A friend of mine, Enrico, bought this boards it was sold as working but it suffers some kind of colors issue. Wrong colors are displayed on the screen, the board graphics seem to be too green and too blue. Here’s the PCB:
… and this is a video shot of the issue:
This is due to a lack of the red component in the video signal, it’s a quite common issue on mvs boards. Usually missing or incorrect colors can be caused by a damged part in the video DAC circuit. The digital part of the video output section shouldn’t be guilty this time. The parts involved causing the issue should be spotted between a pair of 74LS273 latches, a resistor network and a 74LS05 hex buffer-inverter IC.
The usual visual inspection reveals damage in a resistor in the video DAC circuit, it’s a common issue seeing this resistors failing, it’s due to Ohm’s law: the lower the resistance the higher is the current flowing through the circuit, so the resistors with lowest values are the ones which fails first:
The 150ohm resistor is burned and must be replaced, but this may be the consequence of the culprit and not the damage itself. So take a look at the video DAC circuit taken from the schematics at NeoGeoDev.Com site:
On the following picture you can also look how the RGB colors are mangled by the board:
Measuring the resistor value reports a too low value around 24ohm instead of the 150ohm needed by the circuit, but this damage should be caused by something else, so it needs further investigations.
Once replaced the resistor …
… diplaying color bars and measured the output signal intensity of the RED component with a scope the resultulting measurements reveals an unsuitable peek-to-peek value compared to other color (RGB) signals, the BLUE signal is in yellow and the RED signal is in green (LOL), it should be a perfect match:
If you look at the color bars too you can spot a slighter low brightness of the RED color signal:
Also measuring with an ohmmeter the connection between one lead of the 150ohm RED video signal resistor and ground reveals a short circuit, instead of (about) 10Mohm resistance of the GREEN and BLUE signals, this high resistance should be the internal resistance of an IC probably damaged.
Looking at the schematics only one IC can be responsible of this bad connection, so we must take care of it, it’s a 74LS05 Hex Inverter. Promptly removed from the board:
… promptly tested, ad promptly failed, the output at pin 6 is shorted to ground (pin 7) and obviusly the relative logic unit fails the test:
I replaced the IC and tested the board …. SUCCESS!
For correct color balancing i replaced all three 150ohm resistors too:
… another case closed! Enrico should be happy this time too! 🙂