Weird TV fault, any electronics experts / clairvoyants?

that

If you have common sense it is not difficult, but there is one born every minute - apparently

Unplug it, blow out the dust, and you may need a long handle paintbrush to dislodge the stuck dirt, often caused by static.

Mist plugs are keycoded, but if not number them and the board (not with a pencil) before unplugging. You want to turn that board 90 degrees to solder, rather than upside down.

Often some of the larger components have big metal surfaces that sink heat, so a 50 watt iron, or a BIG copper bit will also often work.

I would normally say that the joint will possibly look greyer and sometimes pitted than the others, but due to age you may find this harder to see, they possibly all look grey. it also may have a crack in the solder around the pin.

The task is not difficult, just don't short stuff out.

Worth having a bash before you throw it out. Before you start this make sure your tv has not been running for about an hour as most of the caps will have discharged, but the tube will not have.

https://www.ebme.co.uk/articles/maintenance/the-basic-de-soldering-guide-photo-gallery
https://www.quora.com/Is-a-dry-or-cold-solder-joint-conductive

https://en.wikipedia.org/wiki/File:Cold_solder_joint2.jpg
https://en.wikipedia.org/wiki/File:Gebrochene_loetstellen.jpg

I'm also an ex tv engineer, but crossed over into computers before the VDUs became colour

mart44

About the noise when the TV is switched on: That could be the degaussing noise. Degaussing uses a degauss thermistor, a three-legged device that only works once when the TV is first switched on. It has a low resistance to AC mains when cold and allows an AC current to run through the degaussing coil that is situated close to the CRT's shadow mask. The thermistor quickly heats up and as it does, the resistance goes high, so cutting off the AC to the degaussing coil. The result is an initially high but then fading magnetic field that demagnetises (degausses) the CRT's shadow mask.

The coil can make a brief humming noise as AC passes through it before the thermistor resistance cuts the current off. The degaussing noise won't be heard again unless the thermistor has cooled right down, cool enough so that the resistance has gone low enough for it to pass an AC current again.

It only cools when the TV is turned off for around 10 to 15 minutes, so you wouldn't hear the degaussing noise (if that is the noise that's being heard) when working on the TV and/or turning it off and on again within short periods of time. This won't be anything to do with the fault but it is why you don't hear the degaussing noise at every switch-on.

Intermittent faults are a trial. It could be a dry joint and the best way to tackle those is to try and bring the fault on when the TV is running by tapping around on the PCB with a non-conductive tool (a plastic knitting needle for example). Waggle plugs and socket etc. If the fault can be bought on this way, gradually narrow down the area of tapping to the most sensitive point and then try re-soldering any joints in the area that doesn't look good under a magnifying glass.

It could be a thermal fault. A good way of approaching those is to wait until the fault occurs and then spray freezer over different components in a general area of the PCB. See if cooling any of them down clears the fault. If the freezer does tie the fault down to a certain area, start spraying individual components with freezer until you come across the one that is faulty when warm.

Look out for bulging electrolytic capacitors and for leakage from them, remove any leakage because it is conductive. Above all, be careful of what has already been mentioned as regards safety and high voltage.

steviebabes

Most likely cause is dry joints on the line output transformer, they oscillate at high frequency and cause solder joint failures. They will need desoldering and resoldering with a good quality leaded solder.

droopsnoot

that wrote: »

I would normally say that the joint will possibly look greyer and sometimes pitted than the others, but due to age you may find this harder to see, they possibly all look grey. it also may have a crack in the solder around the pin.

The task is not difficult, just don't short stuff out.

Worth having a bash before you throw it out. Before you start this make sure your tv has not been running for about an hour as most of the caps will have discharged, but the tube will not have.

Thanks, I may have a go at it after Christmas. The TV (for various reasons) is powered off (properly, not stand-by) overnight, so a good time would perhaps be the next day, everything should have settled down by then.


I have some experience of dry solder joints - I have a Mk1 Audi TT, and they seem to have been a design feature in several areas of those.

mart44 wrote: »

...stuff about the popping noise on switch on...

That seems reasonable, so the though I had about the popping noise being linked to whether it will come on again is not actually linked, it probably wouldn't have popped anyway, without the fault.

mart44 wrote: »

Intermittent faults are a trial. It could be a dry joint and the best way to tackle those is to try and bring the fault on when the TV is running by tapping around on the PCB with a non-conductive tool (a plastic knitting needle for example).

I'm not sure I fancy doing that with the set on. I am generally quite careful but catching an exhaust or a weld when it hasn't cooled properly is a different thing.

mart44 wrote: »

Look out for bulging electrolytic capacitors and for leakage from them, remove any leakage because it is conductive. Above all, be careful of what has already been mentioned as regards safety and high voltage.

That's one I have seen on my old PVR, though in that case the device failed to power on. I could look for those.


Thanks to you all for the advice, if I do try something I'll update the thread. It hasn't popped off again since Friday.

DoaM

Regarding leaking capacitors ... wear gloves to clear up any leakage as the substance can sometimes be "not nice" if it gets on your skin.

Heedtheadvice

Lots of sensible posts about possible causes. A lot more could be added. However I will not.


Why....it has already been posted regarding the dangers of working inside a crt tv. The tube is powered by 20 to 25kv with enough energy to give you a serious shock. Even after prolonged disconnection it can 'self charge' and still be dangerous plus it is connected to other parts of the circuitry plus the chassis can be connected to mains phase wire......


Need I write more. If you are not trained and experienced why take the risk?
Anyone who advises DIY is being irresponsible and is not full of seasonal good will!



You could just get the paramedics sitting eating mince pies close by just in case....:rotfl:

Lumstorm

Yes I wouldn't mess with such things when my Philips CRT monitor died last month after 27 years. I took to a local TV repair shop and got them to look at it. A new flyback transformer some diodes and capacitors replaced and it's back in working order with better colour and brightness than it had previously. I guess some parts were dying before it finally popped.

droopsnoot

Lumstorm wrote: »

I took to a local TV repair shop and got them to look at it. A new flyback transformer some diodes and capacitors replaced and it's back in working order with better colour and brightness than it had previously.

Out of interest, roughly what was the cost of the repair?

I like doing stuff myself, but I am already a little apprehensive about getting the back off it. Unfortunately taking it somewhere is a bit tricky as it won't fit in the car, or I don't think it will, at least.

Lumstorm

It cost me £110 in total when they called to to tell me after their initial inspection to diagnose the fault, They said they had good news and bad news, good news they could fix it, bad news the cost wasn't worth it as they could sell me a nice second-hand LCD monitor for £40. I explained that I required an analogue CRT and with the cost of buying another one of unknown quality the repair was worth it to me.

that

if it is a dry joint, and it sound the most likely cause, if you leave it too long it may start to burn the hole, tracks and heat the board so the copper becomes loose. Worse it where it carbonises a section of the board, but does not happen often.

Also sparking on an inductor/coil is not good, as you can get very high pulses.

It is fairly easy to go over all the joints, but some will probably have to have the solder removed, tracked scraped (possibly small needle file or scalpel or exacto knife), and flux and solder reapplied. Slow and methodical with the tv not plugged in is the way to go, I mean not plugged it, do not skimp and just switch it off at the mains.