Could someone help me to fix an old TV ?

October 11, 2008 10:52 AM

I've just inherited an old Hitachi crt TV (15 years old). Since I've turned it on, it has never ceased to make an annoying humming. I thought it came through the speakers, and then realized that the noise was the same even if the sound was turned off. The noise is distinctively louder when the image is bright, and softer when the image is dark. I gather there's a problem with the tube energy supply, but after this point, there could be lions, dragons or monsters behind the tube, it's terra incognita. So could someone provide me with an explanation and some easy instructions to solve the problem ? Thanks !
posted by nicolin to technology (13 comments total)

You really don't want to mess around with the innards of a tv. The capacitor contains a deadly amount of energy, even when turned off and unplugged.
posted by stavrogin at 11:01 AM on October 11, 2008 [1 favorite]

Seconding this -- take it to an experienced repair shop. Both the voltage and current inside a CRT TV are humungous, when it is plugged in. As stavrogin said, the capacitor stores a deadly amount of charge, even when the power is off. You can't touch anything -- not even with insulated gloves -- without putting your life at risk. If it isn't worth spending money on, forget it.
But to answer your question about the probably cause, the humming is probably either one of the color "guns" breaking down (a high-energy valve) or the CRT tube itself breaking down (this is probably worth more than the TV now, even if you can find a replacement). If the TV is 15 years old, it is not going to be cheap to fix. Which is probably why you were given it.
posted by Susurration at 11:35 AM on October 11, 2008

If the hum is 50 hz (I see you're in France), then it's the main power supply transformer.

Big transformers are, in simple terms, two coils plus metal filler. The metal is necessary in order for the transformer to be efficient. The big metal section is made up of small plates which are interleaved (technically known as "E's" and "I's") and in the old days they were held together by shellac or some sort of glue.

Every AC cycle, the metal flexes a bit. 50 times per second. Over a period of years, the shellac eventually weakens, and then the transformer begins to make noise as it flexes.

The only way to make it stop is to replace the transformer, but finding a new one with the proper characteristics could be tricky. It might be easier to try to find a different monitor.

There's a different failure mode involved, but big electrolytic capacitors can begin to sing, too.
posted by Class Goat at 11:37 AM on October 11, 2008 [1 favorite]

So could someone provide me with an explanation and some easy instructions to solve the problem

(1) Unplug tv.
(2) Throw in garbage.
(3a) Spend X Euro on a new but small 16:9 tv with a nice picture instead of spending X Euro to return that ancient Hitachi to a moderately bad old-and-busted 4:3 picture.
(3b) Spend X/4 Euro on a different used tv that works instead of spending X Euro fixing that one.
posted by ROU_Xenophobe at 11:43 AM on October 11, 2008 [4 favorites]

Yea, don't open it. A friend decided to explore the innards of a tv as a teenager and found himself on the opposite side of his bedroom before he knew what happened. Fortunately this was before I met him so I didn't make the same mistake :)

Like Class Goat said, if it's low pitch, it's the AC. If it's really high pitched, it's the flyback transformer. Either way, it's irrelevant. Get a new(er) TV.
posted by jdfan at 11:59 AM on October 11, 2008

Very probably the flyback transformer (as jdfan says):

In television sets, this high frequency is about 15 kilohertz (15,734Hz for NTSC), and vibrations from the related circuitry can often be heard as a high-pitched whine. In modern computer displays the frequency can vary over a wide range, from about 30 kHz to 150 kHz.

If it is it's incurable, but you are somewhat unusual if you are out of your teenage years and can still hear it.

Asthmatics can hear such frequencies as adults much more often than contols.
posted by jamjam at 12:25 PM on October 11, 2008 [1 favorite]

The OP described the sound as a "hum" and that's not a sound that a flyback transformer makes. A failing flyback transformer makes a high-pitched squeal.
posted by Class Goat at 1:44 PM on October 11, 2008

Your observation that the set makes different noise levels depending on the brightness level of the CRT reduces the likelihood that it's a low voltage power supply problem (which is what the transformer hum is, technically).

Refresh rates on the CRT are 30 frames per second, and the video content of the pix, coupled with its intensity can generate frequency artifacts within the audio bandpass of the audio section.

The audio in most of these sets is fixed gain with a variable input signal level. That means that if noise gets into the audio circuit AFTER the volume control, it is amplified at max level.

I suspect that is what is happening. I don't think it's power supply caps, or transformer hum... I think it's video getting into your audio. Verifying this is best done with a video signal generator, if you can borrow one. There are a number of patterns you could experiment with that would show if the noise is content-related, and if it is, I am right.

How to fix? Internal cable rearrangment, adding filter caps to specific points in the audio circuit, and oddly enough, fine tuning adjustments. I have seen slightly misadjusted tuning cause video leakage into the audio. I don't know your set, so I can't say if it's synthesized tuning, varactor tuning, or what. If it's fine tuning, you may be able to improve it by changing from channel 3 to 4 if you are watching it on cable. You can also test this hypothesis by seeing if it is channel specific, if you have access to broadcast signals other than cable.

I agree, it's probably not a good idea to do this without some knowledge of what parts will get you killed. There are several. Do not work on it plugged in.

Do some of the tests above and drop me an email (in profile) and I'll try to help you assess whether it's worth pursuing. These days, TVs are free, almost, so not many are worth fixing.
posted by FauxScot at 7:16 PM on October 11, 2008 [1 favorite]

(2) Throw in garbage hazardous waste recycling channel.

(CRTs have lots of lead and other nasty stuff inside. If throwing them in the garbage isn't illegal where you live, it should be.)
posted by finite at 10:41 PM on October 11, 2008 [1 favorite]

The most dangerous component in a monitor to the uninitiated is the CRT itself. Among its other characteristics, it's a huge Leyden Jar, and it's possible for a CRT to hold a lethal charge for up to a week after the last time it was powered up.

Do not screw with the inside of a monitor on your own unless you're a trained electronics technician. We don't need any funeral announcements, OK?
posted by Class Goat at 11:56 AM on October 12, 2008 [1 favorite]

I think that I get the picture. Thanks to all (it's nice to see that many people concerned about my health). That TV set is going right to the waste deposit centre.
posted by nicolin at 1:15 PM on October 13, 2008

OK, so I finally asked a technician to have a look. It appeared that there was no problem with the TV set for hours, and then the noise resumed. And here's the culprit. Apparently, it began to vibrate after a while and emitted that noise. 50 $ later, the TV is back, and the technician said it probably gonna last for a few more years.
Thanks for your advices, I didn't attempt to open the monster and the technician repaired it quite easily.
posted by nicolin at 3:19 AM on October 25, 2008 [1 favorite]

That part number is described as a 'line filter'. It's not the big transformer you were talking about, Class Goat, but your diagnosis of the problem appears to have been correct in all essential details.
posted by jamjam at 11:09 AM on October 25, 2008 [1 favorite]