Technika LCD19DVDID dead

[tom66]

Re: Technika LCD19DVDID dead

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

Remove the Vcc/startup cap and check it if you can. Sounds a lot like you have a bad startup cap. 12V might be enough for the chip to cut out (threshold is 11.5V for a very similar chip.)

If you can't test it, just replace it. They are cheap anyway and no need to use a low ESR one, a general purpose cheapie will do fine.

eta - Just noticed the datasheet specifies an over-voltage threshold of about 23-25V, so this should be a maximum, not 32V.

[bbjunkie]

Re: Technika LCD19DVDID dead

Had already removed and tested the startup cap, have just swapped it out now to be sure, problem is still there. (assuming the startup cap is C21 in DSCN1803.jpg here https://www.badcaps.net/forum/showpo...7&postcount=15)

[tom66]

Re: Technika LCD19DVDID dead

Okay, I've traced the circuit a bit on the primary side. Test the components and check for AC (preferably with a scope) on the secondary side... as it is isolated no need for an isolation transformer.

[bbjunkie]

Re: Technika LCD19DVDID dead

Ok, got the scope on the transformer secondary.

With the gnd on centre there was nothing. gnd on the output opposite the word "HIGH" and probe on centre one produced a 50hz waveform of approx 30v p-p
Similar was noted on the other output using the same pin for gnd as previous.

I put the probe on the output of the psu and noted the other (smaller) waveform, again 50hz but only at an amplitude of approximately 40mv

Large waveform shown at 5v/div
Small waveform shown at 20mv/div

[tom66]

Re: Technika LCD19DVDID dead

Looks like a ground loop from the mains (@50 Hz). Unplug it and does it go away immediately or take a few seconds to decay?

[bbjunkie]

Re: Technika LCD19DVDID dead

I'll need to go back inside to check (scope indoors, rest of tools outdoors, bit of a pain at the moment)

What I did discover was that my new probe on the scope was set to x10 - the large waveform is actually closer to 300v

I also discovered with the probe on the secondary ground i'm seeing the same waveform (same as the one on the transformer)

With the red lead of the DMM on ground and black floating im seeing a reading of 12v ac

Im very confused! Will go back indoors and check that ground loop theory shortly.

[bbjunkie]

Re: Technika LCD19DVDID dead

Forgot to mention, the sign of damage on the board is right.. however according to the magnifier and dmm the track is still connected

[tom66]

Re: Technika LCD19DVDID dead

I'll need to go back inside to check (scope indoors, rest of tools outdoors, bit of a pain at the moment)

What I did discover was that my new probe on the scope was set to x10 - the large waveform is actually closer to 300v

I also discovered with the probe on the secondary ground i'm seeing the same waveform (same as the one on the transformer)

With the red lead of the DMM on ground and black floating im seeing a reading of 12v ac

Im very confused! Will go back indoors and check that ground loop theory shortly.

Yeah, you'll get that kind of waveform when trying to probe an isolated ground. The ground is floating because of the class-Y capacitor, it creates a voltage divider putting the ground at around half the AC waveform, of course depending on the bulk capacitance and other stuff it won't quite look like a sine but it should have about 300V amplitude on X1. It's very high impedance (>10Meg) but the capacitive divider inside the scope picks up the AC easily.

To fix, you need to connect the COM terminal to Earth using your scope's ground clip.

I bet the DMM error is caused by you holding the probe. Disconnect negative from the meter socket and see if you get the same. Your body has a fairly low impedance to ground.

[bbjunkie]

Re: Technika LCD19DVDID dead

Ok then, taking that as the explanation then with ground on centre pin of transformer and probe on outer pins of transformer, there was no output at all on scope.

How do I check the optoisolator?

You were spot on about the DMM error!

[tom66]

Re: Technika LCD19DVDID dead

Ok then, taking that as the explanation then with ground on centre pin of transformer and probe on outer pins of transformer, there was no output at all on scope.

How do I check the optoisolator?

You were spot on about the DMM error!

To test the opto: Test the secondary side using a diode test you should get around 1200. I've never seen them fail, though...

Have you checked the FET for shorts?

Check the current limit resistor - should be close to zero ohms.

I assume you are getting ~320VDC across the main cap as you have a Vcc?

[bbjunkie]

Re: Technika LCD19DVDID dead

Yes, getting 320VDC across main cap.

Tested the opto back in post #6 with the diode method:

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

Current limit resistor is reading 1.6 ohms in circuit

Checked the switching transistor, it's a MOSFET so im not entirely confident i've tested it right. It certainly doesn't have any internal shorts though.

I'll pull it again and test.

Jim

[bbjunkie]

Re: Technika LCD19DVDID dead

The switching transistor is a FQPF 8N60C mosfet

Datasheet here: http://www.datasheetarchive.com/FQPF...datasheet.html

Definitley not shorted between any pins

[tom66]

Re: Technika LCD19DVDID dead

Current limit resistor is reading 1.6 ohms in circuit

That's a bit high. What do you probes read when shorted together?

Checked the switching transistor, it's a MOSFET so im not entirely confident i've tested it right. It certainly doesn't have any internal shorts though.

Check both ways on every combination of the pins using the ohmmeter function. Before testing short gate to source. All pins should appear open.

[bbjunkie]

Re: Technika LCD19DVDID dead

After shorting G to S I'm seeing a voltage drop of .5v slowly dropping to .3v between D and S

Does this mean it's faulty?

[bbjunkie]

Re: Technika LCD19DVDID dead

corrected error above, voltage drop is between D and S

[tom66]

Re: Technika LCD19DVDID dead

After shorting G to S I'm seeing a voltage drop of .5v slowly dropping to .3v between D and S

Does this mean it's faulty?

Seems strange. Are you sure you are shorting the correct pins? I think Drain is the pin that's bent outwards. Also, make sure to test both ways - one way has a diode integral to the MOSFET.

[bbjunkie]

Re: Technika LCD19DVDID dead

According to the Datasheet the pins are G D S in that order when viewing it from the front.

Therefore, I am shorting the two outside pins then doing the diode test between centre (black wire) and right (red wire)

If I swap the wires or try any other combination i'm seeing an open circuit.

[bbjunkie]

Re: Technika LCD19DVDID dead

missed the question about the meter - reading 0.6 ohms with leads shorted.

[tom66]

Re: Technika LCD19DVDID dead

MOSFET seems okay... but...

Get a 9V battery and connect it gate-source briefly. This will charge the gate-source capacitance. Then check drain-source; should measure close to zero both ways. Short gate-source again, now one way should be open and the other with diode test should be ~500mV. Also make sure to use resistance for all except the diode check.

Current limit resistor is 1 ohms. That seems okay.

Right now the problem appears to be the supply is getting power but not starting up or running properly.

Check the MOSFET gate resistor and the diode for the Vcc supply.

[bbjunkie]

Re: Technika LCD19DVDID dead

Gate resistor - if that's the one you have marked as R? then it's 26 ohms, the diode to it's 11 o'clock checks fine in circuit (drop of approx 0.53 volts)