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Good news, but what is the "ZD803 had fluffed "?
The ZD803 and ZD804 (12V Zener, 1N4742) are copnnected in series and also fed by 1.5K ohm resistance (it means that the voltage on that string can be higher than 24V due to 1.5K resistance in series with the Zeners), if the diode shorted out, the power supply will see 1.5K Ohm load which will not be enough to load down the power supply.
Good news, but what is the "ZD803 had fluffed "?
The ZD803 and ZD804 (12V Zener, 1N4742) are copnnected in series and also fed by 1.5K ohm resistance (it means that the voltage on that string can be higher than 24V due to 1.5K resistance in series with the Zeners), if the diode shorted out, the power supply will see 1.5K Ohm load which will not be enough to load down the power supply.
If I am being totally honest it was a bit odd, I measured across the diode(as best i could with a DMM) and seemed to be alright for some reason I tested it again and the thing broke apart on the board, it just seemed it had gone very brittle, popped another one in and the backlights popped straight on and the voltages returned
1.5k ohm "short" could easily stop the Vcc from rising. It is usually charged with less than 1mA so the limit of the Vcc would be around 1.5V best case. If lower current it would be even lower.
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If the b+ at the output of the rectifier DM803 is around 25V, the filter cap for that supply is CM808 (22UF/50V), If you put 1.5K load to that B+, the resistor will only draw about 16mA (0.4Watt), that is not enough to load down the B+ at all unless it is very weak power supply designed on purpose. I wonder if it is the ZM802 30V Zener which is the regulator for that power supply is bad.
The charge path for the initial start up of Vcc is through the STR-W6252's internal current source between drain and Vcc. If there's a 1.5k ohm short (even through one 12V zener) then the Vcc will not rise much above 12V, which is probably below the startup threshold of the STR-W6252. (I can't locate a full datasheet, but from experience most switching ICs start at around 17~20V, then operate at 11~15V.)
Once it's started I agree 1.5k ohm means nothing to the supply, it's just got to bootstrap itself before it's OK.
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I have not look at the internal of the STR yet, you may be correct that the internal charge pump may not be able to handle that much current (cap charging current plus the 1.5K) at start up.
Istartup = 1.5mA (higher than I expected.)
Startup voltage typ = 15.5V (min 13.9V, max 17.1V)
So 1.5mA across 1.5kohm plus 12V zener = 14.25V.
It may start sometimes, but not very often (or perhaps only certain batches of the IC.)
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I expect if the zener didn't fail short immediately but instead failed leaky, the set may have become harder and harder to start after power was applied. Very strange failure; that zener should not have been dissipating much power at all...
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I expect if the zener didn't fail short immediately but instead failed leaky, the set may have become harder and harder to start after power was applied. Very strange failure; that zener should not have been dissipating much power at all...
When I got the tv with the known cause of power surge and it being as dead as a door nail I instantly thought usual fets took out internal fuse but when they were all ok with no voltages I did think Im in this for a new psu (cheapest £70)
I would never of expected 1 zener to destroy every voltage it did, the strangest thing is how the diode just broke apart on the board.
But ha ho its fixed and that is all the matters....and can I say you two are bloody genius I lost you both many posts back...I have alot to learn...but thank you so much
This has come back to haunt me, the TV has an intermittent fault, the fault is that it shuts down randomly, it could last 1hr it could last 2 weeks.
I have resoldered STR just in case, that didnt cure it
The only thing I can find and can you please tell me if this could cause the fault:-
The two zeners I replaced to get the tv working if I measure them with a DMM on continuity test one reads 400 and the other is sitting around 800, these were new 5weeks ago..could these cause an intermittent fault and what could be causing these to start breaking down after a short period of time
Measure the voltage across the zeners when it is working and when it is not.
Please do not PM me with questions! Questions via PM will not be answered. Post on the forums instead!
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When I got the tv with the known cause of power surge and it being as dead as a door nail I instantly thought usual fets took out internal fuse but when they were all ok with no voltages I did think Im in this for a new psu (cheapest £70)
I would never of expected 1 zener to destroy every voltage it did, the strangest thing is how the diode just broke apart on the board.
But ha ho its fixed and that is all the matters....and can I say you two are bloody genius I lost you both many posts back...I have alot to learn...but thank you so much
Will this blow the 2 MOSFETs QI821 and QI820 and cause them to go short cct?
Replaced FDPF7N5OU with FDPF8NONZF as the first one is obsolete. Blew the 3,15A fuse and the 6.3 A after about 1Second.Cant find ay other shorts on the board.
Last edited by trigger; 08-07-2013, 02:26 PM.
Reason: Short of info
I have an LA37B530 with the same powerboard and it is dead. Local TV repairer told me $330 to repair as he said it needs a new powerboard so I brought it home. I am a total newb to working out what might be wrong with a TV. Can you help me please?
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