LC520 Overclock

kefkafloyd

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Really cool article, thanks for sharing! I definitely can see documentation error leading folks to think the LC 550 and CC II logic boards were substantially different when they actually aren't. I do wonder what controls the different gestalt IDs though?

Macintosh LC 520 = 56
Macintosh LC 550 = 80
Macintosh Color Classic II = 83

The resistors affecting CPU speed would explain 520/550, but LC 550/CC II? Something from the chassis? Video resolution from the sense code?

I believe on these machines the Gestalt is determined from some combination of CPU speed and sense pin detection on the machine's logic board connector. The ROMs are identical between LC 520, LC 550, and Color Classic II and they all use the same System Enabler 403. My LC 550 board inside the Color Classic simply comes up as "Macintosh", but it does show the Color Classic icon. Put the same board in my Mac TV and it shows up as "Macintosh" but with a Hook chassis icon instead.

You'll note in JDW's 67Hz 640x480 mod video that his LC 520 board does identify as an LC 520 inside of the modified Color Classic. But I don't know if he tried that in his machine before the modifications.

Looking at the System Enabler 403 STR# resource the strings for IDs 80 and 83 (LC 550 and CC II) are blank. Of these three machines only the LC 520 has a proper string value at its gestalt. All three of these machines need Enabler 403 to boot, though. A reasonable guess is that the enabler was never updated when the 550/CC II were speed bumped. If I were to edit the names into strings 80 and 83 in it I bet they'd show up correctly.

Apple is not above using jumpers, resistors, or bridges along with the CPU speed to set gestalts—the difference between an LC 475 and Q605 is Jumper J18. Changing chip speeds on other boards will make different gestalts come up as well. http://www.rgaros.nl/gestalt/chapters/ch-19.html

Back to the original question from that article my LC 550 board is Rev B (4.5 Volt PRAM battery) so if you confirmed Rev A (3.6 Volt PRAM battery) then neither have R80 populated.
Excellent, thanks for confirming this!
 
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Fizzbinn

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Back to the original question, from that article my LC 550 board is Rev B (4.5 Volt PRAM battery) so if you confirmed Rev A (3.6 Volt PRAM battery) then neither have R80 populated.

@Fizzbinn you’re amazing!
And @kefkafloyd , thank you too!

I just posted the question about the 550 Rev. B board on FaceBook, so let’s see if that brings forth the final answer more quickly…

View attachment 23762

I think there was some confusion, my LC 550 board is a Rev. B (820-0595-A, 4.5V PRAM battery with velcro) and @kefkafloyd 's LC 550 board is a Rev. A (820-0368-A, 3.6V 1/2 AA PRAM battery) so we have confirmed both lack all 4 resistors, although more confirmation is never a bad idea!

Summarizing my newfound understanding:

SpeedLogic Board silk screenRC67, R75, R79, R80 InstalledPRAM Battery Type
LC 52025 MHz820-0368-A, 1993Yes3.6V 1/2 AA
LC 550 (Rev. A Logic Board)33 MHz820-0368-A, 1993No3.6V 1/2 AA
LC 550 (Rev. B Logic Board)33 MHz820-0595-A, 1993,94No4.5V Alkaline
Color Classic II33 MHz820-0368-A, 1993No3.6V 1/2 AA
 
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kefkafloyd

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Of note, there's another way to identify boards: their serial schemas.

I've noticed two different schemas for 520s: EZxxxxxxDY7 and DWxxxxxxDY7. The DW boards are the later ones (the serial labels say Rev D). I think the difference is that the EZ boards were made in Singapore while the DW ones were made in Malaysia. At least, that's what I've noticed.

Interestingly, both LC 550 revisions have the same schema of DWxxxxxx1X9. All the CC II and 550 boards I've seen have followed it. The newer revision will have "CUDA" on the serial label.

There's also LC 550 boards which were originally meant for half-AAs that have factory bodges to use the alkaline battery. They still have 820-0368-A on their silkscreen, so they're not the later revision boards.
 
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JDW

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You'll note in JDW's 67Hz 640x480 mod video that his LC 520 board does identify as an LC 520 inside of the modified Color Classic. But I don't know if he tried that in his machine before the modifications.
I did indeed test my LC520 motherboard in my Color Classic with the standard 60Hz 68.4V "VGA mod" (640x480 resolution) installed. But as you may know, the 520 and 550 and stock CC boards are NOT compatible with the standard VGA mod, and that's why I more recently released a video about my switch to the 67Hz 84V 13" High Resolution Mod, which allows use of ALL COMPATIBLE motherboards, including the LC575.

When using the stock CC motherboard in the CC with the 68.4V VGA Mod, you get a black screen, but the machine still boots.

When you using the LC520 motherboard in the CC with 68.4V VGA Mod, you get a display on the CRT, but the vertical sync is lost and it endlessly rolls vertically like this:


I've never tested the LC520 board in my CC in its stock condition (i.e., at 512x384 resolution).



I think there was some confusion, my LC 550 board is a Rev. B (820-0595-A, 4.5V PRAM battery with velcro)...

🤦‍♂️ Yes, my eyes sadly and shockingly overlooked that when I first spotted your photos, and subsequent to that I read @kefkafloyd 's reply saying, "No idea if R80 is populated on a Rev B board." And for some reason my silly brain fixated on that statement and neglected to ponder your crystal clear photos!

Ack! I must be experiencing a Brain Cloud.
BrainCloud.png



@Fizzbinn
I overlaid your LC550 Rev. B board's TOP & BTM photos, distorted them slightly so all pads would align, then I flipped the PCB TOP horizontally so it would be super easy to flip back and forth and see where R80 pads lead. Sadly, one teensy tight thru-hole pad (see rightmost arrow below) seems to connect to inner traces, and the other pad (shown by the arrow at left below) leads us elsewhere on the board.

1760409743598.png


1760409726136.png


So I still have no idea what resistor R80 is supposed to do, and without a schematic its much more troublesome to find out.

What seems rather clear though is that we should remove 4 resistors, not simply 3.

@Drake hasn't been replying back in this thread, and Uniserver stopped all online replies years ago and is now unreachable, so it's not clear if either of them experienced issues after their 3-resistor Overclock. I've also not been able to Google-up anyone else whose followed their lead in removing only 3 resistors either.
 

Fizzbinn

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I overlaid your LC550 Rev. B board's TOP & BTM photos, distorted them slightly so all pads would align, then I flipped the PCB TOP horizontally so it would be super easy to flip back and forth and see where R80 pads lead. Sadly, one teensy tight thru-hole pad (see rightmost arrow below) seems to connect to inner traces, and the other pad (shown by the arrow at left below) leads us elsewhere on the board.

1760409743598.png


1760409726136.png


So I still have no idea what resistor R80 is supposed to do, and without a schematic its much more troublesome to find out.

What seems rather clear though is that we should remove 4 resistors, not simply 3.

Hmm, from your sleuthing perhaps the reason why others left off R80 as needing to be removed is that is was vestigial? The Q1 component is missing from both the LC 520 and LC 550. Its possible inter-layer connections connect R80 in a meaningful way to other components but maybe not, it seems within reason that it could be tied up in whatever lead Apple to build but not populate Q1 and J12... Like documentation errors it seems somewhat likely that when Q1 and J12 where deemed not needed the supporting R80 was forgotten to be removed and this was fixed in subsequent versions. Just a guess. If I was trying to convert a 520 board to a 550 board I'd remove it nonetheless.
 
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