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Old 11-13-2011, 02:01 PM   #1
tom66
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Default Hitachi Plasma Monitor 42PD4200 - Dead, no power or standby

Found this at the local dump... I know it's an old 852x480 plasma but I paid 25 for it to see if I can repair it... Worst case I will scrap it for boards or the parts.

Fault: No power. No standby LED. No relay clicks.

Tests: Mains fuse good. 230V AC on all AC side stuff. But no DC link voltage, PFC or standby. All rails 0V.

Service Manual: http://elektrotanya.com/hitachi_42pd.../download.html

Working through the PSU schematic (page 36/37) I checked diodes D005 - D008; they were good. I then proceeded to test the inrush limiters R003 - R005. R003 and R005 measure 10.1 ohms each, but R004 is measuring as a fairly precise infinite impedance. No burn marks, though - just open on an ohmmeter test.

Meter check on resistor after removal shows 8 megohms... pretty close to open...

I'm therefore suspecting R004 is the fault and will replace it.
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Last edited by tom66; 11-13-2011 at 03:06 PM..
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Old 11-14-2011, 11:07 AM   #2
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Default Re: Hitachi Plasma Monitor 42PD4200

I found some spare 25W 22 ohm resistors. I put two in parallel and made an 11 ohm resistor. It worked - the TV now powers on and gives a good picture. However the resistors were too big for the board, so are on flying leads - this is dangerous if the leads touch something so I will wait for my proper 5W resistors before assembling it.
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Old 11-16-2011, 03:19 PM   #3
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Default Re: Hitachi Plasma Monitor 42PD4200

I received the proper resistor and have now installed it. TV turns on fine.

I also found a fuse on the Z-Sustain for the Vs voltage had blown. This was causing the picture to be very dim and slow to update. This was changed and the picture returned to what it should be.

Next problem was a 2 inch black column running down the middle of the screen. This turned out to be a problem with the driver flex which would write off the TV... or so I thought. It turned out the resistors on the FPC had simply separated themselves (cold joint); touching that up fixed the problem.

Finally, I'm now having a problem with sparklies on the screen. Most often they are green but sometimes blue and red (90% green 10% others I would say.) I adjusted the timings to within 1us using my scope (which is just a 20 MHz analog one, but still capable.)

The sparklies were still there, maybe a bit less but not much better. Then I proceeded to check the voltages. Vs 189V (should be 189V), Va 61V (61V), -Vy 75V (75V) but Vsetup was 225V (200V). So Vsetup is out. I adjusted the Ysus Vsetup pot but I could only get it to go down to 218V. And it will go up to 260V. According to service manual, it should have a range 185V - 225V. So, why won't it let me set the voltage down? Could the high Vsetup be causing my problems?

The sparklies are visible on certain scenes only; usually medium brightness.
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Old 11-16-2011, 09:03 PM   #4
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Default Re: Hitachi Plasma Monitor 42PD4200

Quote:
Originally Posted by tom66 View Post
I received the proper resistor and have now installed it. TV turns on fine.

I also found a fuse on the Z-Sustain for the Vs voltage had blown. This was causing the picture to be very dim and slow to update. This was changed and the picture returned to what it should be.

Next problem was a 2 inch black column running down the middle of the screen. This turned out to be a problem with the driver flex which would write off the TV... or so I thought. It turned out the resistors on the FPC had simply separated themselves (cold joint); touching that up fixed the problem.

Finally, I'm now having a problem with sparklies on the screen. Most often they are green but sometimes blue and red (90% green 10% others I would say.) I adjusted the timings to within 1us using my scope (which is just a 20 MHz analog one, but still capable.)

The sparklies were still there, maybe a bit less but not much better. Then I proceeded to check the voltages. Vs 189V (should be 189V), Va 61V (61V), -Vy 75V (75V) but Vsetup was 225V (200V). So Vsetup is out. I adjusted the Ysus Vsetup pot but I could only get it to go down to 218V. And it will go up to 260V. According to service manual, it should have a range 185V - 225V. So, why won't it let me set the voltage down? Could the high Vsetup be causing my problems?

The sparklies are visible on certain scenes only; usually medium brightness.
That indicates a problem with the Vsetup supply, probably with the TL431 reference or the feedback circuit (opto-isolator).

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Old 11-17-2011, 12:28 AM   #5
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Default Re: Hitachi Plasma Monitor 42PD4200

Quote:
Originally Posted by PlainBill View Post
That indicates a problem with the Vsetup supply, probably with the TL431 reference or the feedback circuit (opto-isolator).

PlainBill
I figured that, but I don't see a TL431 anywhere. There is an optocoupler though. No schematic for the Y-sustain, so I could be missing it.

Anyway, the upper Y buffer just decided to go on vacation and blew on of its ICs with a nice puff of acrid smoke. This was probably my fault as I let the metal frame of the TV touch one of the metal contacts on the back of it. Arrgh!
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Old 11-17-2011, 06:24 AM   #6
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Default Re: Hitachi Plasma Monitor 42PD4200

Quote:
Originally Posted by tom66 View Post
I figured that, but I don't see a TL431 anywhere. There is an optocoupler though. No schematic for the Y-sustain, so I could be missing it.

Anyway, the upper Y buffer just decided to go on vacation and blew on of its ICs with a nice puff of acrid smoke. This was probably my fault as I let the metal frame of the TV touch one of the metal contacts on the back of it. Arrgh!
The TL431 is usually found in a TO-92 package. It is also available in DIP, SOT-89, SOT-23 and SOT-23-5 packages.

Tough about letting the magic smoke out of the buffer.

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Old 11-17-2011, 08:28 AM   #7
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Default Re: Hitachi Plasma Monitor 42PD4200

Good point on the TL431 in SOT-89. I found it, right next to the potentiometer for Vsetup. I checked the anode-ref voltage which should be 2.495V but measures only 2.4V. So it's more than 3.7% out (rated maximum tolerance of +/-2%). However, that still wouldn't explain a ~40V rise in Vsetup voltage. Or would it?

I checked the others on the board, each next to their respective potentiometers; one measured 2.43V (for -Vy) which is also bad (2.6% down), another 2.47V (1% down) which is probably close enough, not sure which voltage that is for. Like LDOs fail, they all seem to be drifting downwards.

So now, with a toasted buffer, I have to decide if it's worth fixing. It would seem someone has had a play with a lot of the things inside this TV. Like the Z-sustain fuse going. There's no way you could watch it like that... And the black column shows faint signs of burn in so I'm guessing the owners used it for a while with that column there... . And then the resistor failed, for some unknown reason.

Last edited by tom66; 11-17-2011 at 08:32 AM..
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Old 11-17-2011, 10:21 AM   #8
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Default Re: Hitachi Plasma Monitor 42PD4200

Quote:
Originally Posted by tom66 View Post
Good point on the TL431 in SOT-89. I found it, right next to the potentiometer for Vsetup. I checked the anode-ref voltage which should be 2.495V but measures only 2.4V. So it's more than 3.7% out (rated maximum tolerance of +/-2%). However, that still wouldn't explain a ~40V rise in Vsetup voltage. Or would it?

I checked the others on the board, each next to their respective potentiometers; one measured 2.43V (for -Vy) which is also bad (2.6% down), another 2.47V (1% down) which is probably close enough, not sure which voltage that is for. Like LDOs fail, they all seem to be drifting downwards.

So now, with a toasted buffer, I have to decide if it's worth fixing. It would seem someone has had a play with a lot of the things inside this TV. Like the Z-sustain fuse going. There's no way you could watch it like that... And the black column shows faint signs of burn in so I'm guessing the owners used it for a while with that column there... . And then the resistor failed, for some unknown reason.
I've been through the use of the TL431 in another thread. The output voltage of the supply feeds the 'top' of a voltage divider (two resistors in series, 3 if you count the pot). The midpoint is tied to the ref input of the TL431, the bottom of the divider is tied to ground. When the ref input rises to 2.50 volts the TL431 conducts Cathode-Anode. The key is the voltage at the ref input. Since it is low, something is holding the voltage at the ref input low.

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Old 11-17-2011, 03:32 PM   #9
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Default Re: Hitachi Plasma Monitor 42PD4200

Quote:
Originally Posted by PlainBill View Post
I've been through the use of the TL431 in another thread. The output voltage of the supply feeds the 'top' of a voltage divider (two resistors in series, 3 if you count the pot). The midpoint is tied to the ref input of the TL431, the bottom of the divider is tied to ground. When the ref input rises to 2.50 volts the TL431 conducts Cathode-Anode. The key is the voltage at the ref input. Since it is low, something is holding the voltage at the ref input low.

PlainBill
It's quite possible it's either the dividers or the TL431.

I like to imagine the TL431 as a special transistor which conducts C-A when ref exceeds approx 2.5V. Like an op-amp this would naturally steer the ref to about 2.5V, but no more, as the supply would be constantly off... The fact that I have some control over the voltage (I can adjust it from ~220V to ~265V) shows feedback is working but it's too high for some reason.

Some observations I've made. The Vsetup voltage waggles around a lot with no buffer. It's at 210V now and jumps to 215V occasionally. The ripple is okay, at around 500mVp-p, but could be better.

I'm thinking a dodgy potentiometer which explains the sudden jumps occasionally but it could also be a resistor which has drifted, or the TL431 could be intermittent, or the opto could be intermittent.

Now I'm determined to fix this just for the sake of it.
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