I think I might be able to manage the DAC replacement, though there are some things I need to find out first.
TBH, though it would be nice to practice it on a similar chip on a broken board. Does anyone have a similar board, junk that they could send me (id pay postage)
In other news
So I and a friend (electronics hobbyist) had a pour over the schematics, and the DACs spec sheet and here is what was found.
- The DAC requires a 5v supply to work, I measured this and its 4.84v, so it seems fine.
- The DAC requires a reference 1.2V supply, which comes from a near-by chip, I measured this, and its 1.23V, so it appears fine.
- The DAC has a compensation signal, and thats supplied by (I assume) a 580ohm resistor, so I guess if that was failing it would cause issue. Measured resistor, seems ok.
- The chip has 3 pins for RGB, I measured these and they are 5mV when the machine is booting (black screen) and then jumps to 30mV when displaying Workbench.
The 30mV value is the exact same as the pins on the DB23 monitor socket. So its clear there is no issue between the DAC and the socket.
(later I will measure the same outputs on my Amiga 500, to check 30mV is not correct)
It also appears the DAC is getting all the right inputs as well.
So it appears that the DAC is getting the correct digital signals in, but it is not then multiplying these values out, to the correct voltages?
So, what do you guys think? To me now it could be 2 things.
- Dry solder joint on DAC somewhere, causing odd behaviour.
- The DAC is dying.
Some things I could try
- Heating the DAC, in isolation, to see if its possible a chip failure. I understand warming chips up can sometimes temp fix issues. Showing it IS the chip on its way out
- Re-flow the pins for the DAC, its possible a dry solder joint is causing this.
- Replace the DAC :/