Technika LCD19DVDID dead

[bbjunkie]

Some may recall a recent thread where I mentioned I had got this set given to me not working, I fitted two new caps in the primary side of the psu and the TV part was running again.

Today I decided to attempt repairing the DVD player in it, plugged it in, I nearly jumped off the seat when I pushed the plug into the wall there was a loud splat and I thought oh no...

Sure enough the PSU is totally dead on the output. Got 320VDC across the smoothing cap on the input. I haven't traced it any further yet, could do with a schematic.

Anyone got a service manual or even just a schematic for the APC007-G power supply unit that these use?

I've searched Google, Elektrotanya, eServiceinfo - the lot.

Haven't posted any photos as my camera isn't focusing properly, have borrowed another and have it on charge so pics will follow.

Thanks

Jim

[tom66]

Re: Technika LCD19DVDID dead

Well, splat is good - quite likely, some power component has gone bang, but that should be all. Bad diode?

[bbjunkie]

Re: Technika LCD19DVDID dead

I spent last night reading a few SMPS documents, including the one at the repairfaq site. An extract from it says:

Substitutions of low ESR caps into circuits which had lousy caps is not always the good idea that it appears to be.... Caution is advised, as low ESR caps will not limit surge currents.

The circuits' series impedance drops (compare substituted capacitors ESR when new with the original capacitor's ESR was when it was a new capacitor), which, in turn, lets the surge magnitude rise, the higher currents destroy can semiconductors and other components.

Could this be my problem? the lower ESR allowing a larger inrush causing a failure once connection was made to the wall socket?

Going out to the workshop in a bit to investigate further (have checked the switching MOSFET before tom66's post came in and it seems fine, am planning to check diodes as suggested next)

[tom66]

Re: Technika LCD19DVDID dead

Could this be my problem? the lower ESR allowing a larger inrush causing a failure once connection was made to the wall socket?

I don't think so, as these power supplies are current limited. On power supplies with coils, it might matter more.

[bbjunkie]

Re: Technika LCD19DVDID dead

Ok.. That had me thinking I may need some other caps, thanks for putting my mind at rest!

Checked the 3 pin diode and Zener in the secondary side and the diode in the primary near the switching transistor, All seemed ok (out of circuit). I have however discovered an unstable 5.0 - 9.5VAC on two of the three output pins on the transformer between the primary and secondary.

The pics im taking are still not great, but hopefully these two are suitable.

Jim

[bbjunkie]

Re: Technika LCD19DVDID dead

Only two other pieces of silicon on the board, so may as well pull those and test I thought...

Removed the optoisolator OT1 and checked it on the multimeter. With the diode function I am seeing a voltage drop of 0.9 volts between 1 and 2 (in one direction) however when I measure resistance between 1 and 2 I am seeing an open circuit in both directions.
I am also seeing an open circuit in both directions on pins 3 and 4

The markings on the optoisolator are:

L0701
817C
X

Have I discovered my issue?

[tom66]

Re: Technika LCD19DVDID dead

That seems okay.

Do you get any output voltage, at all?

Faint ticking/whining?

How's the output diode, is it shorted?

[bbjunkie]

Re: Technika LCD19DVDID dead

I'm back indoors now, from memory there was nothing, perhaps 0.01v or so on the output.

No noises at all.

I assume the output diode is the 3 pin device screwed to the heatsink by the output (the marking on the PCB confirms it is a diode device)

[tom66]

Re: Technika LCD19DVDID dead

I assume the output diode is the 3 pin device screwed to the heatsink by the output (the marking on the PCB confirms it is a diode device)

Yes, that's most likely the diode. Does the PSU have only one rail? Most usually have auxillary rails (5V usually, sometimes 3.3V, maybe another rail ~15V-24V for audio depending on output power) because it's more expensive to add step-down converters on the main board. There might be SMD diodes on the bottom of the board.

Test all three combinations 1-2, 2-3, 1-3, 2-1, 3-2, 3-1 for shorts.

[momaka]

Re: Technika LCD19DVDID dead

While this may not be what caused your problem, you have some of that brown/tan glue on the primary side near D8. The brown/tan glue turns conductive with heat and age and also corrodes metal contacts. Remove as much of it as you can.

Post a picture of the underside of the PSU as well. The PWM controller is likely to be found there.

[bbjunkie]

Re: Technika LCD19DVDID dead

Going by the markings at the power connector, there is a 5.3v and 12v rail.

Diode checks showed:

1-2 o/c
1-3 o/c
2-1 0.26v drop
2-3 0.26v drop
3-1 o/c
3-2 o/c

Camera is indoors, next time im out i'll bring it and take a picture of underside.

The brown/tan glue is just residue, I have cleaned it off already (when removing the diode for testing). Perhaps I should use some sort of chemical to remove the residue also?

[tom66]

Re: Technika LCD19DVDID dead

That diode is fine.

Now check 5.3V diode.

[bbjunkie]

Re: Technika LCD19DVDID dead

0.713v drop one way and open circuit the other

[tom66]

Re: Technika LCD19DVDID dead

0.713v drop one way and open circuit the other

A bit high but might just be a silicon diode.

Okay, post a picture of the SMPS controller and read off the part # if possible.

[bbjunkie]

Re: Technika LCD19DVDID dead

Hi tom66,

Extremely difficult to read the numbers on the controller IC
Looks like:

LT 723
082269CP

Still having focus problems with the camera, got some better pics though.

ZD1 was removed for testing when this was taken, and the two main caps were also removed to recheck and gain access to other components.

[tom66]

Re: Technika LCD19DVDID dead

Test R43 and the other pink resistor.

eta- R24 too.

Not getting any hits for that part #, double check it with a loupe or 10X mag.

[bbjunkie]

Re: Technika LCD19DVDID dead

R43 is 67.6k ohms and the other is 2 ohms, which tallys with their markings.

I was using an illuminated desk magnifier to read the controller IC, even after cleaning it's almost impossible to read. Will try again when I go inside.

[bbjunkie]

Re: Technika LCD19DVDID dead

The other resistor I cant recall the value, but I tested it yesterday and it was ok.

The part number of the controller is OB2269CC

Attached extract from datasheet

[tom66]

Re: Technika LCD19DVDID dead

Okay, check Vdd (pin 7) relative to GND (pin 1). It should measure from about 16.5V to 36V. More detailed datasheet here:

[bbjunkie]

Re: Technika LCD19DVDID dead

Okay, check Vdd (pin 7) relative to GND (pin 1). It should measure from about 16.5V to 36V.

Jumping between approx. 12v and 16v looks like it could be DC with lots of ripple. Tried on AC setting but not getting a proper reading.

Haven't got my isolating transformer yet so cant put the scope on it.

Jim