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Author Topic: Video ground isolation  (Read 1066 times)

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Zebidee

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Video ground isolation
« on: January 07, 2022, 11:37:43 pm »
I've just done an RGB mod on an old National 14" CRT from around 1980-ish (I guess), and I've come to a problem.

Because the chassis is an older "hot chassis" design (even though it also has an SMPS), there is no ground separation from the power supply "HOT" side and the rest of the chassis (which we might normally call the COLD side, except that obviously it isn't). So the entire chassis is HOT, and has quite a bit of stray voltage running through it.

It isn't really safe, especially when I connect the RGB+GND+sync from a PC/GPU, because the video ground is simply connected to the shield ground so you can get quite a jolt by just plugging in the cable. On the original RF connector you didn't have this problem as it was well buried into the original plastic casing, making it almost impossible to touch accidentally.

So, I was thinking to use a video ground isolator balun, something like this:




EDIT: I decided that this would do the trick after looking at the specs, rated to 600V, and ordered already
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bobbyb13

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Re: Video ground isolation
« Reply #1 on: January 08, 2022, 01:26:22 am »
BZZZZZT!!

 :blowup:

I'll be waiting for your results on this one of course.

Wonder how many of the ones I have may be like that...?!
Relax, all right? My old man is a television repairman, he's got this ultimate set of tools! I can fix it.

Zebidee

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Re: Video ground isolation
« Reply #2 on: January 08, 2022, 02:06:20 am »
BZZZZZT!!

 :blowup:

I'll be waiting for your results on this one of course.

Wonder how many of the ones I have may be like that...?!


I ordered a pack of six, only about US$10 from local agent, as I have a feeling they will be useful for minimising signal noise as well. Will probably just chop up the BNC ends of one and splice it directly into the GND connection, and leave it inside the TV. Like this:

https://www.aliexpress.com/item/1005003772673736.html?spm=a2g0o.placeorder.0.0.ba49321eXLixwF&mp=1

Even so, I think the hot chassis issue in TVs with SMPS (switch mode power supplies) is rare.

When looking at this TVs innards, at first I thought I must be crazy because the SMPS ground is shared with the main ground. Not by accident either, the PCB is printed that way. I was like "that is an SMPS - so why not HOT/COLD sides?". I scratched my head looking hard for an optocoupler, which normally sits across the HOT/COLD sides and shuts down power if things go wild, but there isn't one. I know that older TVs had HOT chassis but thought that went out with SMPSs.

Anyway, I eventually googled "hot chassis tv" it and found, on some 15-20 year old forums, that some early UK TVs (like National) with SMPS still had shared grounds until around 1980.

I don't have a schematic so have been working this all out by looking over the PCB   :dizzy:

However, it has also been an opportunity go go over all the parts and testing where I can, redoing a lot of cold solder joins (power transistors literally loose in the pads). Surprising all the parts come out pretty good, even the old capacitors (they all test OK on the ESR meter). The Horizontal output transistor still works but is a bit suspect (relatively low ohms from collector to emitter), so I ordered a few of those too.
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bobbyb13

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Re: Video ground isolation
« Reply #3 on: January 08, 2022, 07:23:37 am »
I'm nearly to the point I can start nerding out on chassis myself I hope.

Good fun to read about your adventures of course so thank you!
Relax, all right? My old man is a television repairman, he's got this ultimate set of tools! I can fix it.