TV ON switch won't latch

I have a CRT type SONY 27 inch TV Model KV27T530. It's between 10 to

15 years old and still shows a great sharp picture in excellent color. I have Cable TV for the basic channels (2 to 13) plus the first tier of extra channels. A month or so ago it could show only the first tier channels and would not even display the Channel number for the second tier. So I took off the back cover to reseat all the connectors. After that the ON switch wouldn't latch. It comes on and switches itself off after a few seconds. What should I be looking for to fix it?

I used to be an electronics tech and had fixed minor TV problems before. This TV set isn't worth fixing at a shop. I am sure its basically working OK and I don't like dumping equipment that can be fixed. I'd be happy if it can just receive the first tier channels. I've bought a large flat panel TV to watch the coming Olympics. So fixing the SONY is something I can take my time on to troubleshoot.

Thanks in advance.

Reply to
PaPaPeng
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So are you saying that there is a mechanical problem with the switch? If so, just replace the switch.

Or are you saying that the TV shuts down after a few seconds?

Mike

Reply to
Michael Kennedy

Does this not suggest that in dismantling the set, you moved something, or put something back the wrong way?

Reply to
William Sommerwerck

Look for whatever you messed with when you reseated all the connectors.

Reply to
Meat Plow

Its not the front panel mechanical switch. I can switch on the TV with the front panel switch or the remote. There is a distinct click coming on. Then there is a slight crackle as the static or whatever fires up (to warm up the CRT?). After around five seconds the TV clicks itself off (as if there is a safety cutoff.) I looked around for a solenoid like item and of course there is none. It is something solid state on another part of the motherboard. The cable runs are so tight I cannot pull the MB out far enough to see or hear exactly which component is making that noise. If it is solid state why is there a distinct 'click' then. The question here is what kind of component makes a click when you use the remote to turn the TV ON or OFF?

As for mis-reconnecting the cable all the cables have keyed connectors and each of them is unique in size and shape. The cable runs are too tight to make a mistake running the connector to another pin socket. When I get around to it I will probably undo all the connectors and reseat them again.

The other question is, is there such a thing as a hardware problem that would kill the second tier of cable TV stations? The screen wouldn't display any channel numbers or programs beyond Ch 13. Just a black screen and not even the "snow" of a dead station. This condition lasted for a week. I thought I'd be clever and reseat the connectors to see if that will fix it. The old 27 inch CRT type TVs are hernia boxes. I am no longer that young where I can move this thing around easily to fix things.

Reply to
PaPaPeng

  1. you may have, whilst moving things, exacerbated a bad solder joint. you'll need to pull out the pcb, taking photos on a digital camera along the way, so as not to have trouble later, to get to the botom of this.
  2. re.cable channels - check tuning settings, do a channel search and ensure they are enabled in the menu ('air' or 'cable' option set right) maybe the tuner is duff, but they don't often fail this way.

-B

Reply to
b

A relay.

Make sure all are connected properly.

The tuner is responsible for that.

Reply to
Meat Plow

There isn't one. That would have been the first component to suspect.

Reply to
PaPaPeng

Well then you're hearing something snap, possible in the high voltage.

Reply to
Meat Plow

There are a lot of sets with various relays. It's certainly not out of reason to hear a click. Some don't click, but at least one of the ones I have does.

jak

Reply to
jakdedert

The OP claims there are no relays. I have my doubts.

Reply to
Meat Plow

Okay. Let us assume there is a relay. It clicks ON and it Clicks OFF. This would indicate that the relay works. The question then is which part of the circuitry causes it to click ON or OFF and why? How does such a circuit work? It must be connected to the front panel switch and the front panel IR remote singal detector and pass through some circuitry before it connects to the relay.

The next question is a relay can only close a contact, that is complete a conductor path, presumably to turn ON a circuit which should then latch ON. That ON status lasts 5 seconds, a lifetime in electronics. Then the relay clicks to OFF, an unlatch action. At this point I can't figure out what is happening and why. Is there such a thing as an undervoltage or overvoltage protection circuitry that turns off the TV if the voltage (or some other condition) is not met?

Everything in on a 12 inches by 14 inches motherboard. Thus there is a high degree of IC integration that does not lend itself to component repairs. The TV set worked great for something like 15 years. The last time > 5 years ago,there was an intermittent yet persistent problem, was the spreading of scan lines on the top of the screen. After many tries over a period of a few years I eventually got lucky when a tool accidentally brushed a cable and I was able to reproduce the problem. This allowed me to locate a practically invisible separation of a pin donut on the MB. Reflowed that and the TV worked even better then before. SONY PCBs do have a reputation for cold solder separations with age.

Now back to the original problem of loss of second tier cable channels that is blamed on the channel tuner. The TV set has an electronic tuner. It doesn't have a bank of mechanical contacts to fail selectively as in missing certain channels let alone a block of channels. Is there such a thing as an electronic tuner that has circuitry for blocks of channels, first tier, second tier, third tier and so on. I would think access to those tier blocks are controlled by the cable company. If the cable company cut off the second tier channels, I should at least still have the channel number displayed on the screen and maybe some "snow" not a totally blank black screen. I don't have a conceptual tehcnical explanation for that either.

Reply to
PaPaPeng

did you try the menu idea I mentioned before? what happened? B.

Reply to
b

Sorry, missed that. I have only a generic remote without that option. My house was broken into and the crooks took my stereo, etc. consoles the TV being too big to carry.

I forgot about the controls hidden behind the front panel. But now that I can't even get the TV set to fire up I won't be able to check out the menu options.

Reply to
PaPaPeng

As I remember, there is a high-voltage shutdown in those sets that probably takes about 5 seconds from turn-on to turn off. If the OP hears or feels a static sound just before the shutoff click, it is probably an overvoltage problem.

It could also be a failure of the horizontal stage to turn on, the initial power to the relay depends on the horizontal output stage to fire up, and one of the turns on the transformer is connected to a rectifier which then provides a holding voltage to the power relay to keep it turned on. Without an oscilloscope, it is impossible to be sure which of these scenarios is the case.

If the OP can feel the hairs on his arm react when the set is first turned on and his arm is near the face of the CRT, there is probably high voilatage being developed and it is an overvoltage shutdown. If he doesn't feel any static build-up, then it is probably lack of power to the holding relay that is the problem.

Bob Hofmann

Reply to
hrhofmann

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