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TV/VCR/Stereo Troubleshooting/Zenith TV won't power up when plugged in


QUESTION: Zenith tube TV 1994 23" screen
Model: SMS1917SG
1.2A 87 Watts

Normally, I keep this TV unplugged. It has light use about 1-2 times a week. When you plug it in, it normally makes a noise like it's powering up or sucking in electricity to initialize something, I presume some kind of power supply. It usually does it within 2 seconds of plugging it in.

Now, I plugged it in over a week ago and no power-up noise. Pressing the Power On button on the TV (or the remote) wouldn't turn it on. Yet, after being plugged in for 5 minutes, it finally makes the noise like it's powered up and ready to turn on. Now it turns on, but while I can adjust the channel numbers (I have a digital converter box to receive over-the-air broadcasts for this analog TV by the way) I still can't make the volume go up or down. So I turn it off, unplug it, wait a few minutes, then plug it in (about the only thing the manual suggests that was useful). This time, it powers up and I press the power button, it's on and I use it like normal. (I've treated it almost like a computer as if something in its memory was corrupted.)

Next week, I go to plug it in. No power-up noise. I wait 5 minutes, thinking it will behave like last week. No such luck. I unplug, try different receptacles (I try other machines, receptacles are working fine.) I never can get it to suck in the initial power so I can turn it on. It's now dead.

My question: Is this some kind of power supply unit that has died and can be replaced because the TV is otherwise still like new and I can't understand how it died with such light use and never being left plugged in so there's no chance of lightning or power surge damage. (It is not normally plugged into a surge protector when I do turn it on, just directly into wall outlet.)

Would be very appreciative if you could lead me to how to fix it (i.e., what part and where to look for it). (I have some experience with electronics, replacing computer motherboard lithium batteries, A/C contactors, etc., but never opening a TV.)


ANSWER: The set is getting old and has probably reached its end of life stage.  This is when the electronic components have lost their original material stage and are beginning to fail.  Some fail more rapidly than others.  Those that usually fail first are electrolytic capacitors.

Actually, the electronic parts last longer if voltages are applied periodically and not left unused.  

In your case, based on my understanding of the symptoms you explained I suspect that the electrolytic caps in the power supply are going sour and are causing big drag on the dc voltages and preventing it from turning on properly.  There are usually two or three big electrolytics in the power supply. Often they are bulging or even some electrolyte leaking out of the can.  You can check visually for these deformities.  If so, you should replace them.

Can't say that replacing the lytics will solve the problem as there may be other failures that go along with aging in old sets like this.  

Best wishes.

[an error occurred while processing this directive]---------- FOLLOW-UP ----------

QUESTION: I opened up this Zenith TV and carefully inspected the entire circuit board. From where the power cord is soldered to the board, there's a black electrolytic capacitor 200V 330UF with the diameter of a nickel 1 1/2" tall. Next to it is a much smaller 200V capacitor with the diameter of a dime. Before this, there's also a T4.0A 250V fuse. The capacitors have no physical signs of leakage, bulging, and look brand new. There's no sign of burning or heat buildup. The underside of the circuit board also shows no signs of leakage or loose solder contacts. The fuse filament looked fine visually but my 2.0A max voltmeter told me I shouldn't try testing a 4.0A fuse.

So visually, a diagnosis of physical leakage or bulging seems to be ruled out. (I suppose there could be electrical leakage.) Unless the fuse has gone bad, I'm at a loss now. I did inspect the power cord and there aren't any bends, etc. that should be causing a problem.

This analog TV is hooked up to my Atari 2600 which requires an analog 525-line TV so I'm really wanting to fix it.

Please let me know what else I can check.


You are good!

Since you have a voltmeter next thing to look for is full voltage at those lytics you were looking at.  They should have somewhere around 150v plus or minus on them.  If the voltage is less than 100v it is likely that one or both of the lytics are bad or that something else is dragging down the dc voltage.  

You should also try wiggling and jiggling some of the components or board mountings to see if there is a bad connection or poor ohmic ground or something like that.

High voltage power supply failures and horizontal deflection amplifier shorted output transistor are famous for causing shut downs like this as well.

By the way, don't know your location but I was at a nearby thrift store the other day and they had several good working tvs for under $20.  Two of them were 25 - 27 inch sets.  Sometimes that is a 'cheap ' way to go.  Something to think about.

Best wishes.  

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