Mac 512 rectifier question

[elbaroni]

Hi all

I have a week's leave, so I'm going through my collection of compacts working out what works and what doesn't. I have a nice 512 (M0001ED) which, when switched on, makes what I think is the noise Larry Pina describes as "flup flup flup". No sign I can spot of leaky capacitors. For what it's worth I have an Australian board - 240v, model number 630-0108. Or 820-0107-D. 

Page 24 of Dead Mac Scrolls advises checking CR20, which I gather is this guy:



Checking the resistance I get either 0 or about 80 Ω. Not being enormously confident in my resistance checking ability, I was hoping for a sanity check. Pina says it should be 31/30Ω, not 0. He says nothing of 80. 

I'm putting one lead on the nut/washer you can see in the picture above, and one on the solder patch on the back of the analogue board indicated below. Is that the right way to approach this? Or should I be bridging two of the solder points? Grateful for any advice from anyone less dumb at this than I am (which is probably everyone). 

 

[superjer2000]

I always just replace those matter of course. But my last 512k did flup flup because of a blower fuse in the logic board connector. 

 

[elbaroni]

Thanks Superjer — I’ll give the cable a go. Might be lucky. 

 

[elbaroni]

No dice, unfortunately. The cable has continuity on every wire. 

 

[superjer2000]

That’s too bad. As noted in that other thread I always replace a number of diodes/transistors on every pre-se analog board. There might be some differences in labelling if you’ve got a 240v board but let me know if you want me to send the mouser. part numbers. I’m pretty sure I’ve posted them before here but not sure how best To locate the thread. Other than a dead fly back these parts have resurrected every pre se analog board I own. 

 

[superjer2000]

Found it. 

 

[elbaroni]

Awesome - thanks Superjer. Much appreciated. 
 

All the CR20 equivalents seem to have two leads coming out of the bottom, but the one on the motherboard looks like it only has one — am I looking at the wrong parts? Sorry for the questions. Very new to electronics. 

 

[superjer2000]

You’re looking at the Correct parts. One of the legs gets cut off. If you look at the second pic in that post you’ll see a couple of them mounted to heat sinks but slightly askew. That’s because one of the legs was cut off. The second leg is connected  electrically to the heat sink tab. 

 

[elbaroni]

Thanks again Superjer. Much obliged.