Acorn Atom: Restoration Part 2

In the previous part of this series, I got the Acorn Atom working. But I don’t just want it working, I want it in a good, usable condition. So, there is a little more work to do. Here is where I am at.

Noise killer

David Banks from the RGBtoHDMI project had sent me a couple of PCBs for this Atom. The first is a “Noise Killer” and the second is an RGBtoHDMI.

The noise killer will remove random noise on the video output that is caused by timing issue with the CPU accessing the video RAM. In addition, it replaces all the 12x 2114 video RAM ICs with a single 8KB RAM chip. This is a good thing because the 2114s are very power hungry.

I assembled the noise killer board, which was relatively simple to do.

I then removed all the video RAM from the Atom, as well as two of the logic chips, and fitted this board in their place.

The machine worked fine with this in and has gone down to just over 1A being drawn from the PSU, down from 1.7A. Changing the RAM made a big difference! The board is worth it for that alone.

Resetting expectations

As mentioned in the previous post, the power on reset was not working, and this was a basic RC timing circuit.

I removed the capacitor and gave it a test to see if it had degraded.

The capacitor was completely open circuit, my Peak ESR70 couldn’t even detect there was a capacitor there. Which might explain why the power on reset was not working.

I replaced this with a 47uF axial capacitor, as I have lots of these available from repairing other Acorn machines, and it will hold the reset a little bit longer, making sure the reset works correctly.

After this, the reset triggered correctly on power up every time.

RGBtoHDMI

Now to assemble the HDMI board. This required a small amount of SMD soldering and quite a few passive components.

Unlike other RGBtoHDMI boards, this one needs the CPLD to be programmed using the JTAG header, this is easy enough and I did it whilst assembling using the Tigard board in this blog post.

This then connected to the video header I was using for composite output. I powered this up and…

Well, it is an image, but I was expecting an Atom prompt. I tweaked the variable resistor labelled RV1 on the board, which I believe is the black level detection. I only need to tune this a little bit and I got a prompt!

Excellent. Next up we need tune the colour sync. There is a program you can type in which will generate a test patten to sync this. Instructions are in the RGBtoHDMI Wiki. Once I typed this in and ran it, I got this test image.

You can see the bottom colour is a weird mix of yellow and orange, the colour there wasn’t entirely stable either. I tweaked RV2 until the sync was corrected for this. I also went into the RGBtoHDMI settings and changed the border colour to black. This produced a perfect image.

Power Supply

I happened to have a spare Mean Well GS18A05 5v, 3A power supply. This is plenty for the Atom, and it is centre positive too, which is ideal. Unfortunately, it came with a standard 5.5mm / 2.1mm barrel connector. Whereas the Atom uses a 5.0mm / 2.5mm barrel connector.

I could have resolved this by replacing the connector on the Atom with a more standardised size. But instead I happened to have the correct converter in this kit. These combined give me a long-term power supply for the Atom.

Loading tapes

There is a cassette port on the Acorn Atom, and whilst it looks like a standard Acorn cassette port as found on BBC computers, it is a little different. This is a BBC port:

Now, the Atom has the same 7 pin connector. But it’s pinout is slightly different.

As the BBC, pin 1 is cassette out, pin 2 is 0v and pin 3 is cassette in. But on the BBC pins 1 and 4 are linked together and the BBC has a motor control. Whereas on the Atom the four remaining pins are GPIO pins.

I contacted CoolNovelties and asked if their BBC cable was wired in such a way that it was compatible. I sent them some diagrams and photos and they immediately created a cable to work with the Atom. I ordered it and it is here.

I hooked this up to a cassette player I bought to backup tapes via USB to my laptop. The player also has a headphone socket, so I tried connecting to this. I then attempted to load a game.

With the Atom, you cannot hear the cassette playing through the speaker and you do not get much visual feedback. You type in the LOAD command, it prompts you to play the tape and when you do, you hit the space key to acknowledge. You can also run *CAT to list the files on the tape. This is how I knew things were working. Unfortunately loading a game did not work, checksum errors were hit.

I tried a different game, just in case it was the tape at fault. Tweaking the volume every time.

No joy. It could be these tapes, it could be that I couldn’t find the correct volume, or it could be that the tape player’s audio port is not good enough quality. I’ll be revisiting this later.

Next steps

The key things left are cleaning the case and the ability to load software. In addition I have some lower power ICs coming to try so that this machine runs a bit cooler. With any luck I’ll have everything done for the next post!