TV cap question

[Tom41]

I recently opened up my TV (to check for damage, it fell on the floor), and decided to have a look at the caps as well. The circuit board contains many little caps, and two big caps. Of these big caps, one was Rubycon and the other was Lelon. Neither of them look bad in any way, and the TV is not showing any bad cap symptoms (yet) - but what are these big caps actually used for?

If they're important, is it worth worrying about that Lelon one in case it bulges or vents? What's the worst that could happen if it does?

Edit: It's a 21-inch CRT, not an LCD, designed to run off 240v AC mains power.

[larrymoencurly]

Re: TV cap question

The biggest caps in my CRT TVs are for the main power supply, the ones that work at 60 or 120 Hz, just like the biggest ones in an computer ATX PSU.

I had a 5-10uF, 180V cap in series with one of the yoke coils go bad and cause my TV to show only a horizontal line.

Apparently the high frequency caps are the ones most prone to failure -- outputs of the main power supply (if it works at high frequency, but many TVs don't have a transformer in the PSU) and the horizontal output section (around the big transistor that's probably mounted on the largest heatsink), including around the flyback transformer. But a few months ago I had a 1uF 50V Jamicon cap around the vertical output chip go bad in my old Sanyo TV and cause the lower part of the picture to stretch out.

[repairs online]

Re: TV cap question

Hello,

Sorry Larry Moe and Curly, but ALL TVs have a transformer in the PSU. I've have been in the business for over 20 years and to this date I have not seen a TV without a transformer in the PSU.

However, some of the other comments you are making a correct.


Thanks

[kc8adu]

Re: TV cap question

Hello,

Sorry Larry Moe and Curly, but ALL TVs have a transformer in the PSU. I've have been in the business for over 20 years and to this date I have not seen a TV without a transformer in the PSU.

However, some of the other comments you are making a correct.


Thanks

i have seen lots of cheap tv's with no transformer in the psu.
simple doubler off the ac line for b+ and tube heaters in series.

[arneson]

Re: TV cap question

i have seen lots of cheap tv's with no transformer in the psu.
simple doubler off the ac line for b+ and tube heaters in series.

I double that.

[chipper]

Re: TV cap question

Sometimes we tend to misjudge things according to their size , when the real risks are the conditions under which capacitors are "operated". Since you do realize that it may be risky to continue to operate a tv that has been dropped or knocked onto the floor or whatever , you have taken a good step to check inside.

240 volts can be extremely dangerous , but luckily most tv's/lcd's included , of less than 10 years old are reliant upon the rest of the system (ic's) to make it so that large capacitors are not the direct supply on their own for power. I recently took apart an old (20yrs) amplifier which had 2 - 10000uF @50v capacitors bolted to the underside of the unit , facing across the bottom. They could not be seen from above. Both of them were in pristine condition and measured fine by meter too . Meanwhile the unit weighed about 30lbs. This is because the power transformer was huge (about 4 cubic inches) and the heatsink weighed about 3lbs as well.

It is a good indication today that if the unit is lightweight , it posses no danger under normal circumstances , as it contains the newest style power supplies , not old. The worst that could happen is that the unit could become shorted and shut down. Sometimes small fires do survive the process and will eventually heat up large capacitors so that they can explode. But , as I said , these days you might see a 4700uF @35v capacitor in a monitor unit , but it is not likely to omit sparks or flame/contents outside of the casings , which have not been scaled down like the PCB's inside the units. Making sure that the unit wiring is secure and that there are no loud buzzing sounds coming from the monitor will allow it to continue without incident for a long time.

It's usually the ignorance of excessive heat that causes capacitors to explode , they are heated like a propellant , which exits with excessive force. No wires should touch the outer aluminum casing of any capacitor , that could act as a heater to the chemical compilation inside the capacitor. Every precaution should be taken to ensure that all wires have a secure location and connection inside the set , and none of the wires should be fraid or damaged in any way before the cover goes back on.