5.5" B/W CRT TV - maybe composite mod?

Discussions about Sinclair ZX80 and ZX81 Hardware
Post Reply
User avatar
mrtinb
Posts: 924
Joined: Fri Nov 06, 2015 5:44 pm
Location: Denmark
Contact:

5.5" B/W CRT TV - maybe composite mod?

Post by mrtinb » Mon Apr 22, 2019 4:15 pm

Hi :)

I have a 5.5" B/W CRT TV that has VHF/UHF input. But there's no composite.

I've opened it up today, and see there are "holes" in the back for more outputs. And I see on the board there are a lot of "missing" components. So this is probably a PCB made for different models of the TV, and some extra components added for the models that had composite output.

I have no education in electronics, but I have made the circuits I've shared with the forum. You have been a great help. I am very aware that messing with CRTs are dangerous, and have huge respect and don't touch anything when the lit is taken off the TV.

Would it be very difficult to add composite input to this CRT?

Image

Image

Image

Image

Image

Image

Image

(Edit: These are hires images if you open them in a new tab)
Martin
ZX81, Lambda 8300, Commodore 64, Mac G4 Cube

User avatar
mrtinb
Posts: 924
Joined: Fri Nov 06, 2015 5:44 pm
Location: Denmark
Contact:

Re: 5.5" B/W CRT TV - maybe composite mod?

Post by mrtinb » Mon Apr 22, 2019 7:44 pm

There’s YouTube videos with another version that has composite already. But otherwise they look very similar. https://youtu.be/BmllyvG5c2w

And a full disassembly here, but again the version with composite already. http://jestineyong.com/servicing-rising ... onochrome/
Martin
ZX81, Lambda 8300, Commodore 64, Mac G4 Cube

User avatar
1024MAK
Posts: 2313
Joined: Mon Sep 26, 2011 10:56 am
Location: Looking forward to summer in Somerset, UK...

Re: 5.5" B/W CRT TV - maybe composite mod?

Post by 1024MAK » Mon Apr 22, 2019 8:15 pm

Hi Martin

It may be possible to add a composite in in place of the RF input. After all, all TVs with a RF input convert this to some kind of composite signal internally.

But it will not be very easy job without a schematic of the version that you have.

I’m not sure if you noticed, but the versions in the You Tube video and in the web page you linked to use different PCBs. So it is impossible to know what and how many differences there are between the different versions.

It’s not uncommon for an item that looks similar on the outside to have a different PCB on the inside. [The reverse is also often true as well!].

Alas, there are many, many different circuit designs for CRT televisions alone. So guessing is not a smart way to proceed...

But if I had to guess and wanted to point you to the most likely area, then follow the RF input. If it goes into circuitry contained in a screened metal can, it is likely that the output from this is a composite video signal. But beware, as it is only used internally, it may not be at the same voltage level as a ‘standard’ composite video signal.

Now your TV looks like it does not have a mains voltage input. Is this correct? If it does have a mains level input, it is important to check / test that whatever we think the internal composite video is, that it is not internally connected (directly or indirectly) to a high voltage. [I have to put this warning here, as some older designs had all / or a lot of their internal circuitry connected to the mains, hence are known as life chassis designs.]

Mark

User avatar
1024MAK
Posts: 2313
Joined: Mon Sep 26, 2011 10:56 am
Location: Looking forward to summer in Somerset, UK...

Re: 5.5" B/W CRT TV - maybe composite mod?

Post by 1024MAK » Mon Apr 22, 2019 8:29 pm

It would be helpful if you can list details of any chips used in this TV.

Mark

Post Reply