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Posted on Jun 28, 2006

# Tv does not power up

The 12v standby voltage is not there, checked the 12v power supply and found a shorted zener diode, d982. there are no markings on this diode, so i don't know what voltage this zener is.

• Anonymous Jun 28, 2006

the zener is on the PRIMARY (150V) side of the supply, not the secondary. the 12 volts is regulated on the secondary side of the supply by a 7812 regulator.
the zener is in the base circuit of the switcher on the primary side, creating a reference voltage for the switcher to compare to it's feedback voltage. so the question remains.... what voltage is the zener? if i had a schematic, i wouldn't be asking.

• Anonymous Jun 28, 2006

sorry, but 1n4148's are NOT zener diodes, they are general purpose signal diodes.

c'mon people, it's not that difficult.... does anybody have a schematic? i don't, that's why i'm asking.....

• Anonymous Jun 28, 2006

read my reply again "techman"..... this is a switchmode power supply, the zener is the reference voltage to set the duty cycle on the PRIMARY side which is operating at 150v. assuming a duty cycle of 35-65%, this puts the zener somewhere in the range of 50-100v.

• Anonymous Jun 28, 2006

today's class is "switchmode 101".
120 volts ac comes in off the rail to a bridge rectifier, and is rectified to provide 150v dc. the 150v goes to a "chopper" which creates a 150v p-p 20khz square wave across the primary of a transformer. the square wave is stepped down on the secondary to (in this case) 15v at 20khz, which is rectified to provide about 14v dc, which then goes through a 7812 regulator to provide 12v dc.

in the chopper on the primary side is a voltage reference and a comparator. the comparator samples the dc average of the 150v square wave and compares it against a voltage reference. if the average drops below the reference, the comparator widens the pulse width. if the average goes above the reference, the comparator narrows the pulse width. the voltage reference sets the average duty cycle. in this case the voltage reference is a zener diode. switchmode power supplies are most efficient in the range of 35-65% duty cycle, so the zener voltage is between 50 and 100 volts. if the duty cycle is set wrong, overheating will eventually damage components in the secondary circuit, so it is important to know the proper voltage for the zener.

• Anonymous Jun 28, 2006

tnx, i was going to try about 50 volts, then measure the preregulated voltage on the secondary and see if i needed to go up in zener voltage or down. 47 sounds about right for about 35% duty cycle.....then there's a series diode that makes it 0.7v higher. there was also a dried out electrolytic in the reference voltage source, 47uf@50v. if i have a zener in stock, i should have this beast up and running this afternoon.

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I have seen the schematic, d982 is a 13v 1/2w zener
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You my friend are a ASSHOLE

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### Anonymous

The only diode in the primary with a zener symbol on board vd509 says 1s2 on body, this crosses to a 20v 1a schottky. however this wasnt the problem on the one i fixed. nor was the only diode that checked shorted in the primary vd507, a 1n4148, which is a 100v .2 to .45a 4ns fast sw, and not a zener. it also wasnt bad only accross a ferrite bead coil l504. i was unable to find any 900 series diodes nor any designated d anything. but getting back to reality for a moment. the threee 78xx regulators all mounted on copper coated steel heatsinks wernet being powered, because the 16v from vd554 off the secondary of the swmps t511 through r569 and turn on pnp transistor v554 was lost due to cleverly disguised 1a picofuse r569 looking like a tiny white resistor, that opened. the pico fuse opened because capacitor c572 470uf 10v was a dead short on the output of 5v regulator n552, which ws fed by 12v regulator n551 which should have gotten 16 v into it when power on sig to base of turn on transistor v554 arrived i assume from the micro in response to pressing the power button. the primary was clearly good because 123v b+, 23v and 16v were good out of the secondary of the powersupply. youre right it sint complicated but haire when they got tired of making rootbeer never bothered to learn how electronic components should be designated on circuit boards or how to draw schematics. i cant find a shcematic either, but just decided not to let this one kick my a**. hope this helps you, unclejed. Doug, no title just a tech for 50 years VVL

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I have found that in a lot of cases, if we are uncertain of the value of the Zener Diode, the IN 4148 zener works! It is like a universal diode, which works differently in different areas. As for rating, I suggest you put 2 IN4148s in parralel and solder. the 12V problem should get sorted out.

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You answered on your own. (lol)If its suppose to be a standby 12 volt supply then whats the top voltage to be there?? 12 VOLTS!!! The value your looking for is a 12 to 13 volts 1 watt zener diode.. Im thinking your a handi man trying or attemting a repair?? please remember the HI voltage saftey issues, as these sets hold alot of HI voltage even after they are unplugged for a while. Good luck

• Anonymous Jun 28, 2006

Like i said before. If its a 12 volt supply its a 12 volt zener you crazy tech man! How can a real,or so called tech work on a set and not know what there doing on a circuit without a schematic diagram!!??? Let me ask One more qeustion Please? How much experience you have with solid state electronics? Please answer this first please! If your looking for a free schematic, I dont have one to pdf or other wise send you but lots and lots of sourses on line for a service manual on line but this is part of the job . get a service manual.Sorry i cant give ya one!!

• Anonymous Jun 28, 2006

OOps sorry man I wasent paying attention. Yes I do know what a switchmode ps is. I dont have the exact schematic BUT another Mistubishi is 47 volts. My notes say the error transistor that the zener drives has a high resistance leak But will check ok on a regular meter.

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