Author Topic: Episode about high voltage repairs  (Read 352 times)

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Offline frowin

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Episode about high voltage repairs
« on: May 16, 2019, 07:54:24 pm »
Hi guys,

I'm wondering whether Dave ever talked about basic safety information regarding the repair of devices which include handling of mains voltage or even CRTs! I know, that beginners may should not repair a device like this in the first place...but at some point it is necessary. I have a Commodore CRT here which does not work. I would like to have a look at it (unplugged for sure!) but I know, that there is a high voltage on the CRT.

I binge watched dozens and dozens of episodes now but maybe I miss a episode. Anyway, maybe an beginners episode about safety information regarding the repair would be great! I love his style of teaching  :-/O :-DMM
 

Offline Macbeth

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Re: Episode about high voltage repairs
« Reply #1 on: May 16, 2019, 08:40:19 pm »
The extra high voltage/tension (EHT) on the CRT really isn't a problem when unplugged. You can get a long screwdriver and slip it under the "stethoscope" cap at the top of the CRT and just the contact with the shaft to the aquadag (that silvery powder coating on the back of the tube) is enough to get a satisfying "crack" as it discharges its puny energy. Some people like to attach an earth strap with a large resistor for extra belt and braces health and safety but there's no fun in that.

If the TV has been unplugged for quite some time it will all be discharged anyway. What you really need to watch are the big electrolytic capacitors you find in the mains PSU circuit, good ones can hold a lot of charge for quite a long time if there is a fault on the output side. To be safe, when unplugged you can use a resistor to discharge them from under the PCB or just leave it for an hour or so before working on it.

I used work in TV repair back in the 90's, while Dave is a proper electronics engineer and never did that stuff so I don't see him getting into it now.
 

Offline Shock

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Re: Episode about high voltage repairs
« Reply #2 on: May 16, 2019, 11:22:30 pm »
Randy Fromm has some videos on CRT repair uploaded to youtube. CRT discharging is in the second video. You can find instructions in many service manuals under the safety precautions.

https://youtu.be/YsZ5PJB-w2s

https://youtu.be/En75fI6KioA

https://youtu.be/gs0wLG0f-ps

https://youtu.be/AYoNIoJNVWY
Soldering/Rework: Pace ADS200, Pace MBT350
Multimeters: Fluke 87V, 117, 27/FM
Oscilloscopes: Rigol DS1054Z, Phillips PM3065
 


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