A while back I was repairing a Milton Bradley / GE Vectrex console. It had the infamous ‘white dot’ issue – just a white dot in the centre of the screen. Audio is generally okay when you have this issue. It’s normally caused by a couple of things:
- A faulty MC1408P8 DAC chip
- an issue with D503
- An issue with the LF347
- Capacitors (it’s always capacitors eh?)
In this case, the DAC was indeed faulty. As it’s harder to get the original Motorola MC1408P8 chip, you can use the TI version of the same chip – the DAC0808. Only, as I discovered, you can’t always. Using this chip, the -13v line will not come up at all – but it you switch the Vectrex on, then unlplug the internal J204 power connector and plug it back in, the display will come on and all is well in the world.
Strange, huh?
Turns out that there are some differences between the newer TI variant of the 1408 and the original Motorola 1408. I’m not sure looking at the data sheets for either chip what it is, but I can confirm (after buying some original NOS MC1408P-8 chips) that it does indeed make the issue go away in those Vectrex that show this issue when replacing the 1408. The -13v line comes up just fine… Put the TI chip back in the socket and the problem reappears..
If anyone has any ideas why this might be the case, please let me know.