parts of the test probes with any of the other MosFet's terminals until needed.

 2) First, touch the meter positive lead onto the MosFet's 'Gate'.

 3) Now move the positive probe to the 'Drain'. You should get a 'low' reading. The MosFet's internal capacitance on the gate has now been charged up by the meter and the device is 'turned-on'.

 4) With the meter positive still connected to the drain, touch a finger between source and gate (and drain if you like, it does not matter at this stage). The gate will be discharged through your finger and the meter reading should go high, indicating a non-conductive device.


Check that it is working and does not indicate a dead short.

[Zero999]

Time delay (slow-blow) fuses are used on equipment where there is an inrush current, in other words it needs moor power to start-up. This is typically inductive loads like motors or resistive loads that change rapidly with temperature like light bulbs.

In your TV it is more than likely a standard fast blow fuse.

No! SMPS is the device where very high inrush current is present while primary capacitor is charging. Therefore usually slow blow fuses are used in SMPS. Anyway there is T4A of F4A written on the fuse which will solve the dilemma.

No they are Fast acting fuses not slow blow.

How often do you repair SMPS to say that? In my experience they can be either type.

It depends on where the fuse is. If it's before the main filter capacitor, it's more likely to be slow blow. If it's after the main filter capacitor, it's more likely to be fast blow.

If the filter capacitor is fairly large, there will some sort of inrush limiting.

[edy]

Hi everyone,

Thanks for the help. I bought some fuses and the guy at the electronics shop said try to replace the fuse one time and see what happens, but only once. So that's what I did.... I placed a 4A/125V fast-blow fuse and blow it did!      The minute I plugged in the TV I saw the lights flicker and I think that was basically when the magic smoke was released from the fuse. Obviously these TV's are "always on" the minute you plug in they have a standby power draw to power the soft-touch buttons on the front and also the circuit listening for the remote control.

Ok, so now it is on to a real detective and repair job. First thing is I removed the power supply board from the back of the TV so I can easily work on it. I'd like to somewhat figure out the schematics here so I can understand what is going on. From what I can gather from the board (you can look at the pictures and let me know if this is correct)....

1. Mains enters in LIVE/NEUTRAL and the LIVE immediately passes through the fuse in series

2.  There is a resistor across the other side of the fuse and neutral...  Brown/Red/Green/Gold = 1.2Mohms
  - I tested it, measures 1.08Mohm. I figured if the resistor shorted it would blow the fuse. But it is ok. I don't think resistors fail that way anyways... they would if anything break completely and go to infinite resistance.

3. Next also in parallel with the resistor (crossing from other end of fuse to neutral) is a blue cap-like flat component with part number SVR 10D 471K SEC C. There is a "Z" written on the board under it. I assume this is a Varistor. Reading up on it looks like it is a surge protector, clamping the voltage down if there is a splike.

4. Next, also in parallel with the resistor and the varistor is that large blue box-like component with 470nK275V X2 PCX2 337 MK written on it. Google reveals it is a Pilkor component... Interference suppression film cap. Reading up on it, sounds like it EMI/RFI suppression to reduce noise/interference and spikes as well.

Anyways, none of these devices are shorted.... so if the resistor, varistor or cap were shorted that would immediately short the LIVE and NEUTRAL through the fuse.

So continuing on.... I looked up the next part and guess what.... A service manual turned up!   

HERE IT IS:

http://diagramasde.com/diagramas/otros2/PL12.0%20EN%20040812.pdf

The manual says to check each component on the primary side for a short circuit, and each rectifying circuit on the secondary for a short. So I took out the multimeter and placed it across all the diodes (in both directions) using continuity tester and I had a couple of the diodes test continuous. But looking closely at the circuit, I noticed one of the diodes that was continuous was in parallel with a resistor valued at only about 0.68 ohm.... so I can't test that diode unless I remove it from the circuit. The other one is a bit more complex.

I've attached the power supply block diagram from the manual I linked above....

Any guidance would be appreciated. I think the next step is to begin desoldering diodes and replacing caps? I don't know which direction to go first here.

[edy]

I've attached page 43 from that service manual I linked previously.

[Vito_R]

Check all those diodes for shorts or leaks.  Desolder one leg of each if needed. Check all transistors and what's often the culprit is the Mosfet.  Carefully check that.  Do you know how to test a Mosfet?  Please just re-read my previous posts earlier. 

[edy]

Thanks Vito_R for the Mosfet explanation. I am not exactly clear on how to do this. I found the explanation you gave here as well:

http://www.utm.edu/staff/leeb/mostest.htm

Should I be desoldering the Mosfet or can I test it in-circuit? Here is what I found.... The mosfet has G/D/S for leads 1/2/3 going from left to right looking at the picture when the Mosfet is facing us with the screw head showing that connects it to the heat sink.

a) set multimeter to diode/continuity setting but have it on diode function
b) I put negative probe (black) on source and positive probe (red) o gate... Shows 0.100 V.
c) then I put positive (Red) lead on drain while still holding negative on source.... multimeter shows 0 V.
d) while still in same position as in (c) above... I touch the gate with my finger while trying to holding the probes the same way as before (red on drain, black on source).... Guess what, NOTHING HAPPENS.

Also when I put positive (Red) on GATE, I get the same 0.100 V whether the negative (black) probe is held on drain or source.

Regarding resistance/continuity.... I get 121 ohm between GATE-DRAIN and also GATE-SOURCE.... And I get 0 ohm (continuous) between DRAIN and SOURCE. So... Does that mean there is a short through the Mosfet?

Now regarding the Diodes...My multimeter has the diode and continuity on the same position on the rotating knob. I usually press a function button to toggle between diode and continuity. When I measured some of the diodes I was getting 0.541 V on average for all the bridge rectifier diodes (whether I have the red/black probes in one direction and the other).

[Vito_R]

You must absolutely desolder the mosfet and take it completely out of circuit for that test to work. Only use the diode test function on your meter.  Do that and test it again.  Desolder one leg of each of the diodes and test again. 

[edy]

I removed the MOSFET ( I've attached a picture) and there is continuity between the drain and source (middle and right pins on picture).

I am getting 148 ohm reading between gate-drain (left and middle pin) and gate-source (left and right pins) consistently... but 0 ohm between middle and right... So I believe this is a FAIL.

Should I go about testing everything else too or can I take a chance and just put in a new MOSFET and fuse and see what happens?

Also, I am not sure if I must use the exact same part or if I can get away with a similar spec. For example, I go on Sayal.com and can't find the exact part but there are tons of Mosfets... how do I know? Or can I order a one off part from Mouser? I found it on there... STA4,Q,M and STA4,Q... difference? Package or case? Seems I can buy one part from Mouser but have to buy bulk from Digikey. If Sayal.com had a similar part that would be easier as I can just drive over to their store.

[Vito_R]

Can you repeat the test with the diode function and try to trigger the gate like in the procedure? Just want to be sure.  Yes you can get it from mouser, looks like its the same one on their site.  And yes I would still test the other components too just to be sure you don't have any multiple failures.  I always like to spend a little extra time troubleshooting and double check everything to avoid any unpleasant surprises, but that's just me, others may have their own opinion.  I also like to check for any open resistors around that circuit.  Although if they do not show any signs of burning usually they are ok but sometimes they can still open up internally especially if they are fusible or flameproof resistors.

[edy]

An interesting finding....

After I removed that MOSFET, I have re-tested the 4 diodes from the bridge rectifier on the board (without desoldering them) and I have entirely different results. Here is the initial results:

WHEN *FAULTY* MOSFET WAS ON BOARD:

All 4 diodes measured 0.541 V (using diode test on multimeter) in BOTH directions.

Now that I removed the MOSFET.....

The 4 diodes still measure 0.541 V but only the proper direction. That is, I have to put the red/black leads for the proper polarity of the diode to get a forward voltage reading. Otherwise in reverse I get O L (open) result.

Unfortunately, other diodes still show forward/reverse readings so I will have to desolder one end because there is probably another path through the circuit. But at least the bridge has been rendered "testable" by removing the MOSFET and not having to desolder every one of the diodes, so I should be safe to say the bridge rectifier diodes are ok.