Weird TV fault, any electronics experts / clairvoyants?

droopsnoot

I wonder if anyone has any thoughts on this situation*


I have a CRT TV, made by Sharp. It generally works faultlessly, but on the odd occasion, it will switch the picture off. If I switch it off and on again using the remote (so to stand-by, not actually off) it makes no difference. If I switch it off on the physical switch or on the mains, leave it a few minutes, then re-start it, it will usually come back on. If I haven't left it long enough, then on power-on I don't get the usual "pop"-type noise when the tube powers up.


On one occasion it did this several times over a period of an hour, then worked for days without trouble. The other night it did it once, after it had been on for about three hours, then after being switched off for about five minutes, it came back on and worked for another four hours without trouble.


My first thought was something getting warm, but in the most recent episode (three hours on, five minutes off, four hours on) that surely isn't enough time for whatever it is to cool down?


So, I wondered if anyone had any thoughts on what might be causing it. I've tried to narrow it down to particular circumstances or sequences but haven't managed to so far. I don't know enough about electronics to whip the back off it and start sticking test probes around the place.


(* - yes, I know it's old. No, I doubt I'll be paying a TV repairer - if there is such a thing, our local one closed down a while back - to repair it as it would be cheaper to buy a new TV. But it's annoying me now).

DoaM

Tube electronics are failing somehow ... or there's a capacitor failing on a PCB. (A blown/blowing capacitor on power circuits can give this scenario).

another_casualty

This may work: unplug everything , go to your tv preferences and reset it to factory settings as if it has just left the shop.
I had to do that last weekend , when my remote control was giving me different options than what I wanted and was behaving strangely .

Hope that helps

spadoosh

Do CRT's use some kind of starter like fluorescent lights?

It sounds like a similar situation where atmospheric differences can effect that start up im guessing they work somewhat similarly being vacuum tubes.

This is pure speculation and could just as easily be complete nonsense though.

droopsnoot

Thanks for the responses.

I could look for capacitors, I have learned with my previous PVR how to spot failing (ETA - probably "failed", rather than failing) capacitors. I hadn't thought of doing a factory reset on the TV, not sure I can think of how a garbled setting would cause this but it's worth a try. Not sure about whether the CRT has a starter - I imagine something like that, and I recall when we used to have monitors repaired they would often require a line output transformer, but they would fail and not come back. But in each case, it's been on and working for a while, and I'd have thought a starter would just cause an issue when starting, not during operation. Speculation is welcomed, there is not enough information around for anything else.


Of course, the biggest issue is the infrequency with which it fails. It can go weeks without any trouble at all.

Mutton_Geoff

droopsnoot wrote: »

Thanks for the responses.

I could look for capacitors, I have learned with my previous PVR how to spot failing (ETA - probably "failed", rather than failing) capacitors. I hadn't thought of doing a factory reset on the TV, not sure I can think of how a garbled setting would cause this but it's worth a try. Not sure about whether the CRT has a starter - I imagine something like that, and I recall when we used to have monitors repaired they would often require a line output transformer, but they would fail and not come back. But in each case, it's been on and working for a while, and I'd have thought a starter would just cause an issue when starting, not during operation. Speculation is welcomed, there is not enough information around for anything else.


Of course, the biggest issue is the infrequency with which it fails. It can go weeks without any trouble at all.

CRTs don't have a starter, they are essentially a large valve (vacuum tube) and have a heater that generates electrons from the cathode and the anode is the screen with a very high voltage (25,000 volts plus) provided by the "loptee" (Line Output Transformer - LOPT).


This is driven by a lot of circuitry, any one of the components in there could be going out of tolerance when hot. Your best bet is an old school TV engineer to look at it but a new LCD TV is probably cheaper than a couple of hours of his time and the parts to repair your existing TV. I'd just put it down to "end of life" as you've reached the right hand side of the bathtub curve.


"a heater that generates electrons from the cathode", when they invented this device, they named it a "cathode ray tube", or CRT since the electrons spraying from the cathode towards the anode were thought of as rays.

tonyh66

Just a warning. If you are going to open up the tv to look for failing capacitors, make sure they are all discharged properly, there is enough current kicking around in dormant capacitors within a CRT tv to kill you rather quickly. Even if its unplugged from the mains.

Mutton_Geoff

tonyh66 wrote: »

Just a warning. If you are going to open up the tv to look for failing capacitors, make sure they are all discharged properly, there is enough current kicking around in dormant capacitors within a CRT tv to kill you rather quickly. Even if its unplugged from the mains.

Yes, quite correct. As a TV engineer in my teens, I was always taught to work on TVs (and other high voltage equipment) with one hand in my pocket so as not to make a circuit with my body. Also not to put your hand deep inside a chassis since any shock might cause an involuntary reaction that would cause me to rip skin from my arm.


As they say, it's the volts that jolts but the mills that kills! (Mills = milliamps).



Oh, and don't go up ladders with a screwdriver in your pocket was another useful one!

droopsnoot

Mutton_Geoff wrote: »

This is driven by a lot of circuitry, any one of the components in there could be going out of tolerance when hot.

That's the weird thing, though - it's not as if it can be tied to how long it's been on, for getting hot. It's just done it on me now, switched on at 17.15, popped off at about 6pm, turned off for 2-3 minutes and now back on again. But it's not consistent - I had one Wednesday evening a few weeks ago when it popped off six or seven times in a couple of hours, the nothing for weeks. I guess when components start getting unreliable, then there's no predicting how they will demonstrate that.

Mutton_Geoff wrote: »

Your best bet is an old school TV engineer to look at it but a new LCD TV is probably cheaper than a couple of hours of his time and the parts to repair your existing TV. I'd just put it down to "end of life" as you've reached the right hand side of the bathtub curve.

I think you're absolutely right on that, it just seems a pity. If it went off and stayed off, then obviously there's no choice. As it is, it can work for weeks on end with no trouble. And if I swap it for an LCD, I'll have to find somewhere else for the fan and the little pile of books and DVDs that currently sit on top of it.


And thanks for the reminders about not sticking my hand in the back - I am aware of that, but it's always a useful thing to reiterate.

that

droopsnoot wrote: »

If I haven't left it long enough, then on power-on I don't get the usual "pop"-type noise when the tube powers up.

The tube does not plop and never will plop. What you are hearing is a degausing coil. On switch on, current rushes through the coil, and after a few seconds opens the contacts on something like a bi-metalic strip, to stop the current after the initial degause cycle, you only need a few seconds of degausing. If you switch the tv off and on again quickish the degause will not work as the strip has not sufficiently cooled down enough, so no plop. Only when the strip has cooled it will then close the circuit and you will get the plop

droopsnoot wrote: »

My first thought was something getting warm, but in the most recent episode (three hours on, five minutes off, four hours on) that surely isn't enough time for whatever it is to cool down?


So, I wondered if anyone had any thoughts on what might be causing it. I've tried to narrow it down to particular circumstances or sequences but haven't managed to so far. I don't know enough about electronics to whip the back off it and start sticking test probes around the place.

Safety first: that chassis is often floating mains. touch the chassis and earth, and if you are unlucky you will not be celebrating Xmas, but if you are lucky you will say many word not found in the dictionary.

even unplugged, the capacitors may carry a charge. That tube will definitely store a charge.

A tv that age... Dry joints. The intermittentness, dry joints. Yes it could be something else, but.....

Me: would unplug the tv from mains, get a 50 watt soldering iron, electronic solder, electronic flux. Remove the dust. resolder every single pin, especially the ones around the power area and fly back transformer.

Unless you know what you are doing leave the high voltage lead plugged into the tube. It is not your friend and you do not want to play with it, unless 24000V is really your thing.

If you do not know what the term "solder bridge" is, then leave the tv alone. You do not want solder bridges.

droopsnoot

that wrote: »

The tube does not plop and never will plop. What you are hearing is a degausing coil.

That makes sense, I recall having some CRT monitors in work that had a degauss button on them, and that was a similar noise. So the fact that it isn't doing that when I've only left it off for a short time isn't necessarily connected to whether it comes back on or not.

I might have a look at some point, and then again, perhaps not. I'm not looking forward to that much of an electric shock, and while I've done some basic soldering, it wasn't in the relatively small confines of the back of a TV.


Small footnote - hasn't done it since. Obviously until I press "post" on this, that is.