TV with short circuit - how do I find the short?

A friend of mine gave me an old 27 inch TV that no longer works. Upon inspection of the insides, I noticed the "5A 125V GMA" fuse was blown. I purchased a 10 pack of these fuses, and when I replaced it the fuse blew as soon as the TV was plugged in again. I am assuming that there is a short circuit on the board somewhere. How would I go about finding the short? Is it possible the power cable or its connection to the circuit board is related to this short?

Thanks in advance.

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Reply to
N Cook

Simple way? Put in a much larger time delay fuse then look for the smoke.

Reply to
Jon Slaughter

Does the set have a 'conventional' mechanical on / off switch as well as a standby switch ? If so, does the fuse blow when this switch is set to off ? If yes, you might have a short circuit noise filtering cap across the switch. If the fuse only blows when the main power switch is on, then it gets a bit more complicated. It could still be a noise filtering cap on the 'back' side of the switch. It could also be a faulty posistor, which was quite common on a lot of sets for violent fuse blowing. However, if it's none of those things, it gets a lot more complicated as, if the set is anything up to say 10 years or so old, it will employ a switch mode power supply. These are dangerous to inexperienced hands, and notoriously fickle with their faults. Violent 'instant' fuse blowing is usually down to a short circuit chopper transistor or hybrid, or sometimes the bridge rectifier, but that is often not the end of the story. There may be assorted damaged resistors, diodes, control IC if it uses one, feedback opto, and the whole lot may have been caused in the first place by a faulty electrolytic in the primary side.

If the fuse does not blow instantly (ie set needs to be taken out of standby) or violently, then the chances are that the fault is downstream of the power supply, most likely in the HOP stage somewhere. However, when this is the case, most well designed switchers will detect the excess load, and shut down to a 'safe' mode where they will squeal, tick or chuff, rather than blow fuses. A bit more info is needed really, before any 'reasoned' diagnoses can be made.

Arfa

Reply to
Arfa Daily

If you have electronics experience, especially around high voltage and potentially lethal equipment, then go to the TV Repair Guide at the Web sites below, especially for the SAFETY information. A blown fuse can have many causes.

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Reply to
Samuel M. Goldwasser

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If you're not familiar with TV circuits I'm afraid that this is going to be a difficult undertaking for you. Whatever you do though, don't try the larger fuse trick. That advice was just plain stupid, and so is the person for giving it. Lenny. .

Reply to
captainvideo462002

Yep, it's likely to be a short, but not a simple thing like a paper clip that fell across the power wires.

Most likely a capacitor or transistor shorted, one of the bigger ones, like the power supply switching-mode transistor or its drivers. When that shorts it can blow the horizontal output transistor, or vice- versa. Plus maybe weakening or blowing out nearby diodes and capacitors.

Worse yet, the real cause of the first transistor blowing is often yet some OTHER part being a bit below par, something not easily diagnosed, as it can be something subtle like a capacitor having a somewhat high ESR. So even if you replace the bad transistors ($$$) the underlying cause is likely to blow them again in a millisecond or maybe a week.

And you can't just replace one bad component at a time, as the other bad ones are likely to blow the new component toute-suite.

To do this right you need a source of gentle line voltage, like a Variac, lots of spare parts, lots of patience.

I'd recommend you make the best of a bad situation and throw the TV off a very tall building. Taking all appropriate safety precautions of course.

Reply to
Ancient_Hacker

Electronics run on smoke. When the smoke gets out they stop running.

Reply to
PanHandler

I don't think you know enough to safely troubleshoot it or repair it.

Reply to
Meat Plow

Hardly. There's probably a rectifier built out of diodes somewhere near where the 120V comes in. Probably one shorted. BE CAREFUL, YOU CAN EASILY KILL YOURSELF by touching the anode on the tube.

Reply to
Anthony Fremont

In message , Anthony Fremont writes

You are more likely to kill yourself messing around with the rectifier diodes, especially if the reservoir capacitor is still charged.

--
Clint Sharp
Reply to
Clint Sharp

This can be as simple as a short circuit diode in the bridge rectifier, or as complicated as any one or more of a number of parts in the SMPSU. In CRT TVs the degauss posistor can also cause fuse blowing.

Note that the 'ground reference' rail in a SMPSU is actually a couple of hundred volts negative and can easily supply lethal current!

One trick you can try instead of blowing more fuses and possibly making the damage worse, is wire a 60W mains voltage bulb across the fuse holder - this will limit the current to a value that avoids further damage and provides an opportunity to measure voltages (or at least see where there aren't any that should be). Note that once you have the bridge rectifier working it might be possible to store a hefty charge on the big electrolytic capacitor - that can give a very nasty bite!

In theory you should use an isolating transformer to power the set while working on it, but the important thing is to *AVOID* touching anything earthed while inadvertently touching any live part!

Reply to
ian field

Please don't take this as an unwarranted criticism, or an ad hominum attack. I want to make some observations.

1) Your description of the TV circuitry as "the insides" suggests a lack of electrical experience. Hence, you may not be familiar with either the look of voltage storage components, or the concept of a metal chassis (exposed metal superstructure) that is not the same potential as the ground you are standing or leaning on. 2) Your reference to the blown fuse as "the '5A 25oV GMA' fuse" suggests a lack of knowledge of which circuit the fuse is connected to. Is it the input power, or is it some other section of the television circuitry? Television circuitry tends to be very complicated, and dangerous voltages exist at many places inside the box. Have you ever discharged a CRT's anode? Would you know how to do so, and would you know how to prevent its recharging while working on the set? 3) Your question, "How would I go about finding the short?" is strong further evidence in favor of the conjecture that you lack experience with electrical circuits.

Those observations made, I suggest you rid yourself of this device. Should I be wrong, and you can convince myself and others that you will not maim yourself in the pursuit of this adventure, you may get some good troubleshooting answers. Given that, after three days, you haven't responded to "Arfa Daily" about his simple question on the nature of the on/off switch (which is an excellent first step, imo), I'm going to solidify my proposition that you know next to nothing about electric circuits.

Donate the thing to an appliance recycler. That's your best bet.

Reply to
Charlie Siegrist

Thanks for the advice, but I think I have made an informed decision to cease work on this TV as too many people have told me on the group and in real life that I am more than likely to end up touching something that could kill me. The set does not have a mechanical on off switch, but the fuse blows as soon as the TV is plugged in, before the standby switch is pressed. As I said, I am most likely discontinuing work on this TV. I found a similar one in working condition for 25 dollars on craigslist. Thanks again.

Reply to
Steve Ashman

In all the responses to this posting, the one thing I believe no one has mentioned is the need to obtain a schematic and servicing information for the TV. Without the schematic, you are flying blind. Some of us could make a start at trouble shooting a shorted power supply by a visual check of the components on the various circuit boards and making resistance checks on the obvious power rails. Sometimes wisdom comes from experience and the wisdom helps to identify dangers. But there is usually no substitute for a schematic if the damage is not visible.

Good luck and stay safe.

Ken Fowler, KO6NO

Reply to
Ken Fowler

Or just use foil. He he he.

Reply to
Dave

These days the turnover of new designs is so rapid for TVs & monitors etc that each new model is only in production for a few months before the next one comes along, by the time it wears out the manufacturer might as well have forgotten it ever existed! Very few manufacturers even bother to reply to requests for service information, the few that do will refer you to distributors - most of which only deal with trade.

The OP said he found something similar in GWO for $25 - unless you know where to look a schematic can easily cost that much and may even turn out to be a very fuzzy scan of a creased and stained original.

Reply to
ian field

Although this is fundamentally true for a lot of equipment, unless you have a detailed understanding of switchmode power supplies, the schematics will be of little use to you. There are many techniques used nowadays, for power saving and power factor correction, which really muddy the waters around the front end of a switcher. Even the way that the primary side high voltage rail is derived, may be at best not obvious, and often just downright confusing, if a PFC IC is used, and the supply is a 'universal' type with automatic input voltage sensing. Locating a suitable 'ground' point on the primary side can be a very dangerous undertaking, and often, the schematics can be misleading in this respect.

Given this particular poster's declared skill level, a schematic might have helped to identify if there was a noise filtering cap at any appropriate point just inboard of the power lead, but beyond that, I think that he would probably have been on a loser with this particular repair, so posession of service info would have been irrelevant.

But yes, in general, I would agree that if you have the skills to be working on a particular piece of equipment, schematics are certainly desirable.

Arfa

Reply to
Arfa Daily

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