Component modding CRT TV - "China TV"

[Zebidee]

   please.

I wasn't aware 100Ω resistors were commonly found on RGB and YUV lines... I also maintained 1μF electrolytic capacitors in series and now realize ceramic disk capacitors with lower values are better to maintain the needed AC coupling, but improve frequency response a bit.

A modular approach works best.

I suggest you build a breakout PCB with connectors for your Y Pb Pr inputs. This will allow you to quickly and easily swap the whole unit out, to test new arrangements, avoiding strain or risk to TV components.

Start with this (below), which I already posted on the thread above. Use the 100R inline resistors + 100nf (104) ceramics for Pb Pr, and you can also use it for Y (luma). Don't use 1uF for Pb Pr, signal is too high frequency. Y is lower, so a higher value cap is acceptable, but the main effect will be a slight fuzziness as some of the signal is delayed slightly. If the TV has problems syncing, you could try a 1uF (105) or even 10uF for Y. If using electrolytics ensure polarity is correct, negative to GND.

You should use 75R termination resistors because the video signal is impedance matched for 75R and expects 75R termination. The 100nF cap passes high frequency video signal. Any remaining low frequency noise prefers to go to GND via the 75R resistor.

This base design is a good place to start from. So I recommend you try it first, then you can modify from there.


     

I've done RGB mods before and haven't had any such issues with the signal level, and I read where some ICs may have current limiting built in. But
...seems something with the Y clamp circuit in the chip failed, maybe I burned it out doing this mod incorrectly.

In your case, I'd recommend you add some 12v zener diodes for overcurrent protection, as shown in the circuit below. The zeners give some protection from high voltage spikes on the video inputs. I have learned about this the hard way on a TCL TV mod, click on the picture for a link to the thread.

I replaced the chip since I added a socket, then I encountered another issue where it would power on for about a second after I plugged it in then turn off and not respond. It would do the same thing regardless of what chip I put in it

I also recommend that you look at backing up your EEPROM. This is typically an 8-pin DIP or SOP/SOIC-8 chip near the jungle. Jump online and grab a "CH341a", they are cheap, and you can use this to copy/backup/write/hack the EEPROM. You could even socket it, like you did for the jungle itself. I have another thread on this here.

I also wanted to ask about the 391 capacitors on the cathode amplifiers, these are used for filtering according to the schematic Adrian Black on YouTube scanned in from his China kit board. He bought his in 2023, and he doesn't seem to have any blue streaking issues. I checked and the blue capacitor on mine is indeed a higher value so I'll be replacing it. I'm not sure if I'm seeing any symptom of that though. There is some noticeable smearing of the reds and blues when using s-video but that might just be chroma delay seen on these chips.

What are these "cathode amplifiers"? Are you talking about caps on transistors in the colour drive circuit, on the neckboard? Who is Adrian Black?

  More pics please. What works for me is to pretend that everyone here has either dyslexia or attention span issues (maybe not far from truth).

EDIT: Make sure you have removed any GND clamping capacitors on the Y Pb Pr inputs to the jungle. Often a 103 cap. Like this:

In this pic we can see that the trace connects 19 to 21, and then to C804, a 103 (10nF) ceramic capacitor. This 103 cap clamps the Pb, Pr signal pins to ground, to prevent unused inputs from "floating ". It needs to be removed. The first pic below is untouched and shows which pins are the signal pins; the second pic (from an identical but different TV) has had the trace scraped off between Pr and Pb, and C804 has been removed.


 

[titan-91]

Here's how I had things wired thus far. Most of the Reddit post links in that Pastebin document contain full annotated Imgur album links with many more photos as I went along.

https://pastebin.com/ppVWTe7H





I assume the standard method you found will give a brighter picture with more dynamic range, but thanks for the breadboard suggestions. What I think happened is I allowed to much current to sink into the chip and it fried the Y clamp circuit. My assumption was the inputs were high impedance, so I used a higher impedance termination. Guess that wasn't the case.

I've already taken a detour with the EEPROMs, I had a FlashCatUSB Classic on hand so wiring that up with 10kΩ pull-up resistors on the clock and data lines was simple. I uploaded several EEPROM dumps along with board details and photos to my Archive.org account:

https://archive.org/details/@titan91?sort=-addeddate

At first I was concerned the EEPROM contained the remote code data. I've now learned that is not the case and I also have an old Android phone with an IR blaster I use with a Remotify app from 2024 some guy made. 6 of the 8 emulated remotes work fine with these 8895 chips of mine.

The cathode amplifiers are referring to the transistors on the neck board, yes. I don't have a picture of mine at the moment. But I can tell the blue capacitor is definitely not the same capacitor as the other 2 channels. Very strange how that is.

And yes, before I first did the mod the Y pin was floating on the board (not soldered) and the Pb/Pr pins were soldered directly to ground. No capacitors. Not great to isolate from potential DC voltage transients on the ground plane, and leaving Y floating also exposes the chip to electrostatic discharge through that line. When I have Pb and Pr floating I see red and blue stripes across the screen, so there's definitely some coupling that happens there by default.

[Zebidee]

Here's how I had things wired thus far. Most of the Reddit post links in that Pastebin document contain full annotated Imgur album links with many more photos as I went along.

Excellent! It is good discipline to keep detailed logs.

As you've already worked out, you can link to those archived images from here, to quickly pull together a concise story.

I assume the standard method you found will give a brighter picture with more dynamic range, but thanks for the breadboard suggestions. What I think happened is I allowed to much current to sink into the chip and it fried the Y clamp circuit.

The 75R termination pulls the input voltage to the correct level, which for Y is up to 1.0vp_p, though most ICs will accept up to 1.2v. For Pb Pr the input voltage is a little less. Anyway, without that 75R termination the Y input signal can be up to 2v, which the jungle won't like for sure. So it is probably less about current and more about voltage, though obviously you can't have one without the other.

You have done OK for a first time, but honestly some connections there are pretty dodgy-looking. So you also run the risk of accidentally shorting the input signal with something else and perhaps momentarily creating a voltage spike, which could also damage the IC video channels.

I've done these mods many times now. In rare cases the PCB has the vacant spots already there for the extra components, but otherwise a small breakout PCB is the way to go for safety and convenience. It will save you hassle in the long run, and is:
- Structurally much more sound
- Reduces stress to jungle IC and fragile connections
- Easy to connect/disconnect
- Allows you to implement experimental changes easily
- If you use the circuit I suggested above, especially with the zener diodes, it will help protect your circuit from interference and exposing the jungle to excess current/voltage.
- the connectors are cheaply available from your favourite online retailers. You can even buy them with the wires already connected.

I've already taken a detour with the EEPROMs, I had a FlashCatUSB Classic on hand so wiring that up with 10kΩ pull-up resistors on the clock and data lines was simple. I uploaded several EEPROM dumps along with board details and photos to my Archive.org account:

https://archive.org/details/@titan91?sort=-addeddate

FYI  These China TV boards often have a white 4-pin connector (5v, data, clock, GND) allowing you to read/program the EEPROM directly with a customised cable. If there is no connector, there might still be a footprint on the board in which case you can pop one in! I cover all that in the EEPROM thread linked above.

At first I was concerned the EEPROM contained the remote code data. I've now learned that is not the case and I also have an old Android phone with an IR blaster I use with a Remotify app from 2024 some guy made. 6 of the 8 emulated remotes work fine with these 8895 chips of mine.

Cool! What a great idea.

The cathode amplifiers are referring to the transistors on the neck board, yes. I don't have a picture of mine at the moment. But I can tell the blue capacitor is definitely not the same capacitor as the other 2 channels. Very strange how that is.

I once thought that too. After looking at many CRTs I've found it is actually very common, at least in Asian brand sunset-era CRT TVs, but I cannot tell you exactly why the blue cap is different. I've tried swapping it (for the same value used by red/green) in most of my mods, and in many cases the image seems crisper and the blue definitely less smudged (to the right). Occasionally it looks better to leave it in. It might be because a certain amount of smear on blue makes AV/composite/RF actually look better?  It isn't really noticeable until you use a component or svideo input.