Let's design a new video mod

[+batari]

The FMS6400 requires a 75ohm input impedance and a .1uf ceramic capacitor. If you look at the schematic theres a 75 ohm in series and one pulling it to ground. This sets up the input impedance. The 2n3904 does add gain to the luma signal.

I am looking at the datasheet now, and there are no resistors at input, only the 0.1 uF caps, and I see nothing at all about required input impedance. At output, there is only the series resistor, and the resistor to ground appears to be after a 75 ohm coaxial cable (which I assume means an impedance matching resistor in the television itself.) It's not clear from the diagram alone, but looking in other datasheets for other video chips and other video circuits, this seems to be correct.

 

Read the application information on the datasheet. The Luma input needs to be driven by a low impedance source or a 75ohm terminated line. The 2n3904 causes a really high impedance on the luma line so the 75ohm resistors where necessary in this case.

 

If you remove the 2n3904 from the design as is it will produce a noticeably darker image on the TV. Its mainly for increasing the current load for the luma. The DAC portion pretty much sucks all the current out of the luma signal. If you got a CD4050 that could source more current you might be able to do away with it.

OK, I see that. 75 ohm is not critical then, but low impedance is. I'll shoot for low impedance to try to reduce the part count. I could not find any reference to the current required by the chip. The 3904 does work as a current source but I'm sure it provides much more than needed.

[+batari]

Latest schematic here.

 

Resistor values may change, but this circuit addresses a few things like impedance and p-p voltage levels. Impedence is now lower but it's not clear how low is "low impedance."

 

The current output of the resistor D/A can be increased quite a bit (the 4050 can put out 20ma.)

 

Going to try this out later. If anyone else wants to also try it out, be my guest

 

EDIT: Oops, tiny mistake in the schematic.

Edited February 3, 2011 by batari

[A.J. Franzman]

Depending on whether your mod is to be a "socket" type or a wired type of installation (and if so, exactly how you intend for the user to perform the installation), you may want to put pullup resistors on the inputs to the 4050.

[LittleJoe]

I tried removing the diode from TIA 2 on LHE schematic 2.2f, and it worked fine on one older TV. But on a newer Panasonic it yielded something like inverse video, all white screen. Replacing the diode corrected the issue. Removing the 100uf cap seems to have no effect.

[+batari]

I finally had a chance to mess around with the circuit tonight and got something going.

 

I tried the circuit with and without the transistor, and it made very little difference so it was removed. The diode was also eliminated, and the resistors in the D/A were made smaller to get a better picture.

 

Interestingly, everything worked best when I buffered COL but NOT BLK, which is the opposite of the LHE mod. Some resistors were added to BLK and it was combined with COL after it was buffered.

 

The circuit is shown how I built it. Note that some resistors may change later as some were not quite the values I wanted but were just what I had on hand.

 

 

Since COL is buffered, it is now delayed so you may need to adjust the color pot to compensate. Fortunately, it easily adjusts to get perfect colors that very nearly match Stella.

 

Here is the color bar and grayscale:

 

I compared side-by-side with my incarnation of the LHE mod. The LHE mod looks pretty good, but sometimes has color bleeding on bright areas. It wasn't noticeable on my Commodore 1084 CRT but you could see it on my 42" plasma. The worst example of this I could find was the copyright text on the color bar demo which is white on black, single pixel width text. It's possible that real LHE boards aren't as bad but I did see similar bleeding on some screen pics in the LHE mod thread. The single-pixel white text appears yellow on the LHE mod and I think this has the same cause as the bleeding.

 

Anyway, this is the LHE mod:

 

This is mine:

 

It's not perfect, but I believe the remaining issues are RF interference as it was put together on a breadboard with lots of long wires and some of the artifacts change a bit when I move my hand over the wires. If anyone wants to try it out, let me know how it works.

Edited February 5, 2011 by batari

[LittleJoe]

I've put the new one together. I can tell the greyscale is good, But I've got a sync problem. A rolling B/W screen.

[A.J. Franzman]

@ Batari: Are you using the TIA outputs directly from the chip, with any associated circuitry on the motherboard disconnected? What exact part number of 4050 buffer are you using? I saw where LHE said he did some investigating and found that a 74HC4050 was much faster than the standard CMOS variety.

Edited February 5, 2011 by A.J. Franzman

[+batari]

I've put the new one together. I can tell the greyscale is good, But I've got a sync problem. A rolling B/W screen.

Probably you have pin 2 still lifted. On this mod it should not be.

@ Batari: Are you using the TIA outputs directly from the chip, with any associated circuitry on the motherboard disconnected? What exact part number of 4050 buffer are you using? I saw where LHE said he did some investigating and found that a 74HC4050 was much faster than the standard CMOS variety.

It uses the 74HC4050E which is the same part number (if not the same part) as LHE.

 

As for the connections, TIA pins 2, 5, 7, 8 should come from the board (to use the board's pullup resistors) and pins 6 and 9 (and audio pins) should be lifted.

[LittleJoe]

Using a Panasonic Plasma and getting Pin 2 from the board I have this:

 

It's a little dark, a little noisy, and needs a touch more saturation. IMHO. Excellent start.

Edited February 6, 2011 by LittleJoe

[+batari]

Are you using a FMS6400, or a 6406/6410? If using a 6400, you need to connect pin 2 of the chip to 5v or the picture will be dark.

[LittleJoe]

Are you using a FMS6400, or a 6406/6410? If using a 6400, you need to connect pin 2 of the chip to 5v or the picture will be dark.

 

FMS6406

[RevEng]

I don't know if this is a factor or not, but on some 2600s the longhorn mod seems to have an overly bright display. My 4 switcher is also one of them.

[+batari]

Very well could be, as the test unit is a 4-switch. Also, brightness and contrast may be a matter of preference so maybe these should be adjustable.

 

While messing around I found a few resistors that could be replaced with pots, which could serve as these controls. I don't remember offhand which ones but I'll look into it.

[+batari]

OK, here is a revised schematic with a couple of pots, one in place of a fixed resistor, and one added to the circuit in between the resistor ladder and the sync line. R12 should control contrast and R13 should control brightness. I don't have any pots on hand but I think 1k should be more than enough, or maybe even 470 ohms.

 

I didn't have 1k pots to actually test this (just really large pots) but will pick some up in a few days. I expect that the two adjustments probably won't be totally independent and turning the brightness too far down could cause the television to lose sync.

[Longhorn Engineer]

I don't know if this is a factor or not, but on some 2600s the longhorn mod seems to have an overly bright display. My 4 switcher is also one of them.

 

That was before I switched to the higher resistor values for the DAC resistor ladder. It has been fixed in the newer versions of the board.

 

Parker

[LittleJoe]

I discovered that I had a bad connection somewhere in my build. Perhaps more than one. But now I now have a very excellent s-video output. Colors are great. There's no bleeding whatsoever. The only problems I see are a slight vertical banding and composite is bit darker than s-video. I've not installed the pots yet, but I'll play with those soon.

[Longhorn Engineer]

I discovered that I had a bad connection somewhere in my build. Perhaps more than one. But now I now have a very excellent s-video output. Colors are great. There's no bleeding whatsoever. The only problems I see are a slight vertical banding and composite is bit darker than s-video. I've not installed the pots yet, but I'll play with those soon.

 

Check to see what the vertical banding is in by disconnecting the Chroma or Luma. Vertical banding is usually caused by a over amped chroma line or the luma not being buffered (lack of 2n3904 could cause this).

[+batari]

Could be something in the circuit, but it's worth checking if other televisions have the same banding. Also, if the banding is at the same frequency as you pic you posted above, it looks like it might be the same frequency as the 6507 oscillator (1.19 Mhz) so it could just be interference and caps might help that.

 

The 0.1uF caps should be placed within 0.1" from the FMS64xx and 4050. Also, the FMS64xx apparently does need a 10uF cap placed less than 0.75" from it, though the 100uF cap is probably unnecessary.

[HARMIK]

Batari when you say “though the 100uF cap is probably unnecessary” is that to reduce cost or is it to improve quality. If someone put it in there I would have thought it was in there for a reason. I personally would like the best quality Mod possible and would not care if it cost a bit more. Like the faster whatever chip.

 

I am asking because I am genuinely interested and confused not trying to be smart.

[+batari]

The 100uF cap was specified in the original CD4050 mod (which dates back to the mid-90's) and its purpose was never questioned.

 

Also, my purpose with this mod is to make something simple, possible to build by hand, but still with very high quality output.

 

So maybe it will be possible to produce a mod that uses 3x the parts, costs $100 and gives almost unnoticeably better output, but I'd guess you'd be one of the few to buy one.

[Longhorn Engineer]

The 100uF cap was specified in the original CD4050 mod (which dates back to the mid-90's) and its purpose was never questioned.

 

Also, my purpose with this mod is to make something simple, possible to build by hand, but still with very high quality output.

 

So maybe it will be possible to produce a mod that uses 3x the parts, costs $100 and gives almost unnoticeably better output, but I'd guess you'd be one of the few to buy one.

 

I used the 100uF cap because that is the normal capacitor rating for your power rail in something the size of the video mod. Its convention if anything. You can never know what kind of power that 7805 that is 30 years old is putting out.

[+batari]

The 100uF cap was specified in the original CD4050 mod (which dates back to the mid-90's) and its purpose was never questioned.

 

Also, my purpose with this mod is to make something simple, possible to build by hand, but still with very high quality output.

 

So maybe it will be possible to produce a mod that uses 3x the parts, costs $100 and gives almost unnoticeably better output, but I'd guess you'd be one of the few to buy one.

 

I used the 100uF cap because that is the normal capacitor rating for your power rail in something the size of the video mod. Its convention if anything. You can never know what kind of power that 7805 that is 30 years old is putting out.

I can't imagine this board using more than 10ma or so, and 100uF seems overkill for that. Besides, the biggest concern is high frequency noise, not the low frequency voltage ripple. Larger caps also don't do well at bypassing high frequency noise because they can't charge and discharge fast enough, and smaller caps can.

 

Regardless, if I find a 100uF cap gives any appreciable difference, I'll use one.

 

EDIT: just measured and everything but the FMS64xx chip takes about 10ma, but the FMS64xx chip is a power hog, taking 60ma on its own. However, the datasheet for the 6406 indicates that 10uF is all you need.

Edited February 9, 2011 by batari

[+batari]

Today I tried out a ML6428CS1 IC bought from here (they have over a thousand in stock):

 

http://www.onlinecomponents.com/fairchild_ml6428cs1x.html?p=11366096

 

Someone said in another thread that these were "slightly inferior" because their filters are 4th instead of 5th order, but I found the output quality to be quite good, maybe even a bit better than the FMS64xx.

 

FMS6406:

 

ML6428:

 

So, these look like a suitable replacement if we can't get the FMS chips.

[LittleJoe]

Check to see what the vertical banding is in by disconnecting the Chroma or Luma

 

I pulled the chroma from the s-video jack and I've got banding on the B/W output.

 

The 0.1uF caps should be placed within 0.1" from the FMS64xx and 4050.

 

I've got a .1uf and a 10 uf cap right on the power pin of the FMS64xx, but the caps are not near the 4050's power pin.

[BillC]

Today I tried out a ML6428CS1 IC bought from here (they have over a thousand in stock):

 

http://www.onlinecomponents.com/fairchild_ml6428cs1x.html?p=11366096

 

Someone said in another thread that these were "slightly inferior" because their filters are 4th instead of 5th order, but I found the output quality to be quite good, maybe even a bit better than the FMS64xx.

 

FMS6406:

 

ML6428:

 

So, these look like a suitable replacement if we can't get the FMS chips.

I was the one who said they were inferior when I was pointing out their availability at Online Components. I wrote that because I have no personal experience with the chip, only reading the datasheet and comparing it to the one for the FMS6400, and didn't want to mislead anyone if they didn't produce an acceptable result. I'm happy that they did prove to be an acceptable replacement for the FMS6400.

 

I notice since my posting, #764 in the Atari A/V mod update thread, about quantities available at Online Components that all 85 of the ML6428CS1 have been sold. IIRC they were only $0.18ea. which was a bargain, I would have ordered some but the minimum purchase for international orders is $100.00.

 

The link batari posted is for the ML6428CS1X they do still have in stock .