A400 Composite Mod

[+orpheuswaking]

Okay I have the mod and just have a quick question... The Pokey chip lead has a easy connector on it... Did you use it that way, or cut it off and solder direct?

 

Thanks!

 

(I'll be working on this in a few weeks, and will document it also)

 

 

I soldered a wire from R160 to ourside the RFI shield, and then connected the EZ hook to the wire there and wrapped it in electrical insulation tape.

 

My reasoning was that it was not possible to insert the entire EZ hook assembly through the hole in the RFI shield, and so running a line out seemed simpler.

 

Throughout the installation I was trying very hard not to directly hack the 5200 mod board, since if the upgrade didn't work, I was planning on trying to re-sell the mod to a 5200 owner. This was also the reason for the 40-pin IC socket cradle I made instead of soldering the 8 GTIA lines directly onto the mod.

 

Since I now know the mod does work, you could cut the audio line, feed the cut wire through a hole in the RFI shield, and then re-attach it. But if you use the EZ hook to attach to pin 37 of POKEY, you will have to carefully place the plastic part of the hook so it does not get dislodged when you re-assemble the 400. I didn't like the idea of that large hook assembly "floating about" inside the 400

 

HTH

 

Good to know... I was thinking the same thing about not wanting it "floating about"... I know I'm not going to try and resell it so I may just cut it and solder direct.

 

Not sure when I'll get around to it, but I've downloaded all the photos and pasted the info into a word doc.

 

I'll be sure to update when I do however!

[+orpheuswaking]

ARGHHHH My CPU board has the GTIA in the middle and I get lost at step 4...

 

Can't see the line I need to tap into for L0 as it's completely different...

 

Here is my board (funnily enough I have THREE 400's and all of them have this board - figures right)

 

[+orpheuswaking]

Random thought... can I just solder direct to the pins referenced in the write up?

 

[edit]

 

been talking to people who know about these things lol... Apparently I can go direct to each pin on the GTIA (which is ok as I have spares if need be)...

 

HOWEVER, I'm going to see if I can find better points on the board to tap into like the previous write up (Using VIA's and Resistor legs etc)...

 

hopefully I'll pull it off

Edited September 23, 2010 by orpheuswaking

[Rybags]

I've got some doubts about those instructions on the previous page.

 

Some of the pins don't seem to have any correspondence to those printed in the Hardware Manual schematics, or these ones:

 

400.zip

[+orpheuswaking]

I see

 

Color

Csync

Luma 0,1,2,3

+5

 

and

 

Audio from Pokey...

 

They all seem to match up from what I can see, or am i missing something?

[+orpheuswaking]

Okay I'm half way there I think... I used my DVM to figure out what pins correspond to which traces and found substitute solder points... Obviously direct solder to pins is an option too, but that can get messy and ugly quickly...

 

Once I get this all together I'll post up some new photos

[mimo]

is there anywhere on the 400 to just pick up the composite signal and then add a small amplifier like the simple atari 7800 one?

 

http://www.atariage.com/forums/topic/173828-7800-pal-b-composite-vout-scheme/page__view__findpost__p__2162134

[puppetmark]

is there anywhere on the 400 to just pick up the composite signal and then add a small amplifier like the simple atari 7800 one?

 

http://www.atariage....ost__p__2162134

 

looking at the Atari 400 schematics, it should be possible. You can pick up composite video at R171 or pin 1 of J107 and feed it to an amp, such as the 7800 one, and that should give you composite video out. I don't have a 400, so I am not sure which side of R171 you need to use to get the signal, or where J107 is. Removing R161 and R164 should disable the RF from the audio and video out and may be necessary to remove RF interference. Audio can be picked up at the junction of R161 and C164 and should just need a resistor in between the junction and audio out.

 

I have no way of testing this, but it should work, or at least give someone a starting point to try.

[mimo]

Thanks, I think I will give it a shot

[+orpheuswaking]

If you figure it out post a schematic

[BillC]

is there anywhere on the 400 to just pick up the composite signal and then add a small amplifier like the simple atari 7800 one?

 

http://www.atariage.com/forums/topic/173828-7800-pal-b-composite-vout-scheme/page__view__findpost__p__2162134

I have looked at the schematic, and doing this appears to be fairly simple. The file attached shows the portion of the 400 schematic dealing with the video output, I have labeled the traces for the different signals. This assumes these are standard chroma/luma/audio signals being combined for output to the RF modulator

 

Simply disconnecting the audio at R164 should mean that the signal sent to the RF modulator on the power board would be composite video. Also disconnecting the Chroma signal at R171 should result in only a Luma signal to the RF modulator.

 

The disconnected signals would need to be extended outside the shield and hooked up to output jacks, these signals are no longer passing through inductor L118, I don't know whether an inductor should be used with these signals or not. The composite or luma signal from the input to the RF modulator would also need to be connected to the output jack as well.

 

If the RF modulator is not going to be used I would suggest disconnecting power from it to reduce RF interference. A switch for power to the RF modulator could be installed, still allowing it to be used when needed. Remove RF cable if modulator is no longer going to be used.

 

If separate Chroma/Luma have been used a switch could be installed to connect them for composite video, this should also give a color signal from the RF output if still enabled. Another switch would allow connecting the audio to the RF modulator as well if needed.

 

The audio signal from R164 combines the outputs from both the Pokey audio output and Audio_In from the SIO. This was used in cassette based games, such as Moonbase IO, to allow the player to be given instructions/updated objectives from the audio track. This feature would be lacking from modifications which connect directly to the Pokey audio out.

[BillC]

is there anywhere on the 400 to just pick up the composite signal and then add a small amplifier like the simple atari 7800 one?

 

http://www.atariage....ost__p__2162134

 

looking at the Atari 400 schematics, it should be possible. You can pick up composite video at R171 or pin 1 of J107 and feed it to an amp, such as the 7800 one, and that should give you composite video out. I don't have a 400, so I am not sure which side of R171 you need to use to get the signal, or where J107 is. Removing R161 and R164 should disable the RF from the audio and video out and may be necessary to remove RF interference. Audio can be picked up at the junction of R161 and C164 and should just need a resistor in between the junction and audio out.

 

I have no way of testing this, but it should work, or at least give someone a starting point to try.

I missed reading you reply before I made my response in post# 36. I agree with you on some points, disagree on others.

 

If you disconnect R161 and connect audio out to C164 you lose AUDIO_IN from SIO, and connect before Q105 which I presume is an amplifier, this portion of the schematic is not included in the image I included with post# 36. All audio signal processing is completed before R164, this is where it should be disconnected and used as the audio source.

 

Without the audio the signal to the RF modulator should be composite video.

If you want separate chroma/luma instead of composite just disconnect R171.

 

J107 is the connection between the 400 Main Board and Power Board, the RF modulator is on the power board.

 

I have never gotten around to doing this, in spite of having 2 400s, I just have so many other systems which do have video out.

[puppetmark]

I missed reading you reply before I made my response in post# 36. I agree with you on some points, disagree on others.

 

If you disconnect R161 and connect audio out to C164 you lose AUDIO_IN from SIO, and connect before Q105 which I presume is an amplifier, this portion of the schematic is not included in the image I included with post# 36. All audio signal processing is completed before R164, this is where it should be disconnected and used as the audio source.

 

Without the audio the signal to the RF modulator should be composite video.

If you want separate chroma/luma instead of composite just disconnect R171.

 

J107 is the connection between the 400 Main Board and Power Board, the RF modulator is on the power board.

 

I have never gotten around to doing this, in spite of having 2 400s, I just have so many other systems which do have video out.

 

I looked at the schematic again and I think you are correct about the audio connection. To me, it looks like Transistor Q105 is setup as an oscillator, So I think it is part of the RF circuitry. It looks like both audio and video connect to this oscillator, perhaps some kind of signal combining? Anyway, I was trying bypass it. Q106 is setup as an amp for the SIO audio in, so that should not be bypassed.

 

Thanks for the clarificaton. Like I said, It's a starting point.

Edited January 19, 2011 by puppetmark

[BillC]

I missed reading you reply before I made my response in post# 36. I agree with you on some points, disagree on others.

 

If you disconnect R161 and connect audio out to C164 you lose AUDIO_IN from SIO, and connect before Q105 which I presume is an amplifier, this portion of the schematic is not included in the image I included with post# 36. All audio signal processing is completed before R164, this is where it should be disconnected and used as the audio source.

 

Without the audio the signal to the RF modulator should be composite video.

If you want separate chroma/luma instead of composite just disconnect R171.

 

J107 is the connection between the 400 Main Board and Power Board, the RF modulator is on the power board.

 

I have never gotten around to doing this, in spite of having 2 400s, I just have so many other systems which do have video out.

 

I looked at the schematic again and I think you are correct about the audio connection. To me, it looks like Transistor Q105 is setup as an oscillator, So I think it is part of the RF circuitry. It looks like both audio and video connect to this oscillator, perhaps some kind of signal combining? Anyway, I was trying bypass it. Q106 is setup as an amp for the SIO audio in, so that should not be bypassed.

 

Thanks for the clarificaton. Like I said, It's a starting point.

I checked a copy of the original schematic and the Q105 portion is labeled "SOUND OSC", while the Q106 portion is labeled "AUDIO MIXER", but it does appear that Q105 could affect the video circuit.

 

I also looked over the "Little Brother Grows Up" article, and noticed that it tapped into the luma signals at the resistors on the output of the A111 4050 buffer then fed these signals into another 4050 buffer in their video output circuit. The only reason to do this is to keep the stock output to the RF modulator while adding the video out port. I also found a similar 400 A/V mod, PDF attached, which does the same. With both of these mods the 4050 buffer could be eliminated, just use the one already on the 400 board, if not retaining the stock video signal to the RF modulator.

atari_400_A-V_mod.pdf

[mimo]

wow, thanks for the detailed replys

[puppetmark]

I checked a copy of the original schematic and the Q105 portion is labeled "SOUND OSC", while the Q106 portion is labeled "AUDIO MIXER", but it does appear that Q105 could affect the video circuit.

 

I also looked over the "Little Brother Grows Up" article, and noticed that it tapped into the luma signals at the resistors on the output of the A111 4050 buffer then fed these signals into another 4050 buffer in their video output circuit. The only reason to do this is to keep the stock output to the RF modulator while adding the video out port. I also found a similar 400 A/V mod, PDF attached, which does the same. With both of these mods the 4050 buffer could be eliminated, just use the one already on the 400 board, if not retaining the stock video signal to the RF modulator.

 

OK, that makes more sense. The audio mixer must combine the audio sources, butI am still not clear about the necessity of the sound oscillator. It must have something to do with the input to the RF modulator circuit, Does the modulator have separate Audio and video inputs?

 

I figured, that's why those articles added a second buffer, but personally, I never saw the need to keep the RF. Maybe back in the day, when you might have taken your computer to friends house that didn't have a monitor or TV with AV inputs, but today, I think removing the RF modulator is the way to go.

[+bob1200xl]

A TV gets sound from an FM signal centered on a 4.5mhz carrier IF. (the video is AM on a 45mhz IF) The oscillator is the 4.5mhz audio carrier, I would expect. See the capacitor in series with the 'audio' line? It's 'way too small to pass audio - 4.5mhz would get thru, though.

 

On another note, the existing circuits in a 400 do not have enough drive to interface directly with a monitor video input. You need a buffer.

 

Bob

 

 

 

I checked a copy of the original schematic and the Q105 portion is labeled "SOUND OSC", while the Q106 portion is labeled "AUDIO MIXER", but it does appear that Q105 could affect the video circuit.

 

I also looked over the "Little Brother Grows Up" article, and noticed that it tapped into the luma signals at the resistors on the output of the A111 4050 buffer then fed these signals into another 4050 buffer in their video output circuit. The only reason to do this is to keep the stock output to the RF modulator while adding the video out port. I also found a similar 400 A/V mod, PDF attached, which does the same. With both of these mods the 4050 buffer could be eliminated, just use the one already on the 400 board, if not retaining the stock video signal to the RF modulator.

 

OK, that makes more sense. The audio mixer must combine the audio sources, butI am still not clear about the necessity of the sound oscillator. It must have something to do with the input to the RF modulator circuit, Does the modulator have separate Audio and video inputs?

 

I figured, that's why those articles added a second buffer, but personally, I never saw the need to keep the RF. Maybe back in the day, when you might have taken your computer to friends house that didn't have a monitor or TV with AV inputs, but today, I think removing the RF modulator is the way to go.

[puppetmark]

A TV gets sound from an FM signal centered on a 4.5mhz carrier IF. (the video is AM on a 45mhz IF) The oscillator is the 4.5mhz audio carrier, I would expect. See the capacitor in series with the 'audio' line? It's 'way too small to pass audio - 4.5mhz would get thru, though.

 

On another note, the existing circuits in a 400 do not have enough drive to interface directly with a monitor video input. You need a buffer.

 

Bob

 

Thanks Bob. Yep, I see it now. Something like the 600XL composite video mod AMP should be enough to drive most monitors or TVs, I would think:

 

[mimo]

?

[+bob1200xl]

Yes - something like that.

 

Bob

 

 

A TV gets sound from an FM signal centered on a 4.5mhz carrier IF. (the video is AM on a 45mhz IF) The oscillator is the 4.5mhz audio carrier, I would expect. See the capacitor in series with the 'audio' line? It's 'way too small to pass audio - 4.5mhz would get thru, though.

 

On another note, the existing circuits in a 400 do not have enough drive to interface directly with a monitor video input. You need a buffer.

 

Bob

 

Thanks Bob. Yep, I see it now. Something like the 600XL composite video mod AMP should be enough to drive most monitors or TVs, I would think:

 

[BillC]

I read the Atari A/V mod update thread here, and also looked through the Longhorn Engineers web-site about video mods for the 2600/7800 which have similar video circuits. There is relevant information for modifying a 400.

 

The standard CD4050 IC creates a delay that causes the Luminance signal to be out of sync with the Chroma signal. Adding a second 4050 buffer to the output of the first, like some 400 mods suggest, would aggravate this problem. The resolution is to replace the stock CD4050 with a 74HC4050, which causes a much shorter delay.

 

Before using a 74HC4050 in the socket on the 400 mainboard, it should be confirmed that pin 1 is connected to +5V or changed if connected to +12V. The voltage supply of the CD4050 isn't listed on the schematic, this is only necessary on a 400 or 800 since they are the only 8-bits with a +12V supply.

 

The Longhorn Engineer mods use a FMS6400 IC to create composite video, this uses Chroma/Luma inputs to create Chroma, Luma and Composite outputs. This reduces/eliminates Chroma signal feedback to the Luma signal while allowing Composite video output.

 

There can be problems finding the FMS6400, the FMS6406 or FMS6410 are substitutes, the only difference is the connection for pin 2. These are only available as surface mount devices.

 

FMS6400 - pin 2, connect to +5V to set output to +6DB

FMS6406 - pin 2, Composite video output to RF modulator/driver.

FMS6410 - pin 2, not connected