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Tom Lee's Mac Plus Video Off Modification

equant

equant

6502
Ok, equant, here's an interim method that might end up being the final method. It doesn't absolutely minimize power, but it provides a large reduction with almost no effort.
...

IMPORTANT DISCLAIMER: I have not actually tried this procedure, so I can't guarantee that there isn't a hidden gotcha. So proceed with that it mind.
Click to expand...
Tom,

So I made the modification as you suggested (many moons ago) and ran into a "Gotcha". Curious what you think.

As soon as I flip the switch to disconnect L1 from C2, the video goes out, but the floppy drive starts up. Not sure what it's doing, but it's trying to read a disk. Then, when I flip the switch again (re-connecting L1 and C2) the Mac reboots.

Any ideas?

Thanks,

Nathan

 
Sorry that the hidden gotchas didn't stay hidden.

A likely possibility is that the changes in load are too fast for the power supply. The voltage might overshoot excessively when the load is suddenly reduced, and undershoot too much when the load suddenly increases. I'll have to study the schematics and see if there's a relatively simple way to moderate the demands on the regulator loop.

 
Here's a large photo of what I did, just in case I didn't do what you had suggested...

http://www.retards.org/projects/tmp/DSC_0010.JPG

I looked at the schematic for the analog board from your "Classic Mac Tech Info" document, but didn't see anything. I didn't expect to.

I don't understand why/how a spike would cause it to try and read the disk over and over. The rebooting makes sense.

One last bit of info... if I turn on the mac with the switch open (L1 and C2 not connected) there's no chime, and the disk-drive immediately just starts ka-chunking away.

Also, I double checked the mods with a continuity tester.

 
Your photo was very helpful -- there's some weird wiring there, my friend. It's all my fault for not having been more explicit that the instructions referred to the connectivity as shown in the schematic; the physical layout is different. The fundamental idea is to isolate +12 from +12F. You have properly cut the wire that connects C2 to L1, but you've left many other things tied to the terminal of L1 that used to connect to C2. Heaven only knows what you've got, but whatever it is, it's not going to be happy. :)

What would accomplish what I'd had in mind is simply desoldering that one lead of L1 -- just pull it out and lift it away from the PCB. That absolutely guarantees that nothing will be connected to that terminal, without drilling, cutting or other mods to the PCB. Then solder one switch terminal to that floating lead, and the other to the hole you left behind. Add the diode across L1, and try the experiment again (assuming that the board was not damaged). You may still encounter hidden gotchas, but at least they'll be legitimate ones.

 
Ok, I "believe" I made the change, but the behavior is 100% the same. I know what you should be thinking, "did he do it correctly this time?" Well, let's see, here are some not so helpful pics...

http://www.retards.org/projects/retrochallenge/2008/Gallery/hq/img-13.jpg

...The black wire is to repair the trace I cut.

http://www.retards.org/projects/retrochallenge/2008/Gallery/hq/img-15.jpg

The blue wire goes to a switch. Using the orientation of the photograph, the right side of the toroid goes through the circuit board and the left side is soldered to the wire that goes off camera to the switch and then back via the other wire back into the circuit board.

So tell me what I did wrong this time please ;)

 
Hmm...it still seems to be wired funny, but I can't be sure if I'm interpreting the pictures correctly.

One terminal of L1 connects to one terminal of C2 in the original configuration. It's that connection that you want to break/remake with the switch. That is, one terminal of the switch should connect to the (now free) end of L1 that used to connect to C2. The other switch terminal should connect to the terminal of C2 that used to connect to L1.

Next, the added rectifier should be connected across both terminals of L1, not the two PCB pads that used to connect to L1. From your photo, it seems that the diode's connections have not been updated.

Finally, I do worry that the initial wiring mod may have damaged something, but I assume that you have tested for this by booting with the switch closed, and verifying that all is well.

 
One terminal of L1 connects to one terminal of C2 in the original configuration. It's that connection that you want to break/remake with the switch. That is, one terminal of the switch should connect to the (now free) end of L1 that used to connect to C2. The other switch terminal should connect to the terminal of C2 that used to connect to L1.
Click to expand...
I'm pretty sure that this (how you describe) is how it is. L1 has pivoted 180 degrees on the non-C2 connection in the photo. Don't know if that might be making it hard to interpret.

Next, the added rectifier should be connected across both terminals of L1, not the two PCB pads that used to connect to L1. From your photo, it seems that the diode's connections have not been updated.
Click to expand...
Ah, I see. You are right. I'll fix that and test again.

Finally, I do worry that the initial wiring mod may have damaged something, but I assume that you have tested for this by booting with the switch closed, and verifying that all is well.
Click to expand...
Right. Everything seems well when the switch is closed.

I'll fix the diode and see what happens. Thanks for your reply.

 
The hope is that there is a simple way to reduce the power consumption significantly, by turning off the circuits that drive the display. I think equant's motivation is to use a Plus as a 24/7 server, without having to worry about screen burn and global warming.

His results so far are not encouraging, and I worry that the power supply's transient response is too poor to allow a simple mod to do what equant would like. But perhaps a moderately simple mod (TBD) will still work...

 
The hope is that there is a simple way to reduce the power consumption significantly, by turning off the circuits that drive the display. I think equant's motivation is to use a Plus as a 24/7 server, without having to worry about screen burn and global warming.
Click to expand...
Yes, global warming ;)

I just wanted to try it. It seemed like a useful mod for people. I'd consider using my plus as a 24/7 client if I could get the mod to work.

I fixed the flyback diode, and behavior hasn't changed.

Tom, let me know if you have any new ideas, I'm happy to keep whacking away at this. Perhaps several switches, turning things off in stages?

FYI, when I turn off the video, and it starts accessing the disk, power usage drops from 31W to around 12W. Pretty good if we could get it to work.

 
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