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Computernaut

[Help] Audio amplifier from old CRT

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Posted · Original PosterOP

I have this old CRT which no longer works well, the picture cuts out for a few seconds periodically. Anyway, I wanted to use it as an amplifier for some old speakers I had in storage. I gutted the TV and got out all the circuitry -- a large main PCB, a smaller daughter-board for the rear I/O, and a little board for the front buttons. I disconnected all the things connecting the CRT proper to the main board; they all had convenient plugs. All except two that is; these where connected to the electron guns, I had to cut them, there's a red one and a white one, the red one has very thick insulation given the gauge of the actual wire. These seem to be the main power connectors for the guns, they are at a very high voltage from what  I can tell.

When I switched on the circuitry once it was out of the TV the end of the red wire emitted a blue glow, and there was a constant bolt of lightning between one of the traces on the board and something under the large transformer the red wire is connected to. This seems to be due to the fact that the CRT is disconnected. I hooked the thing up to the previously mentioned speakers and a phone, turned it on, and tried to play some music. It didn't work: Those sparks were a bit sketchy anyway. So I have two options, either take the transformer off the board completely or get a beefy resistor and attach it to the red and white wires. What should I do?

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14 hours ago, Computernaut said:

the red one has very thick insulation given the gauge of the actual wire.

 

14 hours ago, Computernaut said:

red wire emitted a blue glow,

That's the wire carrying the 25000V EHT for the CRT anode. The blue glow is corona discharge. After a while you might start to smell Ozone as well.

 

14 hours ago, Computernaut said:

there was a constant bolt of lightning between one of the traces on the board and something under the large transformer the red wire is connected to.

With the CRT disconnected and thus no load, voltages inside the flyback transformer will greatly exceed their nominal working voltages, possibly causing such arcovers.

This seems to be a very dodgy television set as I'd expect any decent set to prevent the flyback transformer running at all with the CRT completely disconnected. A common technique used back in the day was to to include a jumper wire into the connector for the deflection coils on the CRT so that unplugging the deflection coils automatically turned off the entire EHT/flyback section. Your TV does not seem to have any of that - what a deathtrap!

 

14 hours ago, Computernaut said:

I hooked the thing up to the previously mentioned speakers and a phone, turned it on, and tried to play some music. It didn't work

It's hard to tell why it does not work - it could be many reasons - From the audio amplifier being powered by the flyback section (which is obviously not working right without the CRT, as the sparks imply) to the TV having auto-mute if there is no video signal.

It does not make sense to run a whole television set, now turned death-trap, just to have a audio amplifier.

If you want to play around with the electronics it would make more sense to find the audio amplifier section on the board, if the TV is not ancient it should be a single chip amplifier, extract the relevant parts and use them to construct your own amplifier on a piece of perfboard.

 

If you have no clue how to do any of the above I'd suggest you stop playing around with lethal electronic equipment until you're much more knowledgeable and just buy a cheap amplifier for now.

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Take some nice clear pictures of the circuit boards and maybe we can help you out.  it may be possible to disable the crt portion of the circuit board and leave only the audio part functional and therefore relatively safe circuit board to use.

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Posted · Original PosterOP
7 hours ago, mariushm said:

Take some nice clear pictures of the circuit boards and maybe we can help you out.  it may be possible to disable the crt portion of the circuit board and leave only the audio part functional and therefore relatively safe circuit board to use.

Here are some pictures of the board, I have the area where the spark jumps indicated with a red circle/arrow.

IMG_3412.JPG

IMG_3413.JPG

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+1 Unimportant said

 Appears you're using video circuits, the amplifers/step up transformers that engage the CRT's high voltage deflectors, as a audio amp and yet its dangerous.  This might lead you on a better path http://www.circuitbasics.com/build-a-great-sounding-audio-amplifier-with-bass-boost-from-the-lm386/  and safety stuff nice to know https://www.ccohs.ca/oshanswers/safety_haz/electrical.html

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