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[kick52]

Picked this up at a car boot sale for £5.
It has a dedicated sound chip, so you can swap around the different channels etc. Reminds me of circuit bending drum-machines. One oddity with this is that the sound chip also handles the controls. If you short some pins on the chip, it will act as if you pressed the 1 button etc. You can hook up the RAM to the sound chip to get glitch music.
I made a video of myself probing the sound chip: http://www.youtube.com/watch?v=frMGIqFbCCI
You can glitch up the video by shorting the RAM/VRAM chips, though the trouble with this is that the CPU nearly always crashes, which is quite annoying, as you can't glitch the sound afterwards. I've tried shorting the video chip, sometimes getting some glitches without crashes, but I can't find that bend anymore, and I'm not sure if it was just a chance event. Sometimes, while shorting the VRAM/RAM/video chip the video chip will crash, which is also annoying. (Must find some low pots)
There are some other interesting bends I've found:  a pause/occasional glitch without crash, various sound channel changes around the sound out port, and some strange glitch on the CPU somewhere (looks like a clock bend, I'll have to trace it)
You can short the cartridge pins, though I've only seen crashes, and no stable glitches.

Some pictures:

http://kick52.com/dump/bend/gx4000/1.jpg
http://kick52.com/dump/bend/gx4000/2.jpg
http://kick52.com/dump/bend/gx4000/3.jpg

[Circuitbenders]

What is that synth chip, it sounds very familiar.

[kick52]

The sound chip is an AY-3-8912.

Hmm, it looks like what I thought was the graphics chip is actually the CPU (Z80a), and what I thought was the CPU (which is a SMD) is a ROM chip or something. I still don't know what this Amstrad 40464 chip does though.

EDIT: http://en.wikipedia.org/wiki/AY-3-8912

"The 8910 and its variants became popular chips in many arcade games, and was used on, among others, the Intellivision and Vectrex video game consoles and the MSX, Atari ST, Amstrad CPC, Oric 1, Colour Genie, Elektor TV Games Computer and Sinclair ZX Spectrum 128/+2/+3 home computers as well as the Mockingboard sound card for the Apple II family. It was also produced under license by Yamaha (with minor modifications, I.E. a selectable clock divider pin, and a double-resolution but double-rate volume envelope table) as the YM2149."

[kick52]

Ugh, it's broke.
I don't think I caused it.
Gradually, it started to loose audio, it seemed to be something to do with the cartridge, as I could wiggle it and the sound would be OK.
Though then it started to play the game on its own, and no amount of cartridge wiggling could stop it.
I even made sure the pins on the board were connecting properly, though now that doesn't seem to be the problem, as the sound chip won't work at all.

[computer at sea]

Drag.  That was sounding pretty sweet.

[Gordonjcp]

AY-3-8912s are pretty fragile chips.  They're easy enough to find, though.

They used to be used in Atari STs for the three channel sound and also some additional I/O lines.  I used to have a stock of spares, along with spare floppy controllers, because people used to plug Amstrad 1640 monitors into their STs - the power plug was identical to the ST floppy connector, so this would take out the FDC and the sound chip.  Why the sound chip?  Because one of the GPIO pins on the sound chip was used as floppy select!