Dead Sony KV-T25SZ8 TV

Not familiar with the set so I don't know exactly - but functions related to sound may be powered from a rail generated from the EHT transformer.

Even if not, it is possible that it may be muted or disabled by the CPU if the EHT circuit isn't working correctly. Many other functions may be shut off for safety also.

In the past I have found that the majority of sets/monitors (note - have not worked on many newer than the 1990's) will work once the H out transistor is replaced, however this isn't always the case. The tuning caps (usually across the transistor) can fail or lose capacitance causing the voltage to go through the roof (they often look normal), yokes can burn out (Samsung, usually inside on the H section where you can't see it without removing from tube, usually from corrosion), the EHT transformer can short internally, or can develop an arc through the plastic casing to chassis.

Arcing from out of the housing on some transformers that contains the focus and screen pots. I have seen often

Some of these arcs can be particularly bad, in fact some seem to manage to burn their way through silicon sealant and continue the arc as before. (note -too close to the frame to allow a decent thickness)

In some cases, the only way I could stop this was to remove the aluminium frame from around the transformer, and cut away a section of it near the where the arc was so as to give the transformer nothing to arc to. It also allowed more clearance for a thicker coat of silcone sealant to prevent future problems.

I am leaving out things like dry joints and obviously damaged components as I assume most people on this group would look for these before posting.

Or just to make the damned thing difficult to diagnose, thus increasing the chance it'll be tossed and a new one bought.

At one point, I saw some of the smoke escape, but unfortunately was on the wrong side of the set to see where it came from :(

Sylvia.

d e .

Years ago they probably realised that the sets were likely to last x number of years, and by that time, few would bother fixing. Once you start using tiny surface mount parts (including those QFP chips) the odds of people not bothering with a repair increase exponentially along with the cost if they do pay a serviceman to do it. Robot assembly also allows things to be packed tighter than manual assembly ?

At the same time the costs of these sets have plummeted. Can remember around 2000 buying a new 25" set, and was over $1000. Within 3 years or so, they were down around $5-600, and now if you can still buy 25" CRT sets, I would be surprised if they were even $150-$200 in discount stores or similar. I doubt you would get a repair done for a fault like yours for less than this at a TV service shop. We have 2 nice Sharp 26" ones in the Garage, that work fine and wont get used by me. Cant even give them away.

Interesting that that link to a manual that was posted really only shows how to change the tube, yoke chassis, front plastic push button assembly - but no schematic or details on board level repair. Maybe they would just send exchange boards during the warranty period.

One big thing that would bother me, is paying for and replacing the transistor, and then finding the problem still there or other faults. You could change the CPU and god knows what else in between and still have a problem.

And short of going over everything with a magnifier and strong light you have buckleys chance of finding it - even that probably wouldnt work.

Had the same recently with a Eaton Powerware UPS. Smoke came out, the thing still "sort of worked" .

Couldnt find the smoke source but found a number of burnt tracks, shorted out 5404 3A diodes, and a FET that was shorted on all 3 legs, the part was unobtainable from regular sources. TO make matters worse, it looked as though the FET was designed to put a dead short across the batteries under some circumstance. The battery was also practically dead.

Getting another unit for about $110 ended up being the best fix.

And indeed discover that the still existing fault destroys them again.

No fuse protecting the battery? Sounds like a serious safety issue - shorting out lead-acid batteries would have to be good way of starting fires.

Sylvia.

This brings us back to the Sony problem. On many sets you can use standard transistors and other semiconductors, at least for test purposes, so it is usually worth a punt for a few quid to see if it fixes the set. But an expensive chain failure which necessitates the use of genuine parts (a typical sony scenario!) means that these sets are not often worth bothering with. In this case I had no idea of the set's history, for all I know the tube could have been wasted too - another reason not to put serious time and money into such an item.

It is ironic that latterday CRT 'supermarket sets' tend to have a better chance of getting fixed than some of those costing several times as much!

-B

From memory, and I may be wrong - It was not protected by the fuse, only by the blown PCB tracks. (and there was current limiting from there being a relatively thin wire leading from the battery + to this part of the circuit.)

The circuit was not easy to trace out, and parts like this seemed illogical.

The thing that probably killed it was that I was using it to charge up about 5 SLA batteries, one after the other, and it might not have been designed to be charging for this length of time continuously. once the

5th battery was connected - smoke time.

Still, I'd have expected it to have reached thermal equilibrium long before even the first battery was fully charged. It shouldn't get any hotter from subsequent chargings.

Sylvia.

The set's inability to show any signs of working circuits, with the HOT (transistor) removed (or failed), is normal for most TVs made within about the last 25 years.

Other than a Standby voltage supply, most TVs won't operate (even partially) without a functioning horiz scan/sweep circuit, as in general, several voltage supplies are derived from the IHVT integrated high voltage transformer (some refer to as flyback or other terms).

-- Cheers, WB .............

My Sony KV-1920, purchased ca. 1974, was one of the first TVs with a switching power supply. When the supply failed in the early 80s, a guy from Sony warned me not to try to fix it myself. "If you don't catch all the bad parts the first time, some of the replacement parts might fail. It's best to have a trained tech look at it." It cost me around $75 (which seemed a lot, then) but the set was properly repaired and worked another 15 years.

You are probably right, but this was the only change to "normal" usage I can think of before it failed

On a hunch, I checked the garbage AOL email addy that I use to register for public forums, and there (amongst the ton of spam emails) were more than a couple of emails offering the same advice. But I'm not completely convinced he has mood swings or a personality disorder; I suspect he's a typical internet bully who can only demonstrate bravery from behind the security of a keyboard. If someone were to act like that in real life, you go up to them, grab them by the throat, and watch them fill their Pampers. Still, I've often gotten a good laugh from his posts.

Coincidence - there's probably a yet to be discovered perverse law of nature that says that a machine is most likely to fail shortly after a totally irrelevant change in the way it is used.

Sylvia.

Well, that's possible, though as I've indicated, my TV wouldn't even enter standby mode.

I suppose it's conceivable that since it failed in operation, it always tries to power up in that mode, and that the processor won't allow entry to standby mode unless everything's working. That would seem a bit perverse, but who knows?

Sylvia.

Here's another one... "A machine is most likely to irreversibly fail shortly after you've gone to a great deal of trouble to fix it."

These types of failures are the kinds of problems that TV techs have seen regularly for a lot of years.. a relatively simple fault is capable of causing the "smart chassis" type designs to be completely dead, in many cases.

In sets that develop a fault, and also have some other components that have changed values due to age, the chassis essentially has compound faults, which were commonly referred to as Tough Dogs, among other names. Shut-down circuits have been implemented for many years, to prevent the TV from operating at all, if the CRT's HV potential exceeded a limit, as a safety shutdown.

Some minor faults are often complicated by poor soldering during the circuit board manufacturing process/build, which creates new faults as the boards age and go thru many thermal cycles.

Many of the electronic equipment designs presently manufactured are similar designs of smart chassis design, in that one fault will disable every function.. maybe with the exception of a blinking LED. In many of these designs, either everything operates within normal limits, or the units just shut down and won't restart without experienced technical intervention.

-- Cheers, WB .............

The term probably dates back to at least the 30s. It refers to a problem that refuses to yield to common-sense analysis or trouble-shooting.

I've had dialog with Phil that was very helpful and he's even apologized to me (in his own way) once. On other occasions he's tore into me for no reason whatsoever. And it's not just me but others that don't do things the Phil Allison Way. I have no official proof but this behavior smacks of a sever mental disorder.

--
Live Fast Die Young, Leave A Pretty Corpse

"William Sommerwerck" wrote in news:ilp3r9$p3i$ snipped-for-privacy@news.eternal-september.org:

It is called "the innate perversity of inanimate matter". There was an article on it published MANY years ago in CQ magzine.

It have some ways to use ipoim to do things around the ham shack.

1) in order to prevent stretching from detuning your antenna, cut the wire short to allow for stretch. 2) to fix a complex electronic device: a) go through the parts list and contact all the parts suppliers. You will find one or two parts that can not be found anywhere; it is one of those. b) examine the layout and locate the part that is most difficult to remove and replace. You just found your defective component. I don't remember the rest of the article but it has proven to be quite useful to me, over the years. :)

73 de N5bz