Safely Discharging a Flyback Transformer.

[Spork Schivago]

What are the procedures to safely discharge a flyback transformer? I watched a video on Youtube and the guy didn't want to repair the TV. He used a screw driver with a jumper wire on it going to a ground on the board and touched the screw driver to the anode wire going into the picture tube. If I hook a screw driver up with a jumper wire to something like the tuner box to ground it, would that damage anything? Thanks.

[tom66]

Re: Safely Discharging a Flyback Transformer.

A flyback transformer doesn't have a capacitor. You may mean to discharge the CRT? In which case the typical method is to stick a grounded screwdriver under the anode cap. A spark should be heard, which is the discharge.

[Spork Schivago]

Re: Safely Discharging a Flyback Transformer.

A flyback transformer doesn't have a capacitor. You may mean to discharge the CRT? In which case the typical method is to stick a grounded screwdriver under the anode cap. A spark should be heard, which is the discharge.

I guess I do mean discharge the CRT. Don't I have to run a wire from the screw driver to a ground? Or do I just touch the screw driver under the rubber and it'll discharge?

[tom66]

Re: Safely Discharging a Flyback Transformer.

If you just touch it under the cap, where will the current go? You must ground one end of the screwdriver. Also, it's essential to use an insulated screwdriver.

[Mrx3750]

Re: Safely Discharging a Flyback Transformer.

You would touch the screwdriver and it would discharge.

However that's a really scary way to do it.

On most newer CRT TV's discharging the CRT is not a huge concern because they have bleeder resistors that will drain off the CRT's charge in a few minutes after the set's powered off and unplugged. However discharging is still a good idea.

You could set up a series of high wattage resistors in series from CRT cap to ground if you wanted to discharge without the spark and noise.

[tom66]

Re: Safely Discharging a Flyback Transformer.

On the other hand, hearing the audible "click" is a good confirmation that the CRT has discharged.

[Spork Schivago]

Re: Safely Discharging a Flyback Transformer.

This TV is from 2000, I hooked the screw driver to a wire and the other end of the wire to the metal frame that goes around the CRT. I touched the clips under the rubber and didn't hear a pop. I took the anode clips out of the crt and again, touched the screw driver to the clips. This time, I touched it to both clips...and still no pop.

I'm a little scared of touching the anode clips and seeing if it's discharged. I'd feel a lot safer knowing that it is in fact discharged. Should I assume it has the bleeder resistors?

I checked the continuity of my setup by touching one probe of my DMM to the screw driver, the other end to the alligator clip and it read 0. So I know there's no shorts in my setup...

[Spork Schivago]

Re: Safely Discharging a Flyback Transformer.

What makes the metal frame around the TV a ground but not the screw driver? Is it because the metal frame is square? Wouldn't I have to ground it to something on the circuit board? So it's a complete path?

[tom66]

Re: Safely Discharging a Flyback Transformer.

The metal frame is ground because the CRT is attached to it. There are ground wires running around it, I think the metal coating or edges of the CRT make it ground.

[Spork Schivago]

Re: Safely Discharging a Flyback Transformer.

It must have those bleeder resistors. I ended up touching the clips directly to the tuner's metal box, nothing. Then I ended up touching the clips with my fingers, nothing.

[Spork Schivago]

Re: Safely Discharging a Flyback Transformer.

Thanks!

[Mrx3750]

Re: Safely Discharging a Flyback Transformer.

Might I ask why you're discharging a CRT?

[Spork Schivago]

Re: Safely Discharging a Flyback Transformer.

Might I ask why you're discharging a CRT?

Yup, the OSD doesn't work on this TV. It's a Panasonic CT-36D30B. I cannot find a service menu so I'm going to start randomly pulling components and testing them until I find the bad component.

The volume works but it doesn't show any bars, changing the channels work but it doesn't show any numbers or anything. Hitting the Menu button doesn't display anything on the TV.

I couldn't really find any info on how the on-screen display works. I found some data sheets for some of the ICs. One's called a MPU and some of the pins said something about closed caption. I think that's where I'll start.

When I was trying to work on the board, the flyback's second anode wire that connects to the crt wasn't very long and I couldn't work on it properly. So I had to unhook it so I could move the board out a little more and start unsoldering everything.

[tom66]

Re: Safely Discharging a Flyback Transformer.

The OSD on most of these sets (old CRT) works by switching in the OSD generated video, synced to the video signal. You don't see a black shadow of the OSD, so I'd be looking at why the masking circuit, the part which selects between OSD and normal video isn't working. Due to the highspeed circuits you'd probably need a scope to see what info if any is missing. You can also check the local power supply for the microcontroller(s).

[Spork Schivago]

Re: Safely Discharging a Flyback Transformer.

How much does an oscilloscope costs? How do I tell what and where the masking circuit is? I found what looks like a diode but it doesn't have any markings and it reads open both ways. On the circuit, it's labeled L006. Any idea what this is? My DCA55 meter doesn't recognize it.

[Spork Schivago]

Re: Safely Discharging a Flyback Transformer.

Would the L indicate an inductor? I thought inductors had wires wrapped around them...

[tom66]

Re: Safely Discharging a Flyback Transformer.

L005 is an inductor, and should read shorted.

A decent, used scope can be had for under $50:
https://www.google.co.uk/url?sa=t&rc...51495398,d.ZGU

[Spork Schivago]

Re: Safely Discharging a Flyback Transformer.

Would this one work? It's a 100MHz model. 97$ with 18$ shipping... http://www.ebay.com/itm/Hitachi-Osci...item3cd434b688

Also, here's an Hitachi V 252 2 Channel 20Mhz Dual Trace Oscilloscope Tested for 94.99$ and 39$ shipping. http://www.ebay.com/itm/Hitachi-V-25...item5d436faf95

I don't think the first one comes with cables and not sure about the second one.

I've never used one before. I don't know much about them. Like what's the difference between the various MHz models and stuff like that.

[Spork Schivago]

Re: Safely Discharging a Flyback Transformer.

It's a lot of money for my girl and I. Hopefully the investment will pay off. I don't really know what you can use them for besides fixing televisions.

[Spork Schivago]

Re: Safely Discharging a Flyback Transformer.

On-line I saw something that said for TV repair, a 100MHz one would be best. Is that true?