Leaderboard

The ghost from Christmas past visited the other day in the shape of SYS83 board and it's been giving me nightmares ever since. This particular board I worked on a while back (on the previous page) and I did a repair to the track. The board was returned with a fault of almost booting but not completely. On test it would come out of RESET OK and boot up to all the lockouts energising and then immediately going back into RESET ad infinitum! The problem was the watchdog was firing off but it's taken me the best part of a week to find out why. I changed all the components in the watchdog circuit but it didn't cure the fault. After chasing my tail for a few days I noted that the watchdog 100Hz waveform became very noisy as soon as the boot cycle completed and this is what tripped the RESET. This all happens in a fraction of a second so the only way to monitor it was to disable the watchdog by shorting out the base of TR3 to ground. It was then apparent there was another wave form on top of the watchdog one which was tripping the RESET. Next step was to trace it back and find out where it was coming from. The watchdog pulse is generated from IC43 and it was noisy leaving this chip. This chip had already been changed from the last repair and of course a new one made no difference. Attention was then directed to the supply rails of the chip which were also very noisy! Suspecting there could be a bad supply rail somewhere the scope was moved to the cap in the pic and guess what, no noise? Moving the ground lead of the scope to the left where the repair was done and the noise was then showing. A quick check with the Ohm meter showed it was 0 Ohms but the scope said it wasn't directly connected! So to cut a very long story short what had happened was the copper by the cap was obviously still rotting so a link wire was put across from the cap lead to the new piece of track. When checked again the noise was minimal and what do ya know it didn't trip the watchdog any more. Job done. When Jacob Marley visits again I'll repent all my sins after that one.

Doubt it's the motor. More chance of me having sex with my wife than the motor being fucked. Is it alarming? Bet it's the sensor.

Not sure why you wouldn't want auto hold on a mpu3 machine. Its part of what they are really .