Pioneer PDP-502MXE 50" 720p plasma

[tom66]

Re: Pioneer PDP-502MXE 50" 720p plasma

This TV is actually quite slim, around 2.5", for an older plasma. So, I checked if the resistor I spec'd would be okay, and it looks like it wouldn't... it's too tall (38mm, the max height is around 33mm.) So, I'm using these instead... I'll suck it and see!

http://uk.farnell.com/welwyn/sqp7s-2...-7w/dp/1292522

And the FET was out of stock, chose a similar one:
http://uk.farnell.com/jsp/search/pro...sp?sku=1664092

£4.05 each - pretty pricey! I just hope it doesn't blow it. I'm getting a spare.

[vinceroger69]

Re: Pioneer PDP-502MXE 50" 720p plasma

still going be a cheap set even with the new parts are you going to keep it or pass it on once its working fine?

[tom66]

Re: Pioneer PDP-502MXE 50" 720p plasma

still going be a cheap set even with the new parts are you going to keep it or pass it on once its working fine?

Being a professional video monitor, it is difficult to get it to work with normal sources. But it could be good for someone with a HTPC as it has VGA. And the component supports sync-on-green so it would work with most component sources. It just lacks composite, HDMI and SCART.

If I could fit it in the dorm room, I would keep it... but I probably couldn't.

[vinceroger69]

Re: Pioneer PDP-502MXE 50" 720p plasma

ah i get you know i am sure you will make a good profit on it for the next project.

[Th3_uN1Qu3]

Re: Pioneer PDP-502MXE 50" 720p plasma

That FET is a better choice than the one you originally picked. Yes, they're not cheap (high voltage and high current = high price) but it's likely to last the whole useful life of the PDP. I think the original one failed due to poor heatsinking as it seems to have become the norm. Lower RDS(on) means less heat, so it looks like you're gonna be good.

[tom66]

Re: Pioneer PDP-502MXE 50" 720p plasma

That FET is a better choice than the one you originally picked. Yes, they're not cheap (high voltage and high current = high price) but it's likely to last the whole useful life of the PDP. I think the original one failed due to poor heatsinking as it seems to have become the norm. Lower RDS(on) means less heat, so it looks like you're gonna be good.

It also has lower Qg so will switch faster and have lower switching losses.

The heatsinks are quite small but there's also a heat pad on the main switching transistor heatsink (as well as the PFC & bridge) which "connects" it to the case of the TV. I assume this means debugging with the case off should be limited as it will reduce the heatsink capability. It also has three fans just for the PSU.

It's really surprising to see only one FET used to handle over 400W of pure PDP power. Then again, I suppose that is only 1Arms from the PFC bus but peak currents will be much higher. I wonder why they went for a flyback design, rather than a more conventional two-switch forward as is found in most Samsung and LG PDPs. Cost certainly wouldn't have been much of a concern, being 50" professional HD video monitor... probably around £10k new.

They also use a ringing choke converter to control it, but it's a bit more complex than the standard self oscillating one, it has a few actives (and an op-amp) on the control PCB... and the oddest thing, it has synchronous rectification on the output! For a few amps (rated 170V @ 2.5A) on the Vs bus, that is very unusual. The whole design of this PSU is completely unconventional. Even the PFC inductor is strange; they are normally wound on a transformer core, but this one uses a large torodial core, and the power for the PFC controller & zero current detection comes from a few turns of insulated wire connected to the board through a removable connector.

I still haven't seen the PDP light up, so it could have burn in or be dim, but typical useful life from PDPs is 60,000 hours, so I think it will be okay. The last LG I saw with burn in had it visible with no power, there was a definite shading of the screen where it had been used to view cinema content a lot (top and bottom were clearly brighter.)

Service manual for this TV has PSU schematics (as well as everything else, very nice!): page 114 for PSU.

Looks like those inrush resistors are also used to detect when the caps are charged. There is an optocoupler across them which detects when there is less than around ~1.5V across them, which then activates the relay which shorts out the inrush resistors so the PSU gets full power. There is no microcontroller on the PSU to control all of this, like on newer PDPs. Samsung used a cheap PIC16C27 on one PDP, LG used a custom NEC micro, but Pioneer used good 'ol transistors and RC circuits to do timing.

[tom66]

Re: Pioneer PDP-502MXE 50" 720p plasma

Okay, the transistors arrived yesterday, as did the resistors. Problem is, it looks like I ordered non insulated TO-247 packaged transistors (the original were plastic cased insulated.) So I'll need a mica insulator and insulated bolt... anywhere I can easily get those?

[multimeter]

Re: Pioneer PDP-502MXE 50" 720p plasma

http://www.maplin.co.uk/one-piece-se...nsulator-11889 ignore the availability thing,they do have them

[tom66]

Re: Pioneer PDP-502MXE 50" 720p plasma

Too small unfortunately... I need TO247

[Th3_uN1Qu3]

Re: Pioneer PDP-502MXE 50" 720p plasma

Too small unfortunately... I need TO247

Grab some silpads from a dead ATX PSU. I'm sorry i'm not closer, coz i have at least 30 of them in all shapes and sizes.

[tom66]

Re: Pioneer PDP-502MXE 50" 720p plasma

Grab some silpads from a dead ATX PSU. I'm sorry i'm not closer, coz i have at least 30 of them in all shapes and sizes.

Good idea... there's probably a broken PSU in the junk pile somewhere... time to go diving!

I also need a plastic bolt, need to find one of those.

[tom66]

Re: Pioneer PDP-502MXE 50" 720p plasma

Okay, stole a silpad from an old Sony VCR PSU. (Used a TO-247 NPN for some reason.) Noticed that the bolt hole was insulated, so no need for an insulated bolt.

Fixed the resistors. Used some axial ones (7W ceramic "sandstone" type.) Leads were too short so I took some AWG20 and extended them, soldered them together and heatshrunk the joint properly. Had to bodge in a wire for one of them, but I also heatshrunk that for double insulation. Used a bit of hot glue to stop them moving too much.

Bravely tried it initially with 35V DC to see if anything would smoke. Amazingly, standby PSU will actually work fine from this kind of input, although it does make a hf whine. (It works right down to around 20V although 5V becomes 4.2V.)

Also tried 35V across main caps. No smoke.

Took PSU down to garage, turned on the variac (need a few throws of the breaker, because it's a 4000VA variac it's got -huge- inrush.) Started it at 30% AC (around 70Vac.) No smoke when power button pressed, the relays clicked and Vs and Main light lit (never had that before.) All four fans spun up too.

70Vac is too low for anything but a dim picture (rated 100Vac minimum) so I turned it off and cranked it to 80%. Screen lit up . Using a variac lets me limit any damage from shorted components, in theory.

Now... it's so frigging heavy I haven't been able to see if anything is actually on it. I just saw a glow from it. So, it could still be faulty...

I'm just happy that the FET survived. Primary failures aren't my favourite. Lots of high energy stuff to go wrong!

Stay tuned!

[vinceroger69]

Re: Pioneer PDP-502MXE 50" 720p plasma

thats good news then its never good when new parts go up in smoke. let us know the out come when you have tested it fully.

[tom66]

Re: Pioneer PDP-502MXE 50" 720p plasma

thats good news then its never good when new parts go up in smoke. let us know the out come when you have tested it fully.

Hmm. Well, it's doing some strange things. I've got some form of "no signal" display now and then (no 7 blinks as previously reported.) But, it only wants to fire up now and then. When it does, it will work for about 2-3 minutes but the Vs rail slowly drops from 172V to around 140V then it cuts out with the "MAIN PD" LED on the PSU lit up because it senses an undervoltage on the Vs line (I think.)

I'm suspecting a cold joint or an adjustment that's out of spec.

There may also be an X buffer address failure, as there's a 1cm lit white line on the panel. If this is the case, it is usually unrepairable. However, I will still see if I can fix it.

[vinceroger69]

Re: Pioneer PDP-502MXE 50" 720p plasma

i am sure you will figure it out with time and patience. hope the X buffer address failure is a easy fix.

[vinceroger69]

Re: Pioneer PDP-502MXE 50" 720p plasma

did you manage to fix this? or find out what the problem is.

[tom66]

Re: Pioneer PDP-502MXE 50" 720p plasma

did you manage to fix this? or find out what the problem is.

Working on another TV at the moment and homework for exams, so it's on hold right now.

[tom66]

Re: Pioneer PDP-502MXE 50" 720p plasma

Okay had another look today.

It will usually power up fine if left off for 15 minutes or so.

Once powered up the Vsus rail goes to 171.9V (rated 172V) and the PDP starts sustaining, this causes it to light up and display an image.

Between 30 seconds and 5 minutes later the Vs rail will start twitching a little, sometimes dropping 3-4V at a time, but usually recovering quickly to 172V.

Eventually the twitching will get worse enough that it drops below about 140V at which point the UVP (under voltage protect) circuit kicks in and puts the TV into a MAIN PSU lockout which it remains in until power is pulled.

Initially my suspects were a cold joint of some kind or a loose connector, but there are none. And the voltage variation is so smooth, on a scope, it looks like it actually ramps down, then slowly back up, it's not a sudden drop (a meter doesn't capture it too well.) On the sustain waveforms the variation is substantial and visible, but it doesn't correlate with changes in these waveforms much. I haven't yet tested if it varies with picture content.

My second suspect was the potentiometer for the Vsus adjustment so I've sprayed some contact cleaner in it and I'll see if that fixes the issue.

During the variation the peak drain voltage on the sync fets changes - as does the gate voltage. This is unexpected, but it might be because gate drive for the FETs is developed from the Vsus transformer (I'm not sure if it is.)

Vadr (58V) remains fixed and doesn't vary.

The Vsus bus on this TV provides about 2.4A at 172V and the Vadr provides about 700mA at 58V - so it's pretty power hungry!

I'll look at the addressing failure soon, it may be a permanent fault, I will need to test it with video.

[tom66]

Re: Pioneer PDP-502MXE 50" 720p plasma

Cleaning the pot seems to have helped, at least in preliminary tests. Very hot in shed though (nice day!) so this might be influencing the results.

Now to fix the sustain buzzing.

[vinceroger69]

Re: Pioneer PDP-502MXE 50" 720p plasma

glad you seem to be getting somewhere with it hope you can get it up and running soon and yes i agree with you it was very hot today! got a bad sun burn to prove it lol.