Need More Info On Isolation Transformers

[ppppenguin]

Basic advice so far is OK. Beware of a few sets that use an autotransformer rather than a double wound transformer. These sets will still have a live (hot) chassis despite appearing to have isolation from a transformer.

The main use of an isolating transformer is to allow the safe conenction of earthed (grounded) testgear. if you're just working on a live (hot) chassis set without testgear make sure the chassis is conencted to neutral and use an RCD (GFI).

An RCD (GFI) adds no potection when using an isolating transformer.

Don't trust the switch on the set - always unplug if you want to be sure the set isn't live.

NB: I'm in the UK so using UK terminology with US equivalents in brackets. With 230V over here we can get proper shocks

[Notimetolooz]

TinCanAlley, you said that many TVs before the 60s did not have transformers, that is somewhat backwards. It is more true that most TV before the mid 50s had transformers. The reason for having a design without a transformer is weight and cost. The more portable sets from the mid 50s on wanted to reduce weight so a transformerless design was used. Also tubes that worked on lower B+ became available so rectifying the line voltage or using a voltage doubler would work. Of course reducing the cost was always a factor.
Large screen consoles tended to have transformers.

This all changed however when solid state (transistor) TV came out, the transistors had to have a much lower voltage to operate on, a transformer was the only efficient way to get that.

When on the subject of isolation transformers be careful of confusing chassis ground with the power ground (earth) and power neutral.

[TinCanAlley]

Originally Posted by Notimetolooz

TinCanAlley, you said that many TVs before the 60s did not have transformers, that is somewhat backwards. It is more true that most TV before the mid 50s had transformers. The reason for having a design without a transformer is weight and cost. The more portable sets from the mid 50s on wanted to reduce weight so a transformerless design was used. Also tubes that worked on lower B+ became available so rectifying the line voltage or using a voltage doubler would work. Of course reducing the cost was always a factor.
Large screen consoles tended to have transformers.

This all changed however when solid state (transistor) TV came out, the transistors had to have a much lower voltage to operate on, a transformer was the only efficient way to get that.

When on the subject of isolation transformers be careful of confusing chassis ground with the power ground (earth) and power neutral.

Good to know. I was reading a few sources and one stated that sets from the 70s back were ones where transformers might not exist. I guess it is a case by case basis.

[Electronic M]

Originally Posted by TinCanAlley

Good to know. I was reading a few sources and one stated that sets from the 70s back were ones where transformers might not exist. I guess it is a case by case basis.

It is case by case. Even some BPC era sets have full or partial hot chassis.

Even on properly designed transformer powered sets if a primary to secondary short develops that can be bad for your scope (and the set often will work fine despite the short).
I'm fairly sure I've had that happen before. Sometimes I'll measure AC voltage between my scope ground and chassis onna set before connecting them just to be safe.

[ppppenguin]

Don't know if this has ever been true in the USA but in the UK/Europe there were some sets (1980s?) that used a bridge rectifier and connected one side of the output to the chassis. Hence the chassis was always live (hot) at half mains (line) potential. Definitely need an isolating transformer to do any useful work on those.

[TinCanAlley]

Originally Posted by ppppenguin

Basic advice so far is OK. Beware of a few sets that use an autotransformer rather than a double wound transformer. These sets will still have a live (hot) chassis despite appearing to have isolation from a transformer.

The main use of an isolating transformer is to allow the safe conenction of earthed (grounded) testgear. if you're just working on a live (hot) chassis set without testgear make sure the chassis is conencted to neutral and use an RCD (GFI).

An RCD (GFI) adds no potection when using an isolating transformer.

Don't trust the switch on the set - always unplug if you want to be sure the set isn't live.

NB: I'm in the UK so using UK terminology with US equivalents in brackets. With 230V over here we can get proper shocks

Thanks. I plan on going board to board in the TV looking at the B+ voltage as it passes from board to board. Each board has a ground and the set has a transformer, so I'm going to assume it is save as I scoped it years ago and never damaged the set or my scope. Fingers crossed.