Panasonic Plasma - power supply won't start

Thread Starter

AnalogKid

Joined Aug 1, 2013
8,376
I dearly love my Panasonic 42" plasma TV, TH42PZ85U. It is getting old, and I've resurrected it once, but this time it is putting up a fight. I have the service manual with schematics. Usually when this set has a problem, the On indicator LED flashes a code. This time the entire set is just dead. No clicks, no hum, no nuttin.

I was out of town for one night and severe storms were predicted, so I unplugged the set. When I returned and plugged it in - nothing. This says to me that due to inrush or static buildup or something, something on the primary side of the ps popped. All five physical fuses test good.

The isolated standby power supply is making a nicely regulated 5.07 Vdc, but AC for it comes from before the AC mains DPST relay. Everything else is down. Neither the mains relay nor the PFC pre-charge relay pull in.

The power supply board is two-sided, all PTH on top and all SMT on the botton. The big power transistors and rectifiers are PTH parts on the bottom side, bent parallel to the board and mounted on a heat spreader almost the size of the board. This makes replacing the entire board assembly easy, but it is almost impossible to rework the SMT. There are no visible problems with any topside component.

The service manual is a 67 MB PDF file, so I cannot upload it to the site. I picked it up in 2014, probably from electrotanya, but I don't have a link.

Any ideas? Is this a known thing?

Thanks.

ak
 
Last edited:

Zeeus

Joined Apr 17, 2019
521
I dearly love my Panasonic 42" plasma TV, TH42PZ85U. It is getting old, and I've resurrected it once, but this time it is putting up a fight. I have the service manual with schematics. Usually when this set has a problem, the On indicator LED flashes a code. This time the entire set is just dead. No clicks, no hum, no nuttin.

I was out of town for one night and severe storms were predicted, so I unplugged the set. When I returned and plugged it in - nothing. This says to me that due to inrush or static buildup or something, something on the primary side of the ps popped. All five physical fuses test good.

The isolated standby power supply is making a nicely regulated 5.07 Vdc, but AC for it comes from before the AC mains DPST relay. Everything else is down. Neither the mains relay nor the PFC pre-charge relay pull in.

The power supply board is two-sided, all PTH on top and all SMT on the botton. The big power transistors and rectifiers are PTH parts on the bottom side, bent parallel to the board and mounted on a heat spreader almost the size of the board. This makes replacing the entire board assembly easy, but it is almost impossible to rework the SMT. There are no visible problems with any topside component.

The service manual is a 67 MB PDF file, so I cannot upload it to the site. I picked it up in 2014, probably from electrotanya, but I don't have a link.

Any ideas? Is this a known thing?

Thanks.

ak
Idea :
Follow the current AK
You know what to do

Thanks
 

Hymie

Joined Mar 30, 2018
797
Although you have the standby power, a common fault with switch mode power supplies is the small electrolytic capacitor which starts the main switching, has failed. Once the power supply is up and running the supply voltage for this part of the circuit is normally supplied by an auxiliary winding on the main transformer.

So when the mains power is disconnected, the supply won’t re-start.

The capacitor is normally an electrolytic capacitor of few uF in value, with a rating of maybe 50Vdc. It will be connected to Vcc of the IC driving the transformer fets, and obtain its initial switch on voltage from the rectified mains positive rail (normally via 2 or 3 resistors in series to cover the eventually of a resistor going short circuit).

Since you have the circuit diagram, it should be fairly easy to identify this component based on my description above.

Of course the above may not be the fault, but your description of the unit failing following a power down fits the bill.
 

Thread Starter

AnalogKid

Joined Aug 1, 2013
8,376
The standby supply is actually one controller chip with an on-board switch transistor and a 4-winding transformer making three outputs, one on the "cold" side (isolated from AC) and two on the "hot side" (Panasonic notations). These are the power for all monitors and the main switcher control chips (3 of them, two for the main HV and LV outputs and a third little guy).

Correction - there is no SMT on the bottom side of the main power supply board. All of the high-density SMT is on a series of small vertical boards standing up among the PTH components. NONE of these boards are documented, and the signal processing that turns into the drive for the two power relays is on two of them. Because of that, I think I'm cooked.

ebay used replacement - $90.

ak
 

Thread Starter

AnalogKid

Joined Aug 1, 2013
8,376
The problem is that the relay drive comes through MC552 from MC701, and there is zero data on them. Assuming the coil drivers on 552 are ok, that leaves the brains on 701, and I can't figure out why it is unhappy.

If there is a real problem somewhere and I bypass the mains relay, I'm afraid that a simple ps board swap no longer will be enough. Treading very lightly here.

ak
 

Thread Starter

AnalogKid

Joined Aug 1, 2013
8,376
Your psu has an opto coupler On/Off pc401,control on pin 13 of the control chip on pg 41. Check the voltage on this pin see what's happening, what happens if you bypass the on relay?
Before poking around for that, I re-seated all connectors (again). Viola, power.

Thanks for taking the time to crawl through the schematics. They're not the easiest things to read.

ak
 
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