512K Flup Flup Flup and Schottky diodes

[BadGoldEagle]

Hi

I'm currently trying to troubleshoot a 512K that has a bad 120V analog board. 

The display keeps cutting out every half a second or so and it makes the dreaded flup flup noise. It happened out of the blue and when it's starting to get hot. The dead Mac scrolls points to bad CR20 & CR21 Schottky diodes. 

I tested them but I am not getting the same results as Larry Pina... CR20 should be 31Ω/30Ω. I'm getting 107Ω/84Ω. CR21 should be 52Ω/53Ω and I'm getting 41Ω/43Ω. I got those readings by switching the red/black terminals on the multimeter between the cathode and the diode. Is that how I'm supposed to test them? Shout I try when they're hot or cold? And can you really test diodes in-circuit? 

Thanks!

 

[ArmorAlley]

Are you getting a startup bong? I am but I'm getting any no video at all on my 512K. I had the flup-flup-flup earlier today but now nothing.

I'm currently going through the Dead Mac Scrolls too and the suggestions that seem most helpful so far are without a startup bong and point to the analog board.

 

[techknight]

You cant reliably read Diodes in resistance mode. 

you need to use Diode check mode and check the voltage drop across them. Best test is out of circuit. (by removing them).