Palsonic pdp4225hd plasma tv

[tw2005]

Picked up this PALSONIC PDP4225HD PLASMA TV .

Conditons are : Plugged in: RED STDBY LED . PWR ON: Blue LED + Smokin PSU. No picture or sound.

Tracked the smoke to these RU4YX rectifiers.


Scavanged a couple of FR304 rectifiers from another PSU and got the set fired up but now has no sound.

The 24V appears to be missing and the R2A15112FP audio chip runs off the 24V so most likely the reason nil audio. The headphone output which uses a different chip is fine so all I need to do is figure out how the 24V is produced.

In the first shot you'll see a small 125V 3A fuse (green) directly next to one of the diodes. I saw it there, too lazy (and slack) to test it whilst I was poking around with the diodes. Since it fired up I thought the job was done until the no sound.

Schematics really help luckily I had a copy for the Sanyo equivalent.




Fuse replaced and sound is now back.

This set is electrically identical to the SANYO PDP-42XR9DA vers 1 & 2 with the main board re-located to the bottom instead of the side. Also most likely means another XOCECO built set but at least I can use the Sanyo manual for fault finding. This also means most likely similar to some DYNEX models as well.

If anyone nows what may cause these to fail I'd be interested to hear about it thanks.

[PlainBill]

Re: Palsonic pdp4225hd plasma tv

Good job of troubleshooting. The cause was a shoddy design.

The RU4YX has an absolute maximum reverse voltage of 100 volts, on this design it's seeing at least 50 volts. It's also rated for 4 Amps only if it's on a heat sink; on the board the limit would be 2.2 Amps.

The FR304 you used as replacements are a good choice; a rated reverse voltage of 400 volts, and maximum current of 3 Amps with no mention of a heat sink requirement.

The picofuse is also interesting - they had apparently had problems in the past and added that as an afterthought without considering WHY the diodes were shorting (the fuse blew after the diodes shorted).

PlainBill

[tw2005]

Re: Palsonic pdp4225hd plasma tv

Good job of troubleshooting. The cause was a shoddy design.

The RU4YX has an absolute maximum reverse voltage of 100 volts, on this design it's seeing at least 50 volts. It's also rated for 4 Amps only if it's on a heat sink; on the board the limit would be 2.2 Amps.

The FR304 you used as replacements are a good choice; a rated reverse voltage of 400 volts, and maximum current of 3 Amps with no mention of a heat sink requirement.

The picofuse is also interesting - they had apparently had problems in the past and added that as an afterthought without considering WHY the diodes were shorting (the fuse blew after the diodes shorted).

PlainBill

Thanks for that analysis. Using the FR304 was more out of luck than good decision although naturally I checked the datasheets of both before installation. As you said, the first thing I noticed was the increase in both the voltage and current ratings so i was pretty happy with that.

I did not have the exact fuse which was 125V 3A, so I currently have a 250V 2A in place and it is running fine. I intend to install a 2.5A 125V pico fuse in place of the 3A.

What do you think about that?

[PlainBill]

Re: Palsonic pdp4225hd plasma tv

Thanks for that analysis. Using the FR304 was more out of luck than good decision although naturally I checked the datasheets of both before installation. As you said, the first thing I noticed was the increase in both the voltage and current ratings so i was pretty happy with that.

I did not have the exact fuse which was 125V 3A, so I currently have a 250V 2A in place and it is running fine. I intend to install a 2.5A 125V pico fuse in place of the 3A.

What do you think about that?

I'd tend to go with the original value if you have to order something. Remember, it is highly unlikely that the diodes shorted after the fuse blew, so there is no point in going with a lower current rating on the fuse.

PlainBill

[tom66]

Re: Palsonic pdp4225hd plasma tv

Maybe something like this caused the problem:

One diode shorts due to heat, age or act of a deity. This causes a high current through the other diode, and the fuse due to the capacitor being a short at AC. The fuse blows quickly due to the high current, but now the other diode becomes a path through the secondary winding, killing that diode too. Both split and smoke from the failure. A (somewhat) catastrophic failure, but it stops there. The short circuit could then have prevented the supply from starting properly, causing the original problems. If the design were really poor, it could then take out the primary side, but most supplies have current limits for a reason.

Note: I would suggest a 60-100V Schottky instead of a fast recovery as it won't get as hot.

[PlainBill]

Re: Palsonic pdp4225hd plasma tv

Maybe something like this caused the problem:

One diode shorts due to heat, age or act of a deity. This causes a high current through the other diode, and the fuse due to the capacitor being a short at AC. The fuse blows quickly due to the high current, but now the other diode becomes a path through the secondary winding, killing that diode too. Both split and smoke from the failure. A (somewhat) catastrophic failure, but it stops there. The short circuit could then have prevented the supply from starting properly, causing the original problems. If the design were really poor, it could then take out the primary side, but most supplies have current limits for a reason.

Note: I would suggest a 60-100V Schottky instead of a fast recovery as it won't get as hot.

Your failure scenario is plausible; I disagree with going with such a low voltage diode, however. The reverse voltage across the diodes is going to be over 50 volts, a 60 volt Schottky isn't a good substitute, even a 100 volt is marginal.

PlainBill

[tom66]

Re: Palsonic pdp4225hd plasma tv

Your failure scenario is plausible; I disagree with going with such a low voltage diode, however. The reverse voltage across the diodes is going to be over 50 volts, a 60 volt Schottky isn't a good substitute, even a 100 volt is marginal.

PlainBill

It depends on the SMPS topology, and the transformer.

The typical voltage at the rectifier is Vin x Ns/Np. If the transformer is rated at 325Vdc - 24Vdc or 13.5:1, then the peak voltage could be just ~24V. However, if it is rated at a wide range non-PFC input, it might be 100Vdc to 24Vdc (4.2:1), or at 325Vdc input, it could be as high as 78V.

A "simple" solution is putting two 100V Schottkies in series, but the power dissipation is then greater than a silicon diode, but at least it's split into two diodes. Or, newer SiC (silicon carbide) Schottky diodes are available in 600V ratings (and higher.)

(See http://www.smps.us/topologies.html.)

[pelly]

Re: Palsonic pdp4225hd plasma tv

Have the exact same problem with our less than 2 year old Palsonic pdp4225hd plasma tv. After trying for a warranty with no response, I opened it up for a look. the 2 rectifiers were the same over heated ones as in the pictures. I'm having trouble locally finding rectifier diodes. pictured are 2 types I can source here, would either of them be suitable? I have never even looked inside a TV before, any help would be appreciated.

[tom66]

Re: Palsonic pdp4225hd plasma tv

Those should be fine.

[pelly]

Re: Palsonic pdp4225hd plasma tv

Those should be fine.

Thanks very much, would there be a preference for one or the other?

[tom66]

Re: Palsonic pdp4225hd plasma tv

Either will be fine. MUR420 has a higher current rating, but that probably won't matter too much.

[pelly]

Re: Palsonic pdp4225hd plasma tv

Finally got around to replacing the 2 rectifier diodes and the fuse today, now working. Thanks for the info.