Hello, I'm wondering if there are any other TV engineers here. I've got a very puzzling fault which has stumped a few of us and I thought I'd put it on here to see if anyone can work it out, kind of like a test case, a bit of fun.

So, I'll post the info on it if anyone's interested in the challenge. Should it be fixed before anyone gets it right, then I'd put the answer on here of course.

All the best.

 

what kind of fault is it

 

This fault has us all baffled. So far we have not been able to affect a repair on this 28" television set.

I'll start with the basic fault symptoms:

When no signal is applied to the set via RF or SCART, there is a highish-pitch fuzzy sound from the LOPT at regular intervals every few seconds. Coinciding with the noise each time, a white rough-edged band flashes across the screen for a split second, or the screen dims slightly for a split second. If a signal is applied, the noise and raster disturbance seems to disappear or, at least, become virtually unnoticeable. The picture is excellent and the geometry is perfect. There are slight, faintish, spaced lines extending to the left of individual letters on the screen, but nothing terrible.

Souds like something which you could put up with unless you were a perfectionist who wanted to know why the LOPT was pulsing every few seconds, and put an end to it, but in this case, there is more to the trouble than first meets the eye.

One of the first things I noticed with this set was that at switch off, there seemed to be excessive EHT crackle all around the tube, even the speakers by the side of the tube and the case seemed to be crackling. Thorough cleaning of the anode cap and bare glass area where it attaches made no difference. No arcing or corona apparent. The dag coating is reading to 0V as it should, and aquadag is pressed hard against the coating.

Again, this might not seem so bad, and some would say perhaps normal. Although after hearing it, you might disagree. After switch off (with or without leaving the mains plug plugged in) there was also some arcing apparent. I switched off the lights and saw the blue sparks jumping between two points on the small, separate mains PCB. This arcing doesn't occur when the switch on the unit (on another separate PCB at the front of the set where all the controls are located) is left on and the socket is turned off instead. The reason is that the two points that are arcing are joined together by the switch when it is pushed in. So, you have charge coming from the mainboard down to the small mains PCB, and if it can't pass through the switch junction where it is taken via leads from the mains PCB to the front panel and back because the switched has been turned off, it arcs across from one lead's solder point to the other solder point which lgoes to the mains plug lead. Why it wants to go there even when the mains plug is removed from the socket on the wall I don't know, as it has no further it can go (?).

Also, all the metal 0V points on the chassis, like tuner can, heatsinks, etc., have the same kind of potential on them. Touch them and you get a nasty jolt which makes your arm flick back a bit. The dag coating therefore also has that same charge on it. And the funny thing is, that you can take a screwdriver, holding onto the well-insulated handle, and hold it near the mains plug pins after unplugging the set and it will arc little yellow sparks to the screwdriver, yet the screwdriver is not going to anything and the handle is insulated (I feel nothing at the time). The same can be done with the dag coating, tuner can, etc., although the arcing doesn't occur so much, it's more sparking when the screwdriver touches the metal of these parts.

So there is charge on the mains PCB and charge on the 0V part of the secondary side of the main circuit. Thinking EHT was leaking and causing these strange occurrences, I stuck a screwdriver connected to the 0V aquadag under the anode cap to discharge the tube and got nothing. That's unusual in my opinion, but there is only nothing there after around 30 seconds after switching off the set and the crackling and arcing has stopped. Stick the driver under the cap 10 seconds after switch-off and there's still an appreciable crack.

With the driver on the 0V aquadag, there's crackling when it's passed over the bare glass of the tube, the EHT lead and anode cap, the degaussing coils around the tube, etc. A new EHT lead has been tried with no difference noted.

Another point to make is that when the arcing is occurring on the mains PCB, each spark which jumps across coincides with the loosest degaussing coil wire around the tube suddenly being pulled towards the tube. At first when the sparking is shortly-spaced, the degaussing coil is doing a merry dance back and forth. It does this even when the degaussing coil was disconnected from circuit.

So, where is this potential coming from? What's behind the strange symptoms? If EHT is somehow leaking around, how is it happening and why is potential present on 0V points in that case?

Why is the arcing occurring and why is it jumping to a non-grounded screwdriver at the plug end and from secondary-side 0V points? How is this potential arcing on the mains board getting there, since the primary and secondary circuits are electrically isolated, and the mains board is attached to the primary, yet secondary 0V points are charged too?

I've had shocks from the bare metal cases of non-earthed equipment like satellite receivers when I've been earthed myself (bare feet on marble floor) but not when insulated, such as when touching this set's tuner can (carpet plus thick rubber soled shoes) and when using a screwdriver attached to nothing.

And what's causing the LOPT to pulse when no signal is present, but not when a picture is present. Charge still builds up everywhere regardless. Is the AGC amplifying the noise from the problem when there is no signal and the gain is set to highest, therefore showing the problem on the screen and causing the LOPT to show it with the pulsing noises, so the problem is always there, but only noticeable when there's no signal?

So, there you have it. See what you make of it. All opinions and ideas looked forward to.

Good luck!

 

This should be faulty lopt, but before changing it check to see if the HTdc feed to the line output stage is correct,if it checks out ok go on with replacing the lopt, if not use a load on the HTdc rail of the line output and check ps stage,

 

Ok, I'll have a go...

Maybe there is a decoupling cap going to the hot chassis that has nearly failed, especially somewhere in the LOPT section, or PSU.

Perhaps even a cap with one of those HV breakdown slots cut into it has some dust in it, allowing a premature arc-over.

Sounds like something is loading up the LOPT, arcing, then getting back to normal.

Has this set seen a lightning strike? All sorts of funky things can happen then... Like hard to spot carbon tracking that is conducting at HV.

I had one set that took a nearby lightning hit, had carbon tracking that would arc over when the HV rose on high contrast picture, shutting the set down. Took a lot of "in the dark" viewing to spot the arc.

 

A couple of simple points!.

1) Fill your location in, we've no idea what country you might be in!, which is VERY relevent with regards to TV's.

2) You've not mentioned the make or model of the set, which isn't very helpful?.

One problem I have seen (only ever once) was with a Sony tube, EHT was leaking out from the side of the glass!, actually at the seal between the front glass and the main 'bulb'. Sony had the tube sent back to the factory for examination.

Another set I had once, a friends Panasonic, was doing really 'strange' things - and connecting my scope to it (which luckily isn't earthed) produced even 'stranger' results. The eventual fault was traced to a small piece of metallic foil (off of an artificial Christmas tree) which was shorting across the isolation barrier in the PSU.

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Hello again. Okay, good points. Don't know how I missed adding the make and model!

1) United Kingdom of grey clouds and cold.

2) Sharp DV-6635H

I take it the Sony was airtight but not sealed enough to stop EHT escaping?

This set crackles nicely around the edge of the tube when switched off, even around the speakers next to the tube, but nothing is visibly leaking there.

Nothing across the isolation barrier.

HT is 149V, only 1V out from circuit diagram stated amount of 150V. I use a 60W light bulb on the HT line as a dummy load. Not sure what is meant by ps though?

I cannot know if this set has seen a bit of lightning, unfortunately. I do know it has previously suffered from a listed fault and been repaired - dry joints on deflection coils connector causing two resistors to fail (cook) in the line stage. All rectified, though. I've checked to make sure.

 

Yes, the picture was fine - but with a screwdriver connected to ground you could pull long sparks from the side of the tube :lol:

Even worse it was 'fizzing' across to the metal speaker grill!.

I'm very familiar with these sets (in fact I have the 25 inch version, DV5937, at home) - dry joints on the scan coils (back of the connector on the coils themselves) cause the LOPT transistor to fail. The burn-up is caused by a dry joint on C613, there are also a number of other faults which occur round that part of the set.

However, I've never seen or heard of any problems like yours :cry:

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No one seems to have heard of such a fault, even my father who has been in the trade most of his life, and is now retired.

We've wondered about LOPT failure being the culprit, but my inquisitiveness means I'd like to find out exactly how it has failed if that's the case. The internal diode is one thing which was raised as a possibility.

The fact that the charge still builds up on the 0V points when a picture is present, but the noise and screen disturbance disappear, has everyone so far convinced that the AGC is simply making the fault more apparent when no signal is present. Question is, is the EHT higher than it should be for some reason, even though the HT is normal... And even so, would excessive EHT cause 0V points to be charged.

The isolating components between the primary and secondary did occur to me as being possible culprits, but they check out fine. There must be somewhere from which the high potential is coming, and a reason it is surviving on 0V areas of the secondary, and the points where the mains goes into the set. One idea is that the potential is being produced in the secondary only, and that is it trying to find passage to earth and by doing so, making it's way to the primary side and hence the mains board, or the charge on the secondary is inducing charge in the primary which is flicking from the mains plug in an attempt to get to earth.

I bet the answer will turn out to be simple, if it doesn't find its way to the skip before then...

 

One thought? - what about the components across the isolation barrier?. There should be a special resistor and capacitor across the barrier, to prevent static build-up on the chassis - if these were missing or faulty, it could well build-up and then flash over.

There's a very simple test, the chassis of the TV is isolated from the mains, so connect an earth lead to it and see what difference that makes!.

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Oops! - double post!.

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The isolating components between the primary and secondary consist of two resistors and a capacitor. The resistors are both 8.2 MOhm and the capacitor is on the circuit diagram as 3300P (pF?) and 4KV.

Don't know whether static would build to such high levels in seconds as it does (it flashes if switched on, left for 5 seconds, then switched off), but I do know that the resistors both measured 8.2MOhm on the meter, although the capacitor hasn't been checked on a capacitance meter. Without asking someone, I don't know how the capacitor helps in stopping static build-up myself, and no one I know to have the answer is online at the minute - I live in an isolated place...

I'd hoped the resistors would read open when I measured them a couple of days ago, but no such luck.

Regarding the test, do you mean the earth of the mains? i.e. I take a wire from a plug socket earth or radiator pipe and clip it to the chassis (tuner can, etc)?

 

Yes! - if you're not feeling that brave, you could connect it via a resistor - but a piece of wire will be fine. Bear in mind many aerial systems are earthed anyway (I believe it's a legal requirement in Germany?), and this would earth the chassis (on an isolated chassis set like this one!).

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Stoopid question... have you checked for a cracked neck? The tube could retain vacuum and yet a defect in the glass could allow for some leakage.

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One of the first things I noticed with this set was that at switch off, there seemed to be excessive EHT crackle all around the tube, even the speakers by the side of the tube and the case seemed to be crackling. Thorough cleaning of the anode cap and bare glass area where it attaches made no difference. No arcing or corona apparent. The dag coating is reading to 0V as it should, and aquadag is pressed hard against the coating.
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maybe the tube has gone down with or without the lopt
in that case your moving away from the real problems and looking
for remedies in areas of no substance

for the flashes do a resoldering on the lopt and hout stage

then test tube and lopt

mains leakeage could be coming from a leakeage of the ps power transformer too

do mains leakage test by connecting a 1.5k resistor from chassis to earth ground and measure the voltage then find leakage current vm/1.5k

the 8.5M static discharge resistors are they in series or in parallel
most tvs have a value of about 10M