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gkmaia

Mac plus probing signal at J8 pin 1

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Anyone out there with an open Mac Plus who could send an oscilloscope plot of J8 pins 1, 3 and 5?

 

Specially pin 1 which is video out. Probing it I just got a noisy LOW signal.

 

I was suspicious of my LAG IC but after probing V & H sync I got clear pulses at 60HZ & 22KHZ which seems to be ok. 

 

That will help me heaps to rule out a motherboard issue and focus on the analog board.

 

 

 

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Edited by gkmaia

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Your video signal is kinda weak. Might need to look into that. Check the CRT Neck board for a shorted video driver transistor, and/or check the logic IC on the analog board where the video is gated through. 

 

Also probe the video signal on the logic board as well. 

 

it seems as if the signal for the video is being damped for some reason. 

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Did a few more probing.

 

Lifted R30 to isolate pin #20 (Video out) of BMU2 from the analog board and everything else. Nothing is damping it... it is pretty much weak on it's own.

 

Also did some logic probing on the BMU2. All Is and I/Os are pulsing apart from I14 as it is nodded to OE. 

 

Would you narrow down to a faulty BMU2? Any other suggestions?

 

https://media.digikey.com/pdf/Data Sheets/Texas Instruments PDFs/TIBPAL20L8, R4, R6, R8-25C.pdf

 

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Edited by gkmaia

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It could be. You would have to try a known good PAL from another logic board if you had one. 

 

otherwise, you would have to burn a new PAL which I dont think the bitstream files are out there for the Plus. 

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