RGB modding CRT TV - JVC AV-20N8

[Zebidee]

My latest modding victim was a JVC from early 2000's. This time it's RGB!

The TV had obviously lived a hard life, but the tube is still good.


 

 


   


Can't really tell the model number from that sticker! Seems like "AV-N2088". Whatever it really is, it seems an *almost* perfect match for JVC AV-20N8 (BK). Schematic and jungle datasheet attached.

In case you're curious, I can read Thai and the rest of the label contains some warnings: unplug TV after use (sensible!), don't immerse it in water (there goes my idea for a "Swimmer" cab!), and don't remove the back cover (Oh nose, no warranty for me!). Then there is an address, interesting that a Japanese company houses itself in the China-Thai building.

The RGB mod itself is straightforward. The jungle IC is a Toshiba TB1226EN, which has external analog RGB input pins at 23, 24, 25. The analog RGB blanking (Ys) pin is 22, it requires at least 0.5v for RGB. These pins were originally unused, completely unconnected, except for the Ys (blanking) which has a 10k "pull-down" resistor to ground.

No need for hacking the OSD signal here, So don't have to worry about OSD RGB MUXing stuff.


 


So I made up a 6-pin JLT connector with 30cm wires, and fed them through a convenient hole in the middle of the board. From there I carefully soldered the R,G,B wires directly to the relevant jungle pins. For blanking (yellow), no need to remove an existing 10K terminator on Ys, as it will have negligible effect on the input voltage level, and the solder pad provides a good place to solder the blanking signal wire.

For the sync and ground (white/black wires), I connected them under an existing connector for external video input (to right in pic), close to where it meets the AC coupling input capacitor (10uF, C233). Left the cap as it is. There is a 75 ohm terminator resistor somewhere too (not pictured), so don't have to worry about that either.

Next, we need some RGB termination resistors and AC coupling caps, and a couple of resistors on the blanking voltage. 
To make connecting inputs easy, and to make some space for the input caps & resistors, I made up a mini-PCB with some breadboard. Similar to what I've done previously in component TV mods.


   


I tweaked the mini-PCBs a lot, so multiple versions. I utlimately used the bottom one, with the 68R resistors, except I bypassed the large resistor (470R, 1/4W) you can see on the sync input (I decided it was better to put the resistor on the VGA input wire instead).

The 68R resistors (in series on the RGB inputs) are because this jungle chip expects RGB inputs at 0.5vpp @ 100 IRE, which is a bit less than the standard 0.714vpp. In practice, I found that if the signal is too high then it causes certain distortions (blooming and funky vertical lines) to become noticeable/worse.

So, I did some experimenting by putting a gang of three 500R pots on the RGB inputs and measuring the voltage output with a plain white screen (@ 75%) from the Nokia test program. I've used these pots in other projects. By adjusting the pots and measuring changes in voltage I estimated that there was roughly 75R impedance on the RGB outputs from the video card. This is important for the math to come.


Maths!

If math bores you, skip to the end.

There are already 75R terminators, and plugging these numbers into a voltage divider formula shows that the video card normally outputs RGB at 1.428vpp @ 75R impedance to 75R terminators, ultimately bringing the voltage to 0.714vpp. The math then says it needs 68R to reduce 0.7v to 0.5v. In practice this turned out to be almost spot-on. For those that care about the math, solve for R1, and then Rin:

Vout = (Vin*R2)/(R1+R2))

0.5 = (1.428*75)/(R1+75))

R1 = (Vin - Vout)/Vout * R2

R1 = ((1.428 - 0.5)/0.5) * 75

R1 = 139.2 (R)

We could also ask AI to get the same results.





Because we now know R1, we also know Rin:

R1 = Rin + impedance = 139

Rin =  R1 - impedance = 139 - 75 = 64 ohms

I also confirmed this by doing some testing (with pots) to see what looked best. Went with 68R for the permanent resistors as was the closest value I had.


Coming soon: Rear inputs