Mains off - back on - no TV

[R D S]

I cycled the mains off and back on a few times by means of the RCD, now a TV doesn't work.

could be a coincidence? Or do folk tend to unplug stuff?

[Dave Plowman (News]

Depends what you mean by cycle. If I want to reboot my router, I usually do it via the MCB as that is easier to get at. But leave it off for at least 10 seconds. The router is on the same radial as the AV setup, and not had a problem with anything else doing this.

[Jim GM4DHJ ...]

nothing on at the moment anyway ...

[John Walliker]

The TV might have been powered up for years and just switched to standby mode when not in use. Many switching power supplies have a small electrolytic bootstrap capacitor which is only ever used to help start the power supply. This could have failed ages ago and this would only have become obvious when the mains was switched off and on again.

John

[Paul]

Damage to an inrush limiter on the power front end ?

In this diagram, it is NTCR1 near the plug. NTCR1 tames the rate that C5/C6 charge up (transient amperes through the bridge rectifier).

When an inrush limiter is implemented that way, it needs time to cool off, between uses. Like, 60 seconds of power off, then power on again.

Rapidly toggling mains to things like ATX power supplies, can cause them to pop and fail. in one of the computer groups, we got a report from someone who had done this out of frustration, until they heard a "pop" sound come from the PSU :-)

There is more than one way to implement inrush limiting. On designs with active PFC, you can use the PFC pass transistor as an inrush limiter, at powerup. And then there is no longer a "thermal" issue. The PFC controller recognizes that T=0 and operates in inrush limiting mode, and then a fraction of a second later switches to PFC mode.

There is also fuse F1 in that schematic diagram, but because those are Slo Blow type, they almost never pop. It takes a proper short circuit in the front end, to open fuse F1.

*******

TV sets are partitioned into "boards", and the power function is a separate board. You should be able to recognize the function, by the number of electrolytic capacitors on the board. The power board has SMPS (switched mode power supplies), similar to an ATX supply for a computer, but with different voltages on it.

Power boards are a frequent source of failures, but this was back during the "capacitor plague" when badly fabricated capacitors would corrode through and leak. There were some TVs and computer monitors that suffered for this.

The power boards can be OEM items, and the TV factory did not necessarily make the power board.

Paul

[Peter Able]

Agreed, there are both high and low tech reasons why you shouldn't cycle the mains in seconds.

I take it that you've unplugged the TV completely for, say, a minute - then tried again? Attached boxes, too?

And, yes, it could be a coincidence !

PA

[R D S]

The power was turned off and on at the CU numerous times while I was doing a job.

I've had the back off the TV, there's an IC on the power board with a crater in it.

[Dave Plowman (News]

If off for a reasonable length of time, it shouldn't matter. It was 'cycle' I wondered about. As in switching on and off rapidly.

When I had a not that old TV PS fail. I bought a used board for it from Ebay. Given that it's not easy to get schematics these days.

[T i m]

<snip>

Ah and that's probably let the magic smoke out. ;-(

The problem from my 'I don't generally like to mess with SMPSU's' POV, you can't guarantee that it's or it's_just the obvious that's dead.

I did once chase a fault on a PC monitor PSU where it obviously started with a dry / OC smoothing cap but then escalated to several other components. Various people had tried repairing them and had repeatedly replaced an ever growing list of bits (some not 'consumable cheap') before either hitting the magic component or giving up.

After trying the things that were suggested as the 'most likely' that didn't work I found a complete new PSU from China that cost less than even a couple of rounds of the components ... and that worked straight away. ;-)

I did however replace it's main smoothing cap with the better quality one I'd bought for the faulty PSU so even less was lost and it might then last longer.

As an aside, I saw a stack of 5 cars on the back of a scrap lorry the other day. They all looked undamaged (well, other than where they were stacked directly on each other) and were all newer than our car and imagine many may have been written off for the sake of a few pence worth of components. ;-( [1]

Cheers, T i m

[1] A mate used to repair vehicle 'computers' and very often it was only a few pence worth of parts that turned them from 'working' to 'a very costly replacement and diagnostic re-configuring'.

[R D S]

Indeed, and i'm pitiful at soldering.

I've found a board for $20 odd overseas but I don't want to be without it too long so there's a place in the UK you can post them to for repair/exchange for about £40.

I feel like most would see this as an opportunity to get a new massive telly!

[T i m]

Ah, that might not help.

Quite!

A mate bought such, selling me his 40" shuffled-though (his old lounge TV went into his bedroom, what he sold me came out etc) TV and his new set looked *massive* (to both of us) but now he doesn't see it as large at all.

Once you have the new TV you can bide you time re fixing the old one?

Cheers, T i m

[The Natural Philosopher]

Oops.

[Paul]

When I was a teen, I used to practice soldering on surplus PCBs with chips on 'em. If you have materials to practice on, you can discover the limitations of home equipment.

Even with proper gear (vacuum de-soldering station), some jobs are just about impossible. On power boards, getting out electrolytics that are interference fit (holes only 0.005" bigger than the leads), those suck big time. You can blob solder on those, use the vacuum station to draw up the solder, and the fillet will still not be dry. Then, the leg won't move. And if you pull on it ? The fillet comes right out of the board and it is ruined. Solder wick does no better on those. Even angel hair wick. ChipQuik would not help, because it's not going to void the hole any better than anything else. ChipQuik is good for TSOP SMT maybe (dental floss and gentle heat). That's if you need to recycle an IC and use it again, after you discover it wasn't the broken bit. The dental floss can slide under the foot of the lead, to release it. And it's all because the addition of ChipQuik (Bismuth) reduces the melting point.

My previous employer did not use interference fit mounting for electrolytics. Their stuff was easy to repair. Even a "non-soldering person" could do those. Wouldn't even need a vacuum desoldering station. The interference fit ones are brutal. You can chew the top off the component with large side cutters, but you almost need as much mechanical advantage as Vice Grips to cut cleanly through an electrolytic, then pick the remaining bits off. Then you heat up the fillet with an 80W iron and carefully pull the leg out. But that's not exactly good technique. The Soldering Police will be by, to laugh at you.

Paul

[T i m]

<snip>

As did I. People knew I liked 'taking stuff to bits' and so would give me stuff and as you, I then had things to de-solder and in so doing, learned what to do and what not to do (like flicking solder in your eyes). ;-(

Agreed. I have small reflow and vac desoldering stations plus a selection of irons of all different wattages and tip shapes, desoldering wicks, fluxes and low temp solder and a fair range of tools but some things can be a bugger. ;-)

I have sometimes 'walked' those out, one leg at a time, wetting the joint as best I can with solder, flux and just enough heat.

I have sometimes drilled *though* the pin, leaving a tube and then that can relieve the clearance between pin and hole enough to pull it out (still using the iron etc) without damaging the THP.

For the though hole IC's we would often cut though the shoulder of the leg and then you could confirm if it was the IC or something else on that track. If it wasn't the IC you could re-join the pin.

Yeah. I try to keep some to hand, especially when de-soldering lead-free stuff.

I'm guessing there must be a reason to have the tolerances that tight?

When I was at BT the 300 baud modems (as big as a desktop PC case) had a filter in a sealed tin (like a tobacco tin) and if there was a faulty component inside, you had to de-solder the lid (all round) to get the top off.

I've used a cutting disk on a Dremel but it can get quite messy.

So they might but 'needs must'. ;-)

Cheers, T i m

[Brian Gaff (Sofa]

My crystal ball is not working today.

Not normal for this to occur, normally in standby they recover quite well from unexpected mains failures. However one of mine died between two days, and it seems to be a psu issue as in standby it looks normal, hit the remote, it briefly goes green and there is a couple of clicks, then back to red. Nobody has managed to fix it as its one of those here today gone tomorrow makes. It might be just that you are unlucky and it failed, but might already have been almost ready to die before hand. If its still under warranty shove it back otherwise its just what happens, and it really depends on how much you want to pay to get it fixed Dried out electrolytic are a common cause for inability to start from standby it seems. I had an old video that needed to have been warmed with a hair drier before it could run, No idea why. Brian

[Brian Gaff (Sofa]

Well, that sounds like a switch mode psu to me. Normally though those chips are protected for mains surges etc as early ones would die if a switch to a motor was sparking, or in the case of a Samsung fax machine i had a lightning struck half a mile away!

Brian

[Brian Gaff (Sofa]

Yes my edis sub woofer blue up its bridge rectifier a couple of years ago. When my friend pulled out the offending device and looked it up it was not rated for peak currents over a certain amount, but when the capacitors were charged at turn on this current was much higher as a transient, Hence fitting a more capable bridge slightly raised on wires has been good ever since. Companies trying to save pennies at the expense of reliability. Now its giving problems with its switch on, which is supposed to be triggered by very low levels of audio and kept on for minutes. The only way to boot it up is to unplug it and plug it in multiple times till it hears the sound. Suspect some other component is now changing value. Moor cheap crap I expect. Brian