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Bolacore

Macintosh 128K - no video and humming sound

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I picked up a rather interesting 128K and it finally showed up this week. It's quite an early one as it just has a Macintosh badge instead of the later 128K/512K ones, but was upgraded with a HyperDrive 20 rev 2 and 512K board. I've been trying to get it working, and after patching it's analog board, I'm now getting just a humming sound from what appears to be the analog board (I can't really tell) and no video at all. The sounds reminds me of my aquarium air pump, with a slight whistle to it. No video at all when this happens, but I still see the CRT is on with the glow from the heater.  It did have quite a lot of battery damage in the case, and some rusty bits near the two switches in the top left. Cleaned it before I tried to power it up (but after the picture was taken). 

 

Interestingly, if I swap in a good board from my Plus, the system works fine with no noises, and if I try the 512K board in my Plus it makes the same sounds there. When running the voltages seem fine, seeing 4.89V for 5V,  4.51V on the battery pin, 12.23V for 12V, and -12.4V for the -12V. I've turned the voltage control pot down slightly, but it can go up to 5.2V when I tried. No obvious shorts on the logic board either, lowest resistance I'm seeing is 59 ohms between 5V and ground. 

 

I've pulled off the HyperDrive board for testing (I bent the header pins removing it, so just left it off for now), in case that was the issue, but no change there. Nothing gets warm on the board either. 

 

Any idea what's up with this one? I'm slowly going through the schematic, but I'm not really sure what's up with it. I'd like to get it going again, just because I think the HyperDrive is really weird and neat. 

IMG_20200211_151502.jpg

Edited by Bolacore

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Taking another look after work and there's rotten traces from battery damage near the TSM IC. Started patching wires and now we have video on the screen, albeit just garbage. Does anyone have a nice high res schematic of the 512K board? The  only one I can find is the official apple one, but it's very hard to read. 

Edited by Bolacore

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Almost got this one working! Good few patch wires at this point, and a new 74LS166. We're now booting and are functional beyond a weird screen. 

 

Since it works perfectly other than that, I suspect one of the 74xx ICs for the video generation are also faulty. I will get this stubborn Mac going :)

IMG_20200212_204000.jpg

IMG_20200218_173446.jpg

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9 hours ago, ScutBoy said:

It looks like this when they are done :-)

 

It can be a slog, but worth it in the end. Hope you get it figured out!

 

Really looking forward to getting this one working, never owned an original Macintosh before. Even more so to get the HyperDrive working, would be cool to have one of the few working units in the world :beige:

Edited by Bolacore

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