Need help with a new mod...

[Segasonicfan]

Pretty basic mod principle- I'm just trying to add an RGB input to my awesome 36" JVC CRT television.  It only has Composite and S-Video inputs so I am trying to bypass them and feed RGB straight to the driver.  From what I can tell the encoder is the TA1242N but I can't be sure cause the datasheet is in friggin Japanese and there's isnt a English one online: (http://www.alldatasheet.com/datasheet-pdf/pdf/112571/TOSHIBA/TA1242N.html)

This chip seems like it does too much though...it drives the H and V lines, outputs the high-level RGB drives, amps audio signals AND converts the S vid, RGB, Composite inputs to the high-level RGB output??  Maybe it's just a really expensive chip though, I dunno.  Seems like a lot though.

There really can't be any other encoding chips though since the only other circuit between the inputs is a CXA1545 which appears to just be an AV switcher (switching the source input I presume).  

Everything is controlled via I2c by a main CPU, the MN1874678.  Oddly, this circuit seems to have been designed specifically as a video controller even though it is advertised as just a standard microcomputer.  It does have sync inputs and a built-in sync splitter among other things that make is more monitor specific though.



My Progress So Far:

I have already got full RGB input on the screen.  I just need SYNC, so if anyone can tell me anything about where/how I can input C or HV sync to get the image lined up I would be supersupersuper grateful  I know a little about chassis and monitors but obviously not as much as I should  I have tried inputting sync through the microcontroller, and through the Y input and through pins 37 and 39 on the encoder but no luck so far :/

-Segasonicfan

[viletim]

That's modern technology...all the analog stuff - from the IF to the RGB driver is done on the so called Jungle IC.

From a quick look I'd say pins 37/39 is where the video goes in. So insert your sync here - no bigger that 1Vpp and coupled through a 100n capacitor. If that doesn't work then try the same with pin 43 and try changing the TV between A/V modes...

You have a circuit diagram for the set?

[Segasonicfan]

Hey thanks for the help Viletim  

I tried your suggestions and it looks like pin 43 worked (woot!) however, the TV does an emergency power off when anything bright shows up in the video (like the Neo Geo logo).  I am using arcade PCBs and am assuming the problem is a sync more than 1v p-p, right?  Any idea how I might fix this?  I tried swapping the sync polarity with a 7404 in case that was the problem and I also tried using some diodes to knock down the voltage output, but no success.  

I went aheady and paid $15 for the full service manual.  Here it is (I uploaded it):

http://files.filefront.com/AV36985+JVCpdf/...;/fileinfo.html

Page 35 (of the PDF) has the schematic of the main board with the TA1242N on it.  Please let me know what I can do to fix this if you can....this 36" RGB output is AMAZING!    

[Richter X]

I've been wondering if there was a way to make my TV accept RGB through the component video inputs, saving me the trouble of buying a JRok. I used to easily be able to get into the service menu of my older Sanyo TVs, but I think my new one has a new procedure to get into the mode.

It's a Sanyo DS24425 24" Flatscreen TV. The component input itself looks very good when I play my DVDs.

[Segasonicfan]

I've been wondering if there was a way to make my TV accept RGB through the component video inputs

Very very very unlikely.  The only way to input RGB is the way I did it but cracking open the TV and inputting the signal to the chroma decoder.  I also had to disable a lot of things to get this to work as it's not as easy as it sounds.  

Component inputs are nothing like RGB at all.  The Y line carries the sync signal and it is a complex form of video modulation.  Just get a JROK, they have great output and you will be happier with a working TV

*desperately waits for viletims help on the sync issue* :unsure:

-Segasonicfan  

[viletim]

Segasonicfan,

Get a book on electronics from your library and read the first chapter. It'll contain all sorts of useful information about resistors, ohms law and the like.

Pin 43 is a video input so you'll have to make your TTL sync look somewhat like a video signal. A simple divider should do.

Code Select Expand

.   !CSYNC  ----[R1]--+--[C205]--- PIN43
.   (TTL)       4k7   |
.                    [R2] 1k
.                     |
.                    ---GND
If you want to keep using this as a TV then you can use put a mechanical or CMOS switch (like a 4053 powered from +12v) before C205. To use a game console's composite video signal as a sync source then change R2 to 75 ohms and omit R1.

The RGB video input needs similar attention. I don't think you can just throw in the video signal from a JAMA PCB, you'll have to attenuate it to the usual 0.7Vpp. If you apply a voltage to the Ys pin you can force the TV into RGB mode - could be useful.

[blackevilweredragon]

my HDTVs service menu does let me switch between RGB and Component on the Component modes, but I hate making that change all the time, because it changes ALL Component video jacks..

[Segasonicfan]

Thanks for the info viletim, I hadn't thought to use a voltage divider circuit!

Turns out that circuit didn't do quite the trick but after a lot of testing and trouble shooting I found a setup of values that works well.  I also added pots to adjust the RGB lines and will add one later for the sync line (though it's working quite well without it now).  Anyway, this mod was a complete success and kicks ass to say the least.  I took a pic of the amazing quality:

http://i18.photobucket.com/albums/b134/Seg.../shinobirgb.jpg

that pic doesn't do the image justice but you get an idea of how defined everything is now.  Gawd, I just love RGB  This monitor is really phenomenal too, the convergence is almost perfect and it matches if not beats the quality of my Wells Gardner monitor!  Very, very close to my Nanao in quality.  Of course this guy is 36" though so you could even say it's better for the sheer massiveness!  

a pain in the ass mod but I'm very happy and it was most definitely worth it

-Segasonicfan

[Endymion]

Nice work. I'd feel as accomplished if I could rid my PVM2530 of its topcurl.

[Segasonicfan]

have you tried opening it up?  I use to own a PVM 2530 and there are a lot of adjustments you can make inside (a ton of pots, way more than the later pvms).

-Segasonicfan

[Segasonicfan]

hey, sorry to bump this old thread but does anyone have that service manual I posted on here in the link above?  I had it hosted on filefront for a long time but they have since deleted all my files.  I don't have the money to purchase this manual online anymore...it would be a great help if someone had it on their computer:)
-segasonicfan

[Segasonicfan]

Reup of full Service Manual + Datasheet (JVC AV36985 Television):

https://www.mediafire.com/?78zvldd97fu5u7a

Sorry to bump an ancient thread like this, but I still get emails about this mod asking for this info.  Still my favorite TV to this day, was using it this morning    Still working great after 21 years...
-Segasonicfan

[kbuzz]

Sorry to bump and old thread

I'm working on an rgb mod for an AV-20820 which uses the same TA1242N chip.

Would you mind posting the values you used for the voltage divider circuit? I also notice no mention of fast blanking. Was there no OSD hacking involved in this?

[Segasonicfan]

There was definitely some OSD hacking needed....most likely the Ys pin needs to be pulled to logic high (10k resistor to +5v). 
Unfortunately I did this mod almost 10 years ago and didn't document much.  I have it here but it is way too much work to open up.
The voltage divider circuit for sync i posted here though:

http://assemblergames.com/l/threads/crt-to-rgb-conversion-help-needed.62107/#post-894001

The voltage dividers for the R,G,B lines work but they are not properly configured.  I'm going to redo those soon and then post back here.

-Segasonicfan

[Segasonicfan]

Here's the final version of what works really well for me.  The 500ohm rheostat adjusts the signal strength, which I find very useful.  It can be anything from 1k to 200 most likely, and a trim pot would be ideal.

-Segasonicfan

EDIT: the 4k7 to ground is redundant and unnecessary.  actually a 500-1k pot in series with the signal with 75ohm resistance to GND is a better solution.  my brain wasnt working apparently when I made this  

[kbuzz]

Here's the final version of what works really well for me.  The 500ohm rheostat adjusts the signal strength, which I find very useful.  It can be anything from 1k to 200 most likely, and a trim pot would be ideal.

-Segasonicfan

EDIT: the 4k7 to ground is redundant and unnecessary.  actually a 500-1k pot in series with the signal with 75ohm resistance to GND is a better solution.  my brain wasnt working apparently when I made this

Thanks for this. Finally getting around to this project. A similar RGB mod didn't even need to build a circuit for RGB lines and went straight from Scart to TA124N.
Assuming they lifted the pins or cut the trace before wiring up. Seems the same chip can function differently in various set.

[Segasonicfan]

Awesome.  I'm glad this thread keeps kicking!  I've noticed a lot more love for CRT hacking lately, which is amazing.  People beginning to appreciate the old stuff now.

-Segasonicfan