Hi folks, I'm having issues with a macintosh classic analog board, part 820-0395. The symptom I'm having is lack of vertical sync, so I'm getting a bright horizontal line.
I've replaced the TEA2037, TDA4605, and CNY17 chips which seem to be the most common culprits and I'm still having the issue.
I've noticed the following: 1/input voltage on the TDA4605 pin is measuring only 9 volts when the schematics say 12v, and 2/The transformer output (measured at pin 8) is 0, but 3/on the other side of DP5 I'm measuring 21v (where the schematics say it should be 38v). The is 0v at pin 8 really really confusing to me because I am getting voltage later in the circuit so I'm not getting my head around whats happening there...maybe a backfeed short somewhere else? My current theory is that the high voltage isn't getting generated and this is leading to the lack of vertical sync, but I'm not that experienced with debugging analog issues so really open to ideas here.
Does anyone else have any advice on where else to look? I can poke around with an old oscilloscope and a multimeter if more information would be helpful.
In case it helps: I do get 5v and 12v to the drive connector and main board and I get the startup bong. Also, the first few days after the vertical sync issue first started it would come back with some 'percussive maintenance' by hitting the side of the Mac...but it no longer responds to this.
Also, in pulling out components one by one to test them, I damaged DP-8 so I need to source a replacement. The original diode is labeled PH-73d. Its missing from the Bomarc schematic for 820-0395 but *is* on the schematic for 820-0525 and is labeled 'BY 73d'. Problem is I'm having trouble finding a replacement. It seems to be a pretty standard power rectifier; anyone have any advice on how to choose the correctly rated equivalent? Anyone got an old phillips diode catalog or datasheet book to look up PH-73d?
Thanks,
Don
I've replaced the TEA2037, TDA4605, and CNY17 chips which seem to be the most common culprits and I'm still having the issue.
I've noticed the following: 1/input voltage on the TDA4605 pin is measuring only 9 volts when the schematics say 12v, and 2/The transformer output (measured at pin 8) is 0, but 3/on the other side of DP5 I'm measuring 21v (where the schematics say it should be 38v). The is 0v at pin 8 really really confusing to me because I am getting voltage later in the circuit so I'm not getting my head around whats happening there...maybe a backfeed short somewhere else? My current theory is that the high voltage isn't getting generated and this is leading to the lack of vertical sync, but I'm not that experienced with debugging analog issues so really open to ideas here.
Does anyone else have any advice on where else to look? I can poke around with an old oscilloscope and a multimeter if more information would be helpful.
In case it helps: I do get 5v and 12v to the drive connector and main board and I get the startup bong. Also, the first few days after the vertical sync issue first started it would come back with some 'percussive maintenance' by hitting the side of the Mac...but it no longer responds to this.
Also, in pulling out components one by one to test them, I damaged DP-8 so I need to source a replacement. The original diode is labeled PH-73d. Its missing from the Bomarc schematic for 820-0395 but *is* on the schematic for 820-0525 and is labeled 'BY 73d'. Problem is I'm having trouble finding a replacement. It seems to be a pretty standard power rectifier; anyone have any advice on how to choose the correctly rated equivalent? Anyone got an old phillips diode catalog or datasheet book to look up PH-73d?
Thanks,
Don