Need help on replacing diodes..

[JonathanAnon]

I'm fixing a big TV.. No power, no standby power... After whipping off the cover, I realised that although there was 240V AC on the input pins of the power board, there is only 2.9V AC on the pins of the bridge rectifier.. So while going through all the elements in between the two connections I found two diodes D004 and D009 that are shorted... nb. the rectifier itself checks out okay..


There are no markings on the diodes .. Any of the ones I've replaced previously have had markings on them that I could google and then find equivalents, but not these ones... Any help appreciated..

[tom66]

Re: Need help on replacing diodes..

They are in parallel with a relay so will appear shorted, it is normal.

Looks like a Hitachi plasma PSU. 42PD4200 or similar?

Maybe the common open inrush limit resistors, common problem.

[JonathanAnon]

Re: Need help on replacing diodes..

Hi Tom, thanks for helping again.. Didnt know that about the relay/diodes.. better remember that.. I will check them out of circuit tomorrow, but I assume you're right.

I forgot to take a picture of the board model, which I normally do. I will get more time to look at it tomorrow and I'll find out. I did a quick check of all the rest of the components on the AC side and they all checked out okay... I will go through them thoroughly tomorrow..

[JonathanAnon]

Re: Need help on replacing diodes..

Tom was right on about the diodes.. pulled out of circuit and they test okay..

You were very close with the board.. Hitachi 42PD6700U - PSU - MPF7423 - PCPF0088 55 ... same as this one on another website..

http://flattvparts.co.uk/bmz_cache/2...ge.980x728.jpg

I've tested this device at the AC input, which is marked as Z001 and looks like an inrush current limiter.. It measured 0.5Mohm in circuit, so I took it out and it's shorted (DMM was obviously measuring back circuits)..

[budm]

Re: Need help on replacing diodes..

That looks like MOV to me. You can tell if it is MOV if it is connected between Line and Neutral, or Line and safety Ground, or Neutral and safety Ground. If it is current limiter, it will be connected in series between Line and Load.

[JonathanAnon]

Re: Need help on replacing diodes..

Yeah it is in parallel Bud, which makes it more confusing.. since it is open and in parallel it should have no affect on the circuit.. It must have other additional problems..

[tom66]

Re: Need help on replacing diodes..

On any Hitachi plasma PSU which is completely dead (no standby power) these resistors are your first suspect.

[JonathanAnon]

Re: Need help on replacing diodes..

Yeah I spotted that earlier when I was googling Tom, took both out and they measure fine... Took the rectifier fully out of circuit as well and it measures perfect as well...

I decided to trace the resistance along the paths from the input to the rectifier, which should be 0 ohms..

From the input lead to the power board, the neutral measures 0 ohms to the second pin of the rectifier

The live pin measures 0 ohms up to the point there are a set of two relays and two 10ohm resistors. These are pictured below (the ones that Tom mentioned above)





(I tried overlaying the images to make it easier to view but it was just a mess)

This part of the circuit is open when it is not powered.. I tested from the other side of the relay to the rectifier and it is also 0 ohms... I think the voltage may be being dropped across this section, to leave me with ~2.9V on the AC pins of the rectifier..

So my plan is plug in the power board and to check the AC voltage before and after the relay section....

[tom66]

Re: Need help on replacing diodes..

The problem is somehow the supply isn't getting to the right part of the PSU. The relays aren't clicking right? Check the voltage across the diodes on volts DC. How did you check the resistors? On 200 ohm range, they should measure 10 ohms each.

[JonathanAnon]

Re: Need help on replacing diodes..

The problem is somehow the supply isn't getting to the right part of the PSU. The relays aren't clicking right?

I wasnt aware that they made an audible noise? I spose it makes sense that I should be able to hear the metal touching metal.. I will need to hook it back up to the TV and try it...

Would it be a crazy idea to temporarily jump wire from inductor to rectifier as a test.. ?

How did you check the resistors? On 200 ohm range, they should measure 10 ohms each.

aha... not only did I test with my DMM... but also with my new Atlas ESR60 meter... ... Not sure if I should have bought it, have yet to find any fails caused by caps with high ESR...

[tom66]

Re: Need help on replacing diodes..

Yes relays make a very satisfying click when they turn on. I like relays.
Excessive use of relays: http://www.youtube.com/watch?v=LiJygHkSAYI

I have a feeling your jump will not work and will simply serve to trip a household breaker. The resistors are used to limit the current the TV draws when starting up to avoid tripping the breaker. One is used to switch on the power to the main part of the power supply. The other shorts the resistors out essentially removing them from the circuit once those large caps have reached a high enough voltage.

It's clear due to the topology of this power supply (which is different from the 42PD4200 I worked on) that the relay clicking relies on a separate always-on power supply. As it has a relay to enable the main power - without a control signal that wouldn't work.

See that other large 400V cap on the primary? That's the standby supply. Check if there's a bridge rectifier nearly. It'll probably look like this: http://i6.photobucket.com/albums/y24...S/DSC_0236.gif

[JonathanAnon]

Re: Need help on replacing diodes..

Okay... I took out the relays and noticed that they are triggered by 5V... So I used a DC 5V battery to test each one.. Both trigger and resistance across the contacts is 0 ohms when they are triggered.. Put the relays back in circuit and measured the resistance from live input to rectifier pin 3 and with the battery connected to the coil of the relay, the resistance goes to 0 ohms...

I'll check for the click tomorrow when I plug it in, maybe the voltage is not being supplied across the coils to open the relay... Might just take out the MOV altogether in case it is confusing the situation... It's already sacrificed itself anyway..

[budm]

Re: Need help on replacing diodes..

The MOV is OK because it has real high resistance, it will conduct and blow itself up if the voltage reaches the breakdown point of the MOV, so you are OK. The realy is trun on by the command signal from the main board when you first turn on the TV. it does the self test before truning on the relays to supply the voltage to the main power supply section. There should be a small transistors that drives the relay coils close by.
The contacts of the two relays are connected in series, first relay turn on which will supply the voltage to the main power supply through the in-rush current limitter resistor, then after a few cycle of AC, the second relay will turn on and shorted out the resistor to give full power to the power supply.

[JonathanAnon]

Re: Need help on replacing diodes..

I dont know though but.. My DMM is good to 40M ohms and it still shows up as shorted on that..

I see where you are talking about the first relay. There is a track linking from here right around to the opposite corner of the PCB, and a fairly complex looking circuit.. ..

[JonathanAnon]

Re: Need help on replacing diodes..

Okay got a few minutes more to look at this.. There is no voltage fed back to open up the second relay as I suspected...

From the second relay, part of the live signal is passed through a few diodes and is jumped the whole way across the board to the what seems like a standby circuit... Along this jumping journey there is a 450V filter cap in the middle of the board reading 328V DC so I assume the diodes carry out the rectification (though there is only two of them).. When it reaches the far side of the board There is a cap there that reads 18.6V, so it seems to be okay up to that point..

The problem must be in this section... I have read online a lot of people having problems with Q201, which I cant actually find as of yet!!!! But I will keep looking... thanks for the help so far, I will report back if I make any more progress..

[tom66]

Re: Need help on replacing diodes..

Are you getting any standby output from the PSU? There should be a 5V standby pin somewhere.

[budm]

Re: Need help on replacing diodes..

Can we get good clear pictures of the bottom side of the board?

[JonathanAnon]

Re: Need help on replacing diodes..

Are you getting any standby output from the PSU? There should be a 5V standby pin somewhere.

No Tom, no standby power, which I why I was assuming that it was something to do with the section from the rectifier back to the input cord.. See at this point I had been working with the presumption that standby also use the DC voltage provided by the main bridge rectifier... I didnt realise then that part of the 240V AC is passed down to the standby circuit BEFORE the main rectifier, and the standby carries out it's own rectification on the 240AC .. This voltage is successfully rectified to 330DC, and this is as much as I know at the moment..

I've googled it a lot and this model seems to have quite a few similar errors..

http://www.tv.quuq.org/index.php?ind..._view&iden=189

http://www.avforums.com/forums/plasm...0-died-15.html

I had initially ruled out a lot of the components mentioned on the basis that the problem had to effectively "to the right" of the rectifier... But I'm gonna go back through and check all those aforementioned components when I look at it again, on Sunday..

[tom66]

Re: Need help on replacing diodes..

So, what voltage does the pin marked 5VSTBY or similar measure at?
If it's zero or near zero the problem is there. Standby power is used to make the relay actuate.

[JonathanAnon]

Re: Need help on replacing diodes..

I assume it is close to zero, Tom, as there is no standby LED light.. I have to play with the band tonight.. .. but I'll get a fresh look at it again tomorrow..