While I have the G80 test bench setup I decided to repair some other G80 sets I have.
One set was displaying corrupt graphics, usually this is a sign of a bad video ram so I removed them all and checked them in my IC tester. They all tested good so time to dig a little deeper. Looking at the schematics for the XY control board there are several 74244 chips that deal with RAM data and addressing. I probed a known good board and wrote down the results for each chip. I soon found a problem with U21 74244, once replaced the picture was better but still had problems. I decided to replace each RAM one by one, even though they had tested good. Eventually I found a bad RAM and now the picture was almost perfect. The self test reported all video ram good, bad multiplier.
There is very little info regarding this section of the control board but the multiplier circuit consists of a 25ls14 at U43, 2 x 74ls95's at U45 and U44 and a 74ls244 at U54. I suspect the 25ls14 is bad but as I don't have any spares it will have to wait for now.
Another area I wanted to look at is the X and Y outputs. On the Sega they are +and-4v and + and - 3 Volts. The circuitry is very basic post DAC when compared to the Atari Vector games. A 1k pot is used to adjust the picture size which is in series with a 3.9k resistor. I added a 1k resistor to the chain and wound the variable pot to zero.
I gradually wound up the picture size and found that it stopped increasing before the end of the POTs travel. If you look at the schematics you will see a 1n914 tied to each output and then tied to the +5v rail. this appears to be a limiter....around 4volts max. I disconnected the diodes and increased the picture size to +/-5v and +/-4v ...looking promising.
I need to increase the resistor to 2k and then try again.
I will think I will chain 2 diodes in series to the Y channel which should increase the limiter to a max of 6 and 4 to the X which should allow 8 volts. I'm guessing Sega added these in an attempt to protect the Fireball G08 monitor from over voltage in a game fault situation.Zektor2014-05-05 12:50:34
One set was displaying corrupt graphics, usually this is a sign of a bad video ram so I removed them all and checked them in my IC tester. They all tested good so time to dig a little deeper. Looking at the schematics for the XY control board there are several 74244 chips that deal with RAM data and addressing. I probed a known good board and wrote down the results for each chip. I soon found a problem with U21 74244, once replaced the picture was better but still had problems. I decided to replace each RAM one by one, even though they had tested good. Eventually I found a bad RAM and now the picture was almost perfect. The self test reported all video ram good, bad multiplier.
There is very little info regarding this section of the control board but the multiplier circuit consists of a 25ls14 at U43, 2 x 74ls95's at U45 and U44 and a 74ls244 at U54. I suspect the 25ls14 is bad but as I don't have any spares it will have to wait for now.
Another area I wanted to look at is the X and Y outputs. On the Sega they are +and-4v and + and - 3 Volts. The circuitry is very basic post DAC when compared to the Atari Vector games. A 1k pot is used to adjust the picture size which is in series with a 3.9k resistor. I added a 1k resistor to the chain and wound the variable pot to zero.
I gradually wound up the picture size and found that it stopped increasing before the end of the POTs travel. If you look at the schematics you will see a 1n914 tied to each output and then tied to the +5v rail. this appears to be a limiter....around 4volts max. I disconnected the diodes and increased the picture size to +/-5v and +/-4v ...looking promising.
I need to increase the resistor to 2k and then try again.
I will think I will chain 2 diodes in series to the Y channel which should increase the limiter to a max of 6 and 4 to the X which should allow 8 volts. I'm guessing Sega added these in an attempt to protect the Fireball G08 monitor from over voltage in a game fault situation.Zektor2014-05-05 12:50:34