Hey all, So I've converted a standard US consumer-grade CRT to RGB before many years ago and am trying to do it again on a cute little 9" CRT I found this week. The small ones are hard to find (with color tubes) and this one is old enough that the OSD input (RGB) is going to still be analog. It's also old enough that the main jungle IC doesn't use I2C programming. It's a 1993 Zenith TV. I followed the excellent guide Viletim posted here: http://nfggames.com/forum2/index.php?topic=4159.msg42011 (post reply #2) only difference my RGB inputs appear to be DC-coupled with no caps going in. I made a voltage divider for the sync (pin 45) which works perfect, and the RGB shows up on screen OK it just lacks a lot of color gradients, especially pink and skin tones (see attached pic) I'm not sure what would cause this...it's almost like the jungle IC doesn't process the signal enough because it's only using it for OSD. Does anyone have any ideas? I've tried variable resistors on the RGB signals and various values of caps in case it was an AC-coupling issue. The datasheet seems to show it as DC-coupling as well in the pin description. The OSD microcontroller is unknown and The jungle IC is a TA8879N and the datasheet is over here: http://www.alldatasheet.com/datasheet-pdf/pdf/31572/TOSHIBA/TA8879N.html
Well, I no nothing about this mod, but you might get some interesting here or able to ask Shadow directly. http://www.psxdev.net/forum/viewtopic.php?f=62&t=723
Well, I found a service manual! (attached) It's not the best but it has some useful info.. Hard to track down since there is no model number written on this thing, I just searched some markings off the chassis (like the flyback). It turns out this is a Zenith SMS2049. There are 47pf coupling caps for all the RGB and Ys lines as well as 2k7 ohm in series resistors. The datasheet for the TA8879 also suggest applying voltage to pin 33 (video mode select) when measuring the OSD output. I'm wondering if perhaps my problem is from this chip being in TV Tuner mode instead of Ext Video In mode? I was thinking with Ys tied high (fast blanking) that this would be irrelevant, but maybe it still is. On page 40 it suggests applying 4.3v to pin 33. This is confusing though since on page 9 it shows 3.25v is TV mode and 2.15v is external mode...
That jungle I/C isn't configured to accept .7vPP analog RGB inputs. It has quite a large swing, let's take a look at the data-sheet. I believe it's also missing a clamp on those inputs, too. Are you matching impedence? (Terminating with 75 ohm resistors to ground to match 75 ohm load of signal) That's quite a swing, you have a gain problem. Shouldn't be too difficult to dial that in.
Thanks so much for your help. Any idea on the best way to clamp the signals? I dont know much about it. I can just up the gain with a TH7314 or similar, yeah? or a quad opamp. Im just surprised that would cause missing gradients instead of just a low-contrast image. And yep, I did 75-ohm termination. Since its DC-coupled I should not use caps for the input, right?
"Threshold voltage" in the data sheet snippet seems to suggest to me that the pins may be digital inputs instead of analog - this would allow only 8 colors in total. Good enough for an OSD, but not good enough to feed a console's RGB output into it.
Unseen is correct. That's TTL RGB. I failed to see the truth table for color output combinations in the datasheet. You have a 3 bit color palette there. You'll need to take your RGB source directly to the guns and drive it there.
Bummmmmmmmer. Nuts. Well, I'll have a look at giving it to the guns then, thanks to Armorant's link. Looks iffy and a bit tricky, since service manual says is sends 5.1v signals to the tube. Blagh, these jungle ICs are so hit and miss. -Segasonicfan
That partial schematic printed next to the pin description makes it pretty clear, too - you basically have a long-tailed pair differential amplifier with a reference voltage on the other input, which is a classic comparator setup. On top of that, looking at the service manual it's a hot-chassis design, so potentially hazardous to try and directly feed RGB into anyway.
Something has a hot chassis if there is no electrical isolation between the circuit common and the power line. If you look at the first page of the schematic, you can see that the power line comes in, goes through some EMI filtering components and a relay then into a bridge rectifier (comprised of CR3406 to CR3409) - the -ve output of that rectifier is then connected to the circuit common - so there is a low-impedance path between the power line and the circuit common. If you look on the part of the schematic that deals with the AV inputs, you will also notice that they are fed through optocouplers - this is another strong indication that it's a hot chassis design. In fact, the whole input circuit has it's own separate common and power rail that's derived indirectly from the flyback transformer through an isolation transformer (TX9300 in this case). The only direct electrical connection between the AV input circuits and the rest of the TV is high-impedance - consisting of a 5M6 resistor and a 4.7nF cap in parallel between the AV ground and the circuit common.
wow, thanks so much for explaining all that! Thats nuts...and sounds definitely dangerous for hacking in RGB to the neck board. sounds like I would be better off to test a signal from a system running off of batteries if I do it. Or invest in an isolation transformer.
Your main problem is the design of the jungle IC and the age of the TV. My thread demostrates a successful attempt with a 1999 Samsung with isolation transformer, switch based power supply, and analog RGB inputs as outlined in the service manual. This config hits paydirt. http://assemblergames.com/l/threads/crt-tv-rgb-input-hack-attempt.62175/
Yeah, this CRT isn't so great for hacking, but I must say, I don't think the age has anything to do with it. I've successfully hacked a 1994 JVC CRT for RGB before because it met all the proper conditions (manufactured the exact same year as this Zenith TV). I think the problem lies in this being a portable 9" CRT. So it's less likely to support analog RGB because it will have a very cheap Jungle IC and it will also have a cheaper power supply. Likewise, I have a Sony CRT from 2000 that is not hackable for RGB either...it has analog RGB inputs (digital) for OSD and even analog ones, but it can only switch to analog mode via I2C bus that the jungle IC is programmed to use. So it would take an awful lot of work to make it RGB (which is a shame, cause I love it). So yeah, I think it's really hit and miss with these jungle ICs and other configurations.. -Segasonicfan
I've laid down quite a bit of information and instruction in a thread over at Shmups: http://shmups.system11.org/viewtopic.php?f=6&t=56155 Could be helpful, I cover quite a bit there.