Hi.
I have been working on fixing up an old Macintosh 512k for a while now (as of now I've cleaned up the exterior and completed a full recap of all of the boards), but I've hit something of a dead end. According to my oscilloscope, the HSYNC and VSYNC signals coming off of the motherboard seem to be incorrect (the video signal is OK though).
I've checked for shorts, and strangely it does appear that there is some connection between HSYNC and VSYNC as they have a resistance of 250 ohms from one another (30 ohms when measuring before R44). I'm not sure what to make of this, as looking at the schematic I see little reason why those would be connected at all. If anyone has any info on that front I would appreciate it so I can rule out a short.
Other than that there seems to be nothing wrong with the Macintosh. The CRT filament glows so I know I have high voltage, all of the voltages coming off of the power supply seem to be correct, and I do also get a startup chime. The issue seems to purely lie somewhere in the sync circuitry on the logic board.
If anyone has any ideas of what may be causing this I'd greatly appreciate it.
Thanks!
I have been working on fixing up an old Macintosh 512k for a while now (as of now I've cleaned up the exterior and completed a full recap of all of the boards), but I've hit something of a dead end. According to my oscilloscope, the HSYNC and VSYNC signals coming off of the motherboard seem to be incorrect (the video signal is OK though).
I've checked for shorts, and strangely it does appear that there is some connection between HSYNC and VSYNC as they have a resistance of 250 ohms from one another (30 ohms when measuring before R44). I'm not sure what to make of this, as looking at the schematic I see little reason why those would be connected at all. If anyone has any info on that front I would appreciate it so I can rule out a short.
Other than that there seems to be nothing wrong with the Macintosh. The CRT filament glows so I know I have high voltage, all of the voltages coming off of the power supply seem to be correct, and I do also get a startup chime. The issue seems to purely lie somewhere in the sync circuitry on the logic board.
If anyone has any ideas of what may be causing this I'd greatly appreciate it.
Thanks!