Need More Info On Isolation Transformers - Videokarma.org TV - Video

TinCanAlley · #1 11-07-2021, 07:00 PM

Okay, I understand the basics behind using one, but in my reading I've run across something I need a bit more understanding.

I keep running into not needing one if the device has a transformer. If I'm understanding this correctly, if the device gets power from a two prong and the neutral side goes to the chassis, and the hot goes through rectifiers and such, you need one. If the device has two prongs and goes directly into a transformer to create the voltages necessary for the circuits, you don't need one. Is this correct?

I'll use my Zenith as an example. It has a transformer. If one isn't necessary, I feel better about scoping a few years back. If it did need one, then I feel lucky I scoped it without issue. Is the Zenith Avante with a transformer okay to scope without one?

Same with my Pioneer SX-1080. It has a transformer. So is this an example of a device an IT wouldn't be necessary?

I really don't have anything that doesn't have a transformer, but I understand many TVs from the 60s and earlier didn't.

Thanks for the input. I want to put my Zenith back into diagnostics for the jailbars, but now am not sure it is wise without and IT.

Electronic M · #2 11-07-2021, 11:41 PM

Quote:

Originally Posted by TinCanAlley

Okay, I understand the basics behind using one, but in my reading I've run across something I need a bit more understanding.

I keep running into not needing one if the device has a transformer. If I'm understanding this correctly, if the device gets power from a two prong and the neutral side goes to the chassis, and the hot goes through rectifiers and such, you need one. If the device has two prongs and goes directly into a transformer to create the voltages necessary for the circuits, you don't need one. Is this correct?

I'll use my Zenith as an example. It has a transformer. If one isn't necessary, I feel better about scoping a few years back. If it did need one, then I feel lucky I scoped it without issue. Is the Zenith Avante with a transformer okay to scope without one?

Same with my Pioneer SX-1080. It has a transformer. So is this an example of a device an IT wouldn't be necessary?

I really don't have anything that doesn't have a transformer, but I understand many TVs from the 60s and earlier didn't.

Thanks for the input. I want to put my Zenith back into diagnostics for the jailbars, but now am not sure it is wise without and IT.

You're mostly right.

First off IIRC your Zenith was a 25DC56 or similar which doesn't need an isolation transformer.

Can't comment on the pioneer never seen one or a schematic.

Sets without a power transformer always need an isolation transformer if you want to connect a scope.
Sets with a power transformer often don't need an isolation transformer, but exceptions exist....there were a variety of radios and TVs that had filament only transformers (my Zenith MJ1035 radio is such a set) and some sets that used a non-isolating auto-transformer (primary and secondary are directly connected together...most variacs are this but with a continuously variable tap) and series heaters (some euro radios are like this IIRC). Always a good idea to check the schematic. Also once in a while a transformer will develop a primary to secondary short...I had a 50s bench isolation transformer do that to me and it took a few zaps and big sparks connection test equipment for me to realize something wasn't right.

If one lead of the cord connects to chassis even via a switch it's a hot chassis.
Most hot chassis sets lack power cord polarization so there's only a %50 percent chance the chassis is connected to neutral. Also many hot chassis sets wire the switch such that if the chassis is at neutral when on the neutral gets opened when the set is off and the chassis floats up to hot through the tube heaters. Even if you add polarization and rewire the switch there's LOTS of reverse wired wall outlets in the world...
And even with chassis at neutral the ground and neutral voltages can be different in some circumstances and or there can be ground loop noise that can cause odd scope behavior.

TinCanAlley · #3 11-08-2021, 12:14 AM

Good info. I am sure the Zenith is okay as I scoped the crap out of it and never had an issue. Also, I only clip the ground to a ground on the circuit I'm testing. Each module on the chassis has it's own ground point.

As for the Pioneer, I can't, for the life of me, remember if the chassis hot. I should as I replaced the power cord and I made sure to get the phasing correct and use a polarized plug. Someone told me not to as I might need to reverse the plug if I get hum, but I never have and I thought that was more for externally connected like turntables, etc.

ppppenguin · #4 11-08-2021, 12:51 AM

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 · #5 11-08-2021, 09:13 AM

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 · #6 11-08-2021, 04:14 PM

Quote:

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.

TinCanAlley · #7 11-08-2021, 04:16 PM

Quote:

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 · #8 11-08-2021, 07:27 PM

Quote:

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 · #9 11-09-2021, 01:12 AM

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.