Philips 32PHH4100 faulty power supply (dropping voltage)

[Dannyx]

Good day folks. Got this Philips 32PHH4100 LED TV which has power supply issues. When the chap brought it in, the red power LED would blink rapidly, indicating the set is trying to come on but can't. I took it apart, took some measurements and immediately noticed the 12v output dropping to around 7v when the main board was connected to the power supply. It is an "always on" supply, meaning it doesn't have a STBY voltage - the 12v rail IS the standby voltage as well. To test this theory, I took a computer PSU, connected its 12v output to the appropriate pins on the main board and powered the set on: even though there was no backlight, the TV was definitely on and working fine, as I could see stuff moving on screen with the "flashlight method".
Back on the faulty power board, it is a 715G6550. I couldn't find a schematic, but it's not really necessary at this point, since no components are completely blown apart. When it's first plugged in, the 12v output slowly rises, until it reaches around 11.6v, which is OK, but the turn-on "curve" is WAY too slow: it starts off at 5v and takes about 5 seconds to reach 11v, which is not alright. When you load this output, the voltage goes down to 7v and then slowly rises again upon removal of the load. What I have tried so far to no avail:
-replacing the caps on the 12v output (but not the LED driver too, which runs off a separate tap on the switching transformer)
-replacing the optocoupler, thinking perhaps it doesn't provide correct feedback to the PWM IC
-replacing the 2 small caps on the Vcc pin of the PWM IC in the primary
Could this be a bad PWM Ic, bad MOSFET, bad rectifier diodes ? Suggestions are highly appreciated

[CapLeaker]

Re: Philips 32PHH4100 faulty power supply (dropping voltage)

What do you get on the main filter capacitor?

[dick_barton]

Re: Philips 32PHH4100 faulty power supply (dropping voltage)

What is the voltage on C9801 while the 12V supply starts rising from 5V?

[Dannyx]

Re: Philips 32PHH4100 faulty power supply (dropping voltage)

Right right, a few extra details coming up: I get around 325-327v on the main cap, regardless of the state of the 12v rail. Another thing I forgot to mention is I get 11v on the Vcc pin of the PWM IC (again, irrespective of what the secondary is doing). It's a PF6003AS for which I could not find a datasheet, but is equivalent to FAN6300A from what I read - datasheet here The datasheet for the FAN says it needs at least 16v to start up, which if true for the PF6300a as well, would mean I'm not getting enough voltage on the VCC pin for the thing to fire up....like I said, I've already replaced the caps on this pin. Another strange thing is that a leg of the AC line is fed through some resistors to the same VCC pin, so what the hell is up with this setup ?! I mean, I kinda know what it's doing, but the suggested schematic for the FAN doesn't show this AC leg going to the VCC pin, but rather to the HV pin ONLY :|

[dick_barton]

Re: Philips 32PHH4100 faulty power supply (dropping voltage)

Here is some information on that chip which does show VCC tied to AC line. It's probably to start the chip operating (charge pump) before being taken over by the secondary voltage.

Edit.
You've changed the caps on the VCC pin but it may be worth also checking the resistors around that circuit.

[Dannyx]

Re: Philips 32PHH4100 faulty power supply (dropping voltage)

Here is some information on that chip which does show VCC tied to AC line. It's probably to start the chip operating (charge pump) before being taken over by the secondary voltage.

Edit.
You've changed the caps on the VCC pin but it may be worth also checking the resistors around that circuit.

There's only one resistor in the patch of the VCC and it reads OK. I'm not sure whether it indeed needs 16v or I'm on a goose chase here

[CapLeaker]

Re: Philips 32PHH4100 faulty power supply (dropping voltage)

Here is the service manual for Chassis TPN15.1E I can't upload it as it is 28.8MB... too big.
https://www.electronica-pt.com/pesqu...=TPN15.1E%20LA

[Dannyx]

Re: Philips 32PHH4100 faulty power supply (dropping voltage)

Here is the service manual for Chassis TPN15.1E I can't upload it as it is 28.8MB... too big.
https://www.electronica-pt.com/pesqu...=TPN15.1E%20LA

Thanks, will have a look at it...if all else fails, I'll probably get a whole new PSU.

[Dannyx]

Re: Philips 32PHH4100 faulty power supply (dropping voltage)

On page 59 I noticed U9101 has 2 VCC pins, (pin 8 is VCC as well, unlike the FAN one where 8 is a HV pin) so it makes more sense now. Still, the dummies who made those schematics don't have the common sense to add some bloody readings to let you know how much each point is supposed to read....

[dick_barton]

Re: Philips 32PHH4100 faulty power supply (dropping voltage)

In the service manual there's a 20V zener BZT52-B20 connected to VCC so I'd expect VCC to be somewhere between 13V -18V

[Dannyx]

Re: Philips 32PHH4100 faulty power supply (dropping voltage)

In the service manual there's a 20V zener BZT52-B20 connected to VCC so I'd expect VCC to be somewhere between 13V -18V

I too would expect around 19v on that VCC pin, especially since many other PWM chips that I've encountered run at that, so perhaps that's the issue ? :| Why would the secondary produce voltage though ? Shouldn't it be dead if the PWM IC is below the operational threshold ?

[dick_barton]

Re: Philips 32PHH4100 faulty power supply (dropping voltage)

The capacitor on VCC is charged by current supplied through the resistors tied to the Line voltage. When the voltage at VCC reaches a certain amount the oscillator starts and VCC is now maintained by the secondary supply voltage and the internal supply via line voltage is turned off.

It may be attempting to start but the VCC cannot be maintained by the secondary voltage so it goes back to charging VCC via the line supply and during this time the secondary voltages start falling. This repeats.

[Dannyx]

Re: Philips 32PHH4100 faulty power supply (dropping voltage)

The capacitor on VCC is charged by current supplied through the resistors tied to the Line voltage. When the voltage at VCC reaches a certain amount the oscillator starts and VCC is now maintained by the secondary supply voltage and the internal supply via line voltage is turned off.

It may be attempting to start but the VCC cannot be maintained by the secondary voltage so it goes back to charging VCC via the line supply and during this time the secondary voltages start falling. This repeats.

Ok, so probably bad R9108, D9105 or D9106. Those 11v I see might come from the AC line and might not from the AUX winding at all now that I think about it.

[CapLeaker]

Re: Philips 32PHH4100 faulty power supply (dropping voltage)

I second dick bartons last post.

[Dannyx]

Re: Philips 32PHH4100 faulty power supply (dropping voltage)

Ok, back on this board: I removed and measured both D9105 and D9106 - both had a "slow rise", meaning when I forward-biased them with my meter in diode mode, the V drop slowly went up, instead of *bam* instantly showing the value it's supposed to. I replaced both of them but unfortunately nothing changed: I still get a slow rising 12v output which drops when loaded. For testing purposes, I lifted the cathode of D9106 off the board in order to measure what voltage I get there - I get 18-19v which is more like it, but when I attempt to apply this back onto the VCC pin, it goes to 11, which is what I get there from the AC line even without the diode connected, so it's like the AUX winding of the transformer can't maintain a steady output when loaded (or the IC is shorted internally, though it sort-of works, since it puts out something on the secondary)...

[CapLeaker]

Re: Philips 32PHH4100 faulty power supply (dropping voltage)

Did you check all the components on that VCC line? Resistors for correct values, the zener diode, the 2 caps, R9101, the fet, etc? In what condition are you testing that board? Is it by itself and forced on, or you have it all hooked up?

[dick_barton]

Re: Philips 32PHH4100 faulty power supply (dropping voltage)

Have you checked R9127, R9128, R9142 = 3.3Meg Ohm and R9114 = 110K are within value?

[Dannyx]

Re: Philips 32PHH4100 faulty power supply (dropping voltage)

Did you check all the components on that VCC line? Resistors for correct values, the zener diode, the 2 caps, R9101, the fet, etc? In what condition are you testing that board? Is it by itself and forced on, or you have it all hooked up?

I have checked what I believe to be every component around that PWM IC and they all seem to check OK (resistors in particular). I actually replaced D9105 and 06 and the 2 caps (I must admit I did not have a 10uf one on hand, so I replaced it with a salvaged one...I'm on my way to getting some new ones. It would be so embarrassing if that was the issue all along...tough with no ESR meter). Haven't tried changing the FET though - I'll try that soon as well. I saw a vid of Dave from EEV where he busted his Rigol supply and it had sort-of the same behaviour: slow rising voltage and immediate voltage drop when loaded. As for your other question, about the testing conditions, I'm running just the board, nothing connected to it and I'm loading it with a 12v bulb. It's an always-on configuration, meaning the PS_ON pin leads to a dead trace, so no need to force it on. Only the LED driver needs turning on, which I'm not interested in at the moment.

[dick_barton]

Re: Philips 32PHH4100 faulty power supply (dropping voltage)

What wattage bulb out of curiosity.

[Dannyx]

Re: Philips 32PHH4100 faulty power supply (dropping voltage)

I took a close-up of the area we're talking about, just in case (and oh, don't mind that through-hole diode botched on there - I didn't have a SMD 4148 ). Let's answer some questions:
-resistors: while I have measured all of them, I just noticed something: R9142, 9128 and 9127 are supposed to be 3.3 meg-ohms, but when I put my probes across one of them, it displayed 1.5 meg-ohms and it slowly kept going up, so at that moment I disregarded them, thinking that the number would eventually reach the desired 3 meg-ohm or real close, but now after your suggestion, I took another reading and I'm not getting squat :| Eventually I figured out what happened: initially, I had my probes in one orientation (+ on the right of the resistor and - on the left, let's say), but for my second reading I had them in reverse, so what I was seeing was some leakage going through perhaps the bridge rectifier and the PWM IC....I'll now try pulling them out and measuring them off-board.
-it's a 24v 5w bulb (yes, I lied, it's not 12v now that I look at it )