What hot chassis is and how can I locate it???

Ambrosia · Joined: 01 Mar 2006 Posts: 1 Location: Hong kong

I am interested in repairing some electroinic stuffs such as TV and amplifier. However when I borrow some related books to read, I can't understand what the differences between "hot chassis" and "cold chassis" are. And how can I know that is "hot chassis" instead of cold one and where can I locate this "hot chassis" in TV and computer monitor.

Besides I have read a book that tell me we can measure the voltage from point to point. However where can I connect the negative probe of the voltmeter to when I am doing so. (The book said the -ve probe should be connected to the earth) But the main problem is where/how can I find the ground/ground reference.

I have a CRO with bandwidth only 20MHZ, will it be enough for TV repairng and ;Is it enough for DVD repairing

Finally I want to know why an isolation transformer can protect me from electric shock and will I be safe from electric shock when I use this stuff

Thank! Question

Guest · Posted: Tue Mar 14, 2006 11:30 pm Post subject:

In many parts of the world, the power company grounds one of the feeding power lines to the earth. If one of these power leads is connected to the chassis, and your AC plug is inserted wrong way into house socket, that makes the chassis energized to the same voltage as the power mains. If that is so, full line voltage is available from chassis to the earth. If a part of your body is connected to earth (like a water pipe) and you touch the chassis, you will get electrocuted.

An isolation transformer outputs the same voltage as the input. Only, the primary and secondary windings are separate and electrically not connected to each other. This removes the danger of electrocution on touching the chassis.

Guest · Guest

If you are starting out, I would unplug what you are working on. To be safe. If it is a TV discharge the tube, make sure to unplug the TV first. The jolt for a TV tube is unpleasent. Do not do any repairs if you don't know what you are doing.

gasman0_11 · Guest

You are obviously very new but also show great interest in this topic.

However I think you should curb your enthusiasm in this subject until you understand more, try repair of battery operated equipment first until you gain experience, otherwise there may be tragic results.

A safety isolating transformer will only protect you from a circuit to earth shock, ie. if you touch a live part whilst in contact with mother earth. It will NOT protect you against a full circuit shock such as having one hand on the chassis and the other on a live part.
A good safety guide is to:-
a, use an isolating transformer
b, wear rubber shoes
c, keep one hand in your pocket (may sound stupid but remember that a hand to hand shock is accross the chest and can stop the heart)
d, remember that the crt final anode holds a charge (up to 25000 volts) even if the tv has been switched off for days and must be discharged several times before handling.
A 20mhz CRO is adequate for your needs.

gasman0_11 · Guest

I forgot to explain this in my previous post.

A Hot Chassis is one that is directly connected to one pole of the mains supply. This would normally be the Neutral but if a plug is miswired it can be live.

A cold Chassis is one that is isolated from the mains and therefore provides similar (but not to the same safety standard) protection to an isolating transformer