TV picture not coming on but sound is ok

[Dundonald]

I have a 2 year old portable TV where when switched on occasionally the screen will go blank, but power is still on and sound still comes through the speakers. More recently the screen doesn't come on at all when switched on, though the power light is on and again sound is coming through the speakers. We found that when the screen went blank if we lightly tapped the sides of the TV the picture would sometimes reappear.

Rather than mindlessly going out and buying a replacement I'd rather open the TV up and see if there is anything that I can fix - has a cable come loose, a part can be replaced etc ... ? But I've done no TV DIY before so I'm not sure where to start.

The TV is well looked after, i.e. for the duration of the time that we've had it it's simply been placed on a wall mounted TV stand, so it's not as if it's been knocked about.

I've googled this group but can't find any immediate obvious answers to similar problems. Any pointers please?

[Harry Bloomfield]

Dundonald expressed precisely :

It sounds like a dry soldered joint, however there are some leathal voltages inside TV's (25 to 45Kv) - so I would not advise the inexperienced to go probing about in there.

[Set Square]

Almost certainly a dry joint - but you might well electrocute yourself while trying to fix it if you don't know what you're doing.

If it's only 2 years old, take it back to where you bought it and get them to fix it for free. Forget the maker's guarantee - which is probably only a year - and invoke your common law rights!

[EricP]

Are you in the habit of sticking a scart lead in the back a lot?

If so, you have probably broken the solder around one or more pins on the socket to main board joint

[Lee]

Could be a dry solder joint.

Read this:

the safety information.

Lee

[Dundonald]

What's a dry solder joint? :) (I'll google that)

I've checked this link out (posted by someone below)

and I'm guessing the raster there is no power to the raster (whatever that is! :) because there's no picture at all and there is no sound like there is when there is a picture.

I have a couple questions:

1. how would I identify a dry solder joint and how would it cause a lack of power?
2. regarding voltage, surely if I unplug the TV there'll be no voltage to worry about?

Cheers

[Dundonald]

I bought the TV from Argos, not too sure if we still have the receipt, I'll have a look. But any tips/recommendations on what to say to Argos to see if they'd fix / swap it?

[Harry Bloomfield]

It is a solder joint which is not making full and proper contact between the two parts it is intended to connect. A poor or intermitant connection in other words.

The raster, is what makes the spot on the screen move across the screen.

These can be difficult for even the expert to spot. Personally, with your described fault - I would be working with the set live and the back off - tapping PCB', components, connections and wires with something insulated, hoping to reproduce the fault and thus hope to help localise it. Then you get the magnifying lense out to inpect the area.

The fault you are describing - The loss of whistle + loss of picture would lead me to the line timebase area and probably the EHT unit. If you are really determined to have a go yourself - Look for a PCB upon which is mounted a large component soldered on to the PCB by an almost circular ring of perhaps 8 to 10 connections. To further confirm you have the right part, you should be able to trace the EHT lead back from the tube to the same component. At the tube end it will disappear into a large insulating cover.

Check out the 8 to 10 soldered connections and you may well find one or more of these has been burnt and making poor contact with the PCB.

The TV can store the high voltages for quite some time after it has been unplugged.

[The3rd Earl Of Derby]

Yep! even on a 1950s bubble TV, that I found in the loft of a house which I was working in. Plugged it in and up came "all our yesterdays". :-)

-- Sir Benjamin Middlethwaite

[The3rd Earl Of Derby]

Switch off TV and turn setupside down, if you can see the PCB? this is the soldered(printed circuit board), get some methylated spirits, tooth brush, and a magnifying glass, locate the EHT transfomer area(big black thing on other side of board), rub meths on soldered joints with tooth brush and wait till it drys out, then get magnifying glass and look over the joints they will be greyish now and any dry joint will show as greyish with a thin black circle in the joint...this is a dry joint.

[john]

etc ... ? But I've done no TV

*The* nightmare service problem. dry joints are usually microscopic cracks in the printed board copper or progressive corrosion in a badly soldered joint. Multilayer PCB's may have been used which hide everything. Integrated circuit chips can also develop internal connection problems with similar symptoms in which case replacement is necessary. The chips may be difficult to obtain outside the trade and need special tools to replace.

In any case dry joints are not usually visible unless you're extremely lucky. Mechanical pressure or tapping is required to locate. Even then they may be next to impossible to find. If an intermittent fault can't be repeated at will, it can't be repaired.

The most dangerous voltage is about 25 KV on the back of the picture tube which could cause cardiac arrest if it goes through the body trunk, especially for an older person. This voltage can persist for weeks even when the TV is switched off and unplugged and must be discharged before doing anything. It will damge either yourself or do in the rest of the electronics if it discharges where it shouldn't.

John

[Set Square]

Tell them that they have a legal obligation to supply goods which are of merchantable quality. This TV clearly wasn't - since any reasonable person would expect it to last more than 2 years without failing.

Google for Sale of Goods Act, et al

[Blueyonder]

With questions like that, I would say DON'T OPEN THE SET UP.

Really.

-- JJ

[Blueyonder]

Sorry - that sounded a bit flippant, but it was meant seriously.

If I walked into a car mechanics, and told them my car had stopped working, but I had hear of some thing called "an injection", but wasn't sure what it looked like, so pulled a few bits out of the engine with a hammer, and could they just tell me how to recognise whether the "injection" I pulled was worn or not, then I'm sure I'd not be taken seriously.

Some thing applies to TVs. As well as very high voltages, that can can stay in the set for *days* and can *kill*, a TV set is a very complicated thing. The problem probably is just a dry joint, but identifying it, fixing it, and not bugg*ring up the whole insides of the thing, and putting it back together *safely*, as an absolute beginner, is only going to end in tears.

Take it back - it should last longer than two years. Make a fuss - you have nothing to loose ;-)

-- JJ

[Dundonald]

No problem I understand.

I will attempt to take it back, I need to speak to argos.

[Dundonald]

No problem I understand.

I will attempt to take it back, I need to speak to argos.