Colecovision Component Video

[raisinland]

So... I got ahold of a Colecovision with a bad RF modulator wich seemed a perfect candidate for video upgrading. The TMS9928 video chip used in the Colecovision outputs component YUV video directly. Looking at the datasheet, it appears the pins are capable of driving the output without amplification. I soldered wires to the apriopriate pins, added 470ohm pulldown resistors to ground as indecated in the data sheet, and voila component video! There are very faint vertical bands in solid color blue areas, but that may very well be due to the fact I'm using unshielded wire.

If anybody's interested I'll put pics of the mod up on the wiki

Though it works fine would it be a good idea to throw in a buffer to take the load off the chip?

[NFG]

I've heard that the CV is a very easily modded machine.  Would love to see details of your mod.

[raisinland]

Great! I will post.

It appears the Y-R and Y-B lines are unused, but the Y line runs to the RF modulator. Details details. When everything is sorted I'll put a post up on the wiki.

[Lost Monkey]

I would like to see some screenshots too - anything but Donkey Kong.  I did this mod  a few months ago, and I was never able to get the colours down 100%...  even using a buffer.

[viletim]

No need to bother with a pull down resistor, there's a 75 resistor to ground already inside the TV. It's bit dodgy to drive the TV directly with the chip outputs, what happens if the output is short circuited? (which is inevitable when phono sockets are involved).

I know Y is ment to be 1 volt (peek white to sync bottom) but what about Y-R and Y-B? I recall them being quite small, something like 0.3Vpp. Though I can't think of any place to look this up at the moment.

According to the electrical specs there's a least a volt and a half of DC on the video outputs so a buffer is trivial, just and transistor and a series resistor to drop the level to what's required.

[raisinland]

Lost Monkey - As soon as I get it looking as good as I can I'll post screenshots.

Viletim - I looked it up and Y is indeed 1V, (0.7V+0.3V sync)
R-Y and B-Y are both 0.7V p-p.   Could you point me to an example of the transistor and series resistor circuit you're refering to? I don't have any formal electronics training. I'm learning as I go along so I'm still mostly in the dark when it comes to circuit design.  

[viletim]

Code Select Expand

.                 ___+5V
.                  |
.               |  / (collector)
.               | /
. input o---+---|<  Q1
.           |   | \
.           |   |  > (emitter)
.    +-[R1]-+      |
.    |             +---[R2]---o output
.   ---GROUND

R1 = 470 ohm - it's there because the outputs are only spec'd with this R in place
Q1 = pretty much any small NPN transistor like 2n3904, BC548, 2sc1815, etc
R2 = series output resistor.

The The Y/R-Y/B-Y outputs levels are listed in the datasheet page 5-3, last table. "white-black" means that the the measurement is from the peak white to black level, sync on the Y isn't included. What worries me is the ultra loose spec on the voltage, min = .7v, nom = 1v and max isn't even listed! This makes chooseing R1 a suitable value of R2 a bit difficult and what works for one Colecovision many no be ideal for another. Try and see i suppose... One thing to remember is this resistor is the only thing to limit the current in the case that the output is short circuited. Better not go lower than about 15 ohms.

[chjmartin2]

Not entirely related to the thread... but the TRS-80 uses a Motorola 6847 chip which also has Y, R-Y, B-Y outputs... I've tried connecting those directly to no avail - but I'd imagine that you could use those through an amplifier.  Could one of you really smart people take a look at the spec sheet at the link below and just tell me what kind of circuit I need?

<a href=http://sunpal7.mit.edu/6.111/mc6847/mc6847.html>Here for some specs</a>

[viletim]

chjmartin2,

Try this, might work, no guarantees.

[ManekiNeko]

Hey Raisin, do you have those Component mod instructions up yet?

[carlosemandrade]

Tried Viletim's tansistor schematic with a TMSxxx on a PAL MSX1.0 machine, R1 already there (measured 470 ohms from VDP output pins to ground), R2 with 75 ohms and BC337 transistors. With this setup, picture was very "bluish" (black backgrounds turned to blue and some distortions). So i added two 2k pots from R2 on both R-Y and B-Y, few adjusts and a much better picture, including minimal distortion (from 0 to 10 i give a 8.5) 

[carlosemandrade]

two 1k pots... sorry