Philips Brilliance 420p Fluctuating PSU Voltage

[NipZ]

I have an old Philips Brilliance with a fluctuating voltage. When the tv is powered on the power LED goes from red to green and you hear all the fans startup but a few seconds later a relay clicks and the tv shuts down.
The voltage is suppose to be 380v (at the point the red arrow marks on the picture) and links the 1st power board to a 2nd power board but I am reading a fluctuation between approx 150v-220v but it peaks at over 300v as it shuts down. I have tried with everything possible disconnected but see no difference.

I have tested a few caps out of circuit and replaced one. The 2 caps marked with a yellow circle should be 450v 330uf but only read 285-286uf, would these be what is affecting the 380v?

[Ashroyer05]

Re: Philips Brilliance 420p Fluctuating PSU Voltage

Same problem. Are you getting output voltages from any pins while its on?

[Kiriakos GR]

Re: Philips Brilliance 420p Fluctuating PSU Voltage

Do you have a true LCR meter? Or a 40$ DMM ?

[Caleb]

Re: Philips Brilliance 420p Fluctuating PSU Voltage

Caps are generally +/- 20%, so a 330 could read as low as 264 or as high as 396 and still be within spec. I would be looking at the pfc ic and the associated circuitry, did you find a schematic for the power board? Other likely candidates are those zener diodes to the right of the caps in question.

[NipZ]

Re: Philips Brilliance 420p Fluctuating PSU Voltage

Thanks for the responses. I am using 2 meters, a cheap maplin meter and an esr meter from china

http://www.maplin.co.uk/ut60e-rms-au...nterface-46458
http://www.ebay.co.uk/itm/Transistor...item4175c28c77

I have been checking around the PFC circuit as I have unexpected readings, but I'm unsure how or what else to check. I have very limited knowledge, but very eager to learn more.
I checked and replaced a 25v 1000uf cap with a 35v 1000uf cap and checked a voltage regulator(L7815CV marked in the pic) and had very strange results of input -150v and output -150v. Also similar strange results from the MOSFET (also marked in the pic)
I have a voltage drop of 0.490 and OL on the 2 diodes you mentioned, and 0.380 and OL on the rectifier diode(BYV29F) marked with a yellow arrow.
Would the PFC(MC33368P) be prime suspect or is there a way to test and prove?
I've attached a service manual I found

[Caleb]

Re: Philips Brilliance 420p Fluctuating PSU Voltage

Looked over the block diagram on page 21 and it looks like the pfc ic, mosfet and diode 6611 are the primary components in the pfc circuit. If the diode and mosfet test ok then the pfc is probably not functioning properly. The schematic on page 39,40 shows the voltages expected on each pin of the ic. I have to say it is a very good service manual even has traces for each of the test points if you happen to have an oscilloscope?

[NipZ]

Re: Philips Brilliance 420p Fluctuating PSU Voltage

Unfortunately i haven't invested in an oscilloscope yet. I appreciate your guidance to checking the pfc and i will check voltages against the pages you pointed out and forward my findings.

many thanks

[jetadm123]

Re: Philips Brilliance 420p Fluctuating PSU Voltage

I would consider replacing the startup cap connected to pin 12 (Vcc) of the MC33368P. It's probably something like a 22uf-100uf @50V.

[NipZ]

Re: Philips Brilliance 420p Fluctuating PSU Voltage

I did change cap 2662 from 25v 1000uf to 35v 1000uf.
I am not getting any expected voltages anywhere on the board, at the pfc ic, voltage regulators, mosfets etc..
With my very little knowledge I've been checking voltages and the diagram attached and I'm coming to a conclusion that relay 5690 isn't being activated. Could this possibly be causing my issues? I'm not sure if its the relay or the mosfet 7681 and surrounding resistors that can be causing the issue as I'm not getting readings I'd expect from them. I was going to take them out of circuit and test them.

[budm]

Re: Philips Brilliance 420p Fluctuating PSU Voltage

Since you are working in the Primary (Hot) side of the circuit, what are you using as the Ground ref for the meter?

[jetadm123]

Re: Philips Brilliance 420p Fluctuating PSU Voltage

I believe the 1000uf cap is the filter cap for the 7815 voltage regulator and not the startup cap.

You can check the startup voltage of the MC33368P by measuring across pins 10 (ground) and 12 (Vcc). You should see something like 12-14V. If it's fluctuating, it looks like the startup cap is 2664.

[NipZ]

Re: Philips Brilliance 420p Fluctuating PSU Voltage

I've seen people use the negative of the main filter caps, the heatsinks are connected to the same ground. Are the heatsinks ok as ground?

[NipZ]

Re: Philips Brilliance 420p Fluctuating PSU Voltage

I've checked the start up voltage using pins 12(Vcc) and 10(ground), the voltage I am seeing is a fluctuating between 9.5v and 12.5v(approx). The start up cap was a 25v 100uf but not within tolerance(only 75uf) so its been replaced with a new 35v 100uf.
I also measured the voltage at pin 16(380v) using the same pin 10(ground), but again I am seeing a fluctuating voltage between 320-380v(approx)

[tom66]

Re: Philips Brilliance 420p Fluctuating PSU Voltage

This indicates the PFC IC cannot start up properly. Check 7641, 6665(?) diode from 7815, and output of 7815.

[budm]

Re: Philips Brilliance 420p Fluctuating PSU Voltage

Do both relays turn on? Both of them have to be on so the soft start current limiter resistors will be bypassed otherwise the main power supply will not get the full power.

[NipZ]

Re: Philips Brilliance 420p Fluctuating PSU Voltage

I have checked the diode that tom66 suggested, it has a voltage drop of 0.135v and OL the other way, and I have fluctuations on the voltages he suggested testing.
When I check voltage at pin 1 of the bridge rectifier I am again seeing fluctuations.
I have my doubts that relay 5690 is turning on, I can hear clicking but it only seems to be coming from relay 5680. I have taken out of circuit mosfet 7681(STD12NE06L) and followed a guide to test it and its seems to be working. I also took out of circuit resistors 3680 and 3681 and they also test ok

Am I right using pin 3 Hot_Ground to test the voltages i.e. pin 1 bridge rectifier?

[budm]

Re: Philips Brilliance 420p Fluctuating PSU Voltage

Negative leg of the main filter cap will be the Ground ref point (hot ground as shown in the diagram) for the primary side circuit. If those resistors for the inrush current limiter are not bypassed by the relays, when the TV will try to pull more power, there will be more voltage drops on the resistors which will cause the B+ to drop. Those two relays are turn on by the main board and the coil voltages are supplied by the standby power supply.

[NipZ]

Re: Philips Brilliance 420p Fluctuating PSU Voltage

when you say the the relays are turned on by the mainboard is there a signal for each relay or one signal for both? so if one relay is turning on the second relay is also receiving a signal and its a power supply issue why the relay isn't working or am I looking at a possibility of a main board issue?
Is the signal POR(power on relay?) in the diagram for the relays?

Thanks for all the help and guidance, i'm loving the fault finding and learning from you guys even though i have a few sleepless nights thinking about it all and piecing everything together

[budm]

Re: Philips Brilliance 420p Fluctuating PSU Voltage

If you look at the block diagram on page 21, when power switch is activated, the 'SUPPLY_ON' signal (around 2.5~5vdc = on) from the main board will first turn on the Relay (STBY SWITCHED) that runs off the 5V, then the second relay (runs off 12v) is delayed turn on which will bypass those 2 resistors 3600, 3601.
BTW, from the service manual, I cannot see where POR is connected to. Where did you get that single page of the diagram? The manual I download has a lot of splitted pages.

[NipZ]

Re: Philips Brilliance 420p Fluctuating PSU Voltage

It was 2 split pages(39 & 40) I joined together to make it easier to read.
Thanks for the explanation, makes sense now. Is it likely that the relay is the faulty component? If I take it out of circuit can I add a voltage and see if it works?