Sunday, 28 October 2018

Atari 400 Composite Video Mod

This is the first of a series of posts on upgrading some Atari 400 machines.
This first one is in a bit of a sorry state, so is going to need a good clean up before we're finished. The Atari 400 only came with an RF output, so the first job was testing it was working.
OK, so far so good. The PRINT FRE(0) test shows how much RAM is installed, here 13596 shows it is a standard 16K machine (more on that later).
Inside you can see there is quite a lot going on, on the right is the power input / RF output board, with the RF modulator on a separate daughter board.
In the centre is a large diecast aluminium box (like an Atari 2600), and inside that is a motherboard and two plug in cards.
One card is the CPU, the other 16K of RAM. The brother of this machine, the Atari 800 had five card slots, usually populated with one CPU, one ROM, and three 16K RAM cards, giving it 48K of RAM.
The first step here is to carry a composite video output modification. The modulator board removed, and reveals more of the board below. It looks like it was designed for a larger modulator module, similar to that seen here in an Atari 7800.
The 400 power board is quite complicated affair. At first glance, it should just take in the 9V AC power, rectify and smooth it, and then feed the 7805 to get 5V. However, it also needs 12V and -5V for the 4116 RAM chips, so has a complicated arrangement of charge pump voltage doubling diodes and capacitors to raise (and invert) the supply voltage to generate the other rails.
At the back of the board, there is a channel switch. Not sure why it's fitted on the PAL machine as it's not connected to anything. It does look like an ideal place for the composite video connection, but the heatsink is in the way.
I am planning use or the 'Deluxe Composite Video Mod' from The Future Was 8 bit, so the socket should allow the cable to be unplugged for storage.
The metalwork around it acts as a heatsink to two voltage regulators, a 7805 and a 78M12.
I could have cut a section out, but I thought I would take the opportunity to remove the need for the heatsink and install some switching regulators.
I wired it into the 5 pin header that used to connect up to the modulator board. It is important with mods like this to keep the wiring short. The cables are supplied quite long for cases where that is required, but in general, cut them as short as you can, and keep them away from anything noisy.
The composite mod cable plugs in neatly at the back of the case when it's reassembled.
The ground, 5V and composite video signals where there already. I wired the white wire for the audio. That's where it gets a little complicated. The Atari has three audio sources. The first drives the built in speaker - you don't get speakers like that in computers these days. This is mainly used for things like the loud beep when you switch on with start pressed to load a tape.
That is not connected to the TV output, so will be ignored. The second is the normal audio output, and the third is the audio from the cassette deck. The Atari uses stereo tapes. The left track contains the data, and the right channel on the tape sometimes has loading music, diagloue or silence (or if the tape duplicate company has cheaped out and used mono equipment, data).
None of this makes it to the RF output board, the two sources are mixed and then added to the combined video signal that feeds the modulator. To get around this, I have made use of one of the spare pins on the 5 way header, this connects to a track that goes to a space on the board where a vertical phono connector would have been installed for the RF cable, on boards with the larger modulator module that didn't have it's own output connector.
I fitted a 0.1" pin header in the footprint of the phono connector, and will tap the audio and connect it there, to allow the boards to be separateid in future as this is quite a modular system. (N.B., note the top quality Nichicon and Nppon Chemi-Con capatitors, these all measured well in spec and will likely still be superior to a many modern replacements, so there is absolutely no need to replace them, and I will shout at anyone who does)
And yes, that is an elastic band. The microswitch is used to detect if the cartridge cover is open, and turns off the power if it is. This was for testing without any of the metalwork.
The next step is to split the audio and video, and it's is remarkably similar to the 2600 and other Atari systems of that era. You may recognise the large red inductor from those systems. As with them, that and the associated audio subcarrier oscillator transistor below are removed.
Also removed is a capacitor (C183) that joins the audio and video sections. The white wire taps off the audio and will connect up the the output / power board.
The wires can run out a channel in the metal casing. you may just be able to see it at the bottom of this photo.
That then plugs into the pin header on the power board. Note the resistor below, R202, is also removed as this was apparently used to bypass the 5V regulator. This is no longer required with the switching regulator, and it just gets hot, so I removed it.
The microswitch I was holding down during testing has a plastic piece attached which disconnects the power when the lid is open (and the cartridge could be installed or removed).
When the lid is closed, power can be applied.
With no cartridge installed the Atari 400 brings up the Memo Pad.
This allows you to type in messages, which will be lost when the power is removed, so it's sort of pointless. On later systems this would be replaced by the built in self test.
To use BASIC, you need to install a BASIC cartridge.
But soon you get sick of that and install a different cartridge.
The 400 can should be able to play the entire range of cartridges for the 8-bit Atari range (not sure of any that it cannot).
A lot of which are very good ports of classic games.
However, when it comes to tapes, it is limited by it's 16K RAM, and a lot of cassette games will not load.
I've been testing a lot with a few games I found that did load in 16K. This is Desmond's Dungeon.
In the next part, I'll see if I can address that and give the 400 a RAM upgrade.

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