Background: I have a portable TV which I want to add a headphone socket to.
Plan: Open TV. Find wires (probably just 2, I doubt it a 14", £80 TV has stereo) Add chassis mounted switch to the live (?) wire. Add chassis mounted headphone socket to the casing Wire the socket to the other 'side' of the switch
This will (I hope) leave me with a TV which I can switch between headphone and speaker modes.
thoughts anyone? M.
Be VERY careful! Lethal voltages can be present inside a TV hours after it is turned off. Not a job for the novice.
The headphone socket will incorporate a switch. There are 4 terminals on it.
You break the speaker cable at a convenient point and reconnect them, passing through the appropriate terminals (you may need to install additional cable if there is insufficient slack to position the socket where you want it. The terminals are -ve and +ve feed from the amplifier, and the switched -ve and +ve outputs to the speaker. The internal switch disconnects the speaker automatically when a plug is inserted.
Christian.
Hi Michael,
Buy one of these :
This is really a simple enough job but as others have said in their replies " PLEASE BE CAREFUL OF THE HIGH VOLTAGES INSIDE THE TV " and be careful not to touch any of the capacitors on the PCB or you'll get a shock that will badly burn on both the entry and exit points of your body.
Good luck with it.
*** BEWARE CHARGE IS STORED IN A TV FOR A LONG TIME AFTER IT IS SWITCHED OFF, STAY AWAY FROM THE BACK OF THE TV TUBE ***
I did this to my cheap Matsui TV 15 years ago. No need for a switch the headphone socket cuts the sound to the speaker as it has 4 terminals. My TV died 5 years ago, sound played up just before it totally packed in, don't think it was connected with my mod.
Chris
Is this just for your personal tv? If so you are probably ok with the advice from the other posts. But I do remember the technician at my university language lab telling me that when he did the tv's there, he had to install isolating transformers between the headphone socket and the speaker wires (I guess due to the possibility of live chassis).
Plus you will need a resistor to make the volume acceptable in your headphones, otherwise it will be too loud.
...or for bonus points add a headphone volume control too.
In message , Tim Mitchell writes
How long ago though?
TVs have moved on a bit in the last 50 years. It's very unlikely that a
14" portable would have a live chassis nowadays
I've heard tell some sets with a crude SMPS do still have a 'live' chassis, although I've not come across one. I'd guess if its got any form of AV connections it should be safe, though.
Millions of sets in use today dont have that, and do run with live chassis. It is not an outdated practice at all.
To connect a headphone to a chassis thats either live, or not live but not particularly well insulated from live, is a /really/ stupid idea.
Regards, NT
I just gave away a 10 year old JVC and that had an isolated chassis as does my new Sony and the two 5 year old small bedroom sets. BTW they all had/have headphone sockets too. How could they run modern electronics without a transformer of some kind?
IMHO that defeats the idea of an isolation transformer.
I'd agree that all the quality makes I've played with have been fully isolated.
Most quality makes do - and also AV connections, so they'd have to have an isolated chassis in practice.
If there were no external connections, you could use a form of auto transformer which is marginally cheaper?
I did this to a small portable TV 15 years ago - I used a 5 terminal socket ( stereo & it disconnected 2 terminals when a plug was inserted). As other people have warned, one or more of these contacts could be at live potential, I didn't check, because I was using an audio isolating transformer (I think it was from Tandys). I had to move the loudspeaker connections to the other side of the transformer (so the cut-out would work correctly). Trouble was the transformer lost some power, so you had the turn the volume up louder than before, which caused it to distort if you wanted it loud (but normal viewing was ok).
Tony.
I'd say the ideal cheap transformer would be a telephone line isolating type if quality isn't that important. A good quality audio isolating transformer won't be cheap.
That depends on where you connect the EARTH wire. I was thinking of connecting it to EARTH. What about you? Live? Chassis? Why not be adventurous and connect it to that thick wire that goes into the side of the CRT?
Looking in an old RS catalogue I saw a 3W 1:1 speaker isolation transformer for about 5 quid. Put that before the internal speaker and you can use a switched 'phone socket.
If you're OK with a separate switch you could use a stepdown transformer after the internal speaker and do away with the resistors.
Roger.
Hi
several people write:
isolates the
right. and millions dont.
TVs have been running transformerless for decades. There is more than one approach to it.
you dont need any transformer. There are stacks of silicon based TVs around running mostly on HT. I have 1 right here. The few bits that can only be LT dont eat much power, and either HT circuits produce low voltage at various points to run them, or a low voltage loptf wind is used to supply them.
Another option is to use a self oscillating lop stage running on HT. That starts up with no LT and the lop produces the LT needed for the rest of the set. The LT circuits then pull the lop into sync. Flywheel sync has been standard for a long old time. There are many approaches.
does that mean all TVs are isolated? Maybe you're overlooking the consequences of 240v to the head.
those kind normally are, tho again exceptions exist. If 'most' is good enough for you to apply 240v to the head in 5% of cases, I guess that's your funeral.
I'd say the level of advice in this thread is just plain dangerous.
Perhaps you don't understand what isolation means? Would you also earth one side of a mains isolation transformer in a workshop?
No but I'd earth the frame. Wouldn't you? WTF does that have to do with the problem of the live chassis TV? You can either have the headphone side of the transformer floating at some voltage determined by the leakage capacitance, or stick an earth lead on it (Not on the TV side!) and ensure that the sleeve is always at 0v.
Roger.
ps. Would you like to hear the story of Big Ears and Noddy's home-made electrostatic headphones?
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