Component modding CRT TV

[Zebidee]

This thread is to document component-modding CRT TVs. Thought I'd kick it off with a "China TV" that I bought new a few years ago.

Here it is, new and sealed in the box. Well, it is mostly new - the CRT tube is actually recycled/refurbed. But the rest of the TV, including chassis and shell, is completely new. The brand/model is "Distar CTV1121-20" and it is assembled in Thailand.

I'm going to put up a few posts with pics to break up the text, so please be patient with me. To start with, here is the TV I'm going to be component modding today:




 

[Zebidee]

Here are some pics of the TV inputs for the Distar CTV1121-20. The only options are RF and AV/composite.

There are 2 AV inputs, at the back and the side (the second set of AV connections at the back are for output). Both back and side AV inputs are the same, the TV doesn't differentiate them internally.

I'm going to repurpose the side AV inputs into component/YPbPr inputs, so that is why we have a close-up.


 


Also here are some pics to show that the TV works. You can see the normal menu via remote control. I've already accessed the service mode and enabled "YUV" there.

 


I'm not going to get into how to access the service/factory mode here. This is covered in the attached the service/factory adjustments manual for a TV very close to this one (not identical, but in English and close enough that it works).

[Zebidee]

OK, enough of the introductions. Now for some nitty-gritty.

I pulled the back off the TV and removed the chassis for closer inspection.

The area circled in yellow is where we will be focusing our efforts (below).





As an aside - this chassis, in various forms, seems to be officially called the "China TV" chassis (thus the CTV prefix), and it is very common especially in Asia and especially in developing countries. So in addition to China this includes India, Philipines, Indonesia, Malaysia, Vietnam, Thailand... and that is just what I've seen so far, so probably many more. Anyway, back to the modding.

First thing to look for is the jungle chip. On this TV it is the big one in the middle. In general, when looking for the jungle chip on any TV look for the big one first.

In this case the jungle IC designation is "8895CSNG7GG9". This chip incorporates OSD (On Screen Display) functions. There are no RGB inputs and no OSD RGB inputs to hijack (as they are all internal to the chip).

A close inspection of the schematic reveals component inputs (labelled "Cb", "Y", "Cr" at pins 19,20,21).





Now for a spoiler alert - this TV chassis is built onto a PCB that already has component input capability, but the manufacturers decided to leave all that out to save some money. Fortunately for us, we can use that to our advantage.

Following the PCB traces back from those jungle component input pins brings us to the part of the board we will be working on. The pins connect through to the 3 x ceramic disc capacitors (103, 0.01uF, C801, C802, C803). These 103 capacitors simply clamp these unused component inputs to ground, best practice is to not let them "float".





For the next step, we are going to remove the 103 capacitors (C801, C802, C803), and we are also going to remove the 6 wire bridges (R804-R809) you can see above them


NEXT - Action

[Zebidee]

OK, now for the action you have been waiting for.

Let's start by removing some stuff - those 3 x 103 ceramic capacitors (C801-C803) and 6 wire bridges (R804-R809), as framed in yellow below.




Here is what it looks like once they are gone. I've added some blobs of colour for tracking signal paths

 


Next step is to replace the 103 caps with 2 x 104 for C801 & C802, which correspond with Pr & Pb. I'm going to use a 1uF electrolytic cap for C803 (Y).

Also need to replace 3 of the wire bridges with 75R resistors (termination to ground) and the other 3 with 100R resistors (in series with signal). The 75R are the ones with the distinctly purple/violet end, the 100R are the ones that have black bands in the middle (and brown at the ends). I used permanent markers for the Pr/Pb/ground connections, but didn't have a green marker sorry.




Here is a pic with the inputs attached





This is where those inputs will be connected to. I've modified the side AV input for component! Using a tiny screwdriver I carefully replaced the yellow/red/white facscia with green/blue/red. I also rearranged the connections a little on the back of the PCB (not pictured). Had to add a ground wire and rearrange the connector slightly after taking this photo.




OK, that's all for now. Will be back with more progress soon.

[Zebidee]

So, it all works! here are some shots of it all connected up. On the second pic you can see the standard test colour bars on the screen via PC input.


 


Here is the TV connected to a standard DVD player by component. No GreenAntz required for this, it is completely standard input from the DVD player.


 


Finally, some gratuitous shots of arcade game on a standard CRTEMU PC setup + GreenAntz


   


Please ask questions

[Zebidee]

Finally, a shot of the modded TV with the cover back on, and component input (from PC/CRTEMU + GreenAntz) via side input panel. Neat and tidy, you wouldn't know that it wasn't done in the factory!

[Malenko]

Great write up and great pictures too.

How did you come up with the values for all those resistors and caps, and what do you do if the chassis doesn't have the holes for the caps? I am assuming you just tap the legs of the IC and wire it to a breadboard?

[Zebidee]

Great write up and great pictures too.

How did you come up with the values for all those resistors and caps, and what do you do if the chassis doesn't have the holes for the caps? I am assuming you just tap the legs of the IC and wire it to a breadboard?

Thanks Malenko

I had no specific schematic, but came up with the resistor and capacitor values by looking at many datasheets and schematics for similar TVs with this class of jungle chip. I did a fair bit of experimentation and used an oscilloscope too, not just my eyeballs. Also, this is not the first time I've done this mod for this TV (though definitely the neatest!).

Using 75R for termination is obvious and then 100R in series is very common, in almost every comparable instance I've looked at. Certainly no other value. In any case, the spots were there for the 75R and 100R resistors and there is nothing else that would conceivably go there. And it looks good, better, with scope and eyeballs.

As for the capacitors, 104 (ceramic 0.1uF) is very commonly used after the 100R resistor in similar TV schematics. There are some exceptions of course, one of which is where I've seen a 1uF electrolytic cap used on the Y signal instead of 104, either with or without the 100R resistor. In this case the dead giveaway was the electrolytic cap symbol printed onto the PCB for the Y signal (C803) vs the ceramic cap symbol symbol used for Pb/Pr (C802/801). On the virgin chassis PCB, all three just had 103 caps to clamp the unused signal pins. So I whipped out the 103's and replaced with 104 for Pb/Pr and 1uF electro for Y.

So I'm confident that I've chosen well for this TV - however other TVs may/will be different. It is a good starting point for your own projects, but won't work exactly the same with every other TV.

You don't necessarily need any breadboard. Probably not, I'd try to avoid it myself. If there aren't too many bits to add, they can be put above or below the PCB. It is also possible to add to twist 2 or more components (e.g. 100R resistor+104 cap in series) together so that they fit into the space for 1 part on the PCB. Grounding 3x75R resistors together isn't too hard either. You can be very creative with this stuff.

I try to keep my work neat and tidy, especially for the pics. But some of the stuff I've prototyped looks like a gymnastics hall. Electronics in 3D.

With all that in mind, it'd be great if other people can contribute with their own component-modding projects.