LC520 Overclock

Drake · Sep 23, 2021

In regards to the LC520 (68030 @25mhz) it can often be overlooked in favor of the LC575 (68040 @33mhz).
For those unaware these motherboards are a convenient direct swap-in replacement for the sluggish Color Classic motherboard (68030 @16mhz).
*While I am not in favor of cabalistic practices, I do happen to have the models mentioned above and will swap them for fun.*
The following is a simple, quick and easily reversible mod to bump the LC520 from 25mhz to 33mhz (likely what should have been done in factory) but we won't get into Apples intentional sand bagging of systems.

The above resistors are all that stand in the way from unleashing your blazingly fast 33mhz!
simply remove R67, R75, R79.

Once completed your LC520 board may have an identity crisis and wish to be addressed as a "Color Classic II" with a 33mhz 68030!
Congratulations Speed Racer, you're at the front of the pack now!

JDW · Oct 3, 2021

Amazing info, @Drake!
No ill effects on serial port communication after those resistors are removed?

Drake · Oct 3, 2021

JDW said:
Amazing info, @Drake!
No ill effects on serial port communication after those resistors are removed?

Nope! If we look at the actual performa 275 (Color Classic 2) these resistors aren't installed from factory. Further evidence that the LC520 was just the Color Classic 2's big slow brother!

JDW · Oct 3, 2021

Drake said:
Nope! If we look at the actual performa 275 (Color Classic 2) these resistors aren't installed from factory. Further evidence that the LC520 was just the Color Classic 2's big slow brother!

Very interesting indeed!

I'm quite curious as to what the removal of those 3 resistors is doing in the circuit. Do you have a schematic?

The stock Color Classic motherboard is quite similar, but I'm guessing the same thing cannot be done on it. If it could be done, one wouldn't need to swap with an LC520 to get better performance.

Mr. Fahrenheit · Oct 3, 2021

JDW said:
Very interesting indeed!

I'm quite curious as to what the removal of those 3 resistors is doing in the circuit. Do you have a schematic?

The stock Color Classic motherboard is quite similar, but I'm guessing the same thing cannot be done on it. If it could be done, one wouldn't need to swap with an LC520 to get better performance.

Similar things can be accomplished with the LC575 and allowing it to work at 512x384 resolution, and the LC3 (overclock to 33mhz making it an LC3+), and the LC475/Q605, which can be overclocked easily to 33Mhz from 25mhz.

Drake · Oct 3, 2021

Mr. Fahrenheit said:
Similar things can be accomplished with the LC575 and allowing it to work at 512x384 resolution, and the LC3 (overclock to 33mhz making it an LC3+), and the LC475/Q605, which can be overclocked easily to 33Mhz from 25mhz.

I have done the overclock to a couple pizza boxes as well, a marginal improvement but why not!? On the '040 machines I usually add a heatsink just in case but typically those units don't get too hot.

Mr. Fahrenheit · Oct 3, 2021

Drake said:
I have done the overclock to a couple pizza boxes as well, a marginal improvement but why not!? On the '040 machines I usually add a heatsink just in case but typically those units don't get too hot.

I bought a LC475 board on eBay last year that had a 128MB SIMM and a full 68040 33mhz on it. I think I paid $125 USD for the board with RAM and the CPU. I recapped it, and cleaned it up, and of course OCd it to the 33mhz the CPU was made to run at. I put heat sinks on all of the chips that warm up, not just the CPU. I then put a 36GB SCSI drive (using a SCA80 to 50 pin adapter) and a PDS ethernet card into it. It's my go to machine besides my Q650.

alxlab · Oct 29, 2021

Just thought it might be useful to also have a reference of the original source of the mod:

https://web.archive.org/web/20160416050801/https://www.thinkclassic.org/viewtopic.php?id=600

The mod is very similar the the LC 630 / LC 580 mod. Makes you wonder if there's any other boards that might have similar undocumented abilities

Fizzbinn · Nov 29, 2021

Drake said:
In regards to the LC520 (68030 @25mhz) it can often be overlooked in favor of the LC575 (68040 @33mhz).
For those unaware these motherboards are a convenient direct swap-in replacement for the sluggish Color Classic motherboard (68030 @16mhz).
*While I am not in favor of cabalistic practices, I do happen to have the models mentioned above and will swap them for fun.*
The following is a simple, quick and easily reversible mod to bump the LC520 from 25mhz to 33mhz (likely what should have been done in factory) but we won't get into Apples intentional sand bagging of systems.
View attachment 22
The above resistors are all that stand in the way from unleashing your blazingly fast 33mhz!
simply remove R67, R75, R79.
View attachment 23
Once completed your LC520 board may have an identity crisis and wish to be addressed as a "Color Classic II" with a 33mhz 68030!
Congratulations Speed Racer, you're at the front of the pack now!

Awesome, I had been thinking about whether this might be possible after noticing how similar (seemingly identical) the LC520 (25MHz) logic board is to the LC 550 (33Mhz) and Color Classic II (33Mhz) logic boards (which seem identical themselves except built in VRAM?).

The similarity reminded be of the LC III (25MHz) and LC III+ (33MHz) boards where you just need to move a resistor to turn a III into a III+ and the base LC520/LCIII architectures appear closely related looking at the apple dev notes.

I got a LC520 board in a trade a while back and while it works in the Color Classic (I call it a Color Classic+

I definitely will try this out!

Fizzbinn · Nov 29, 2021

JDW said:
The stock Color Classic motherboard is quite similar, but I'm guessing the same thing cannot be done on it. If it could be done, one wouldn't need to swap with an LC520 to get better performance.

That would be nice, unfortunately you are correct, the Color Classic board's base architecture is from the LC/LC II. The LC III architecture used in the LC 520/550 and CC II, is quite different/improved (32 bit bus, 72-pin SIMM RAM with no 10MB limit, etc.).

One thing that the LC520/550 (and 575 and up) boards have over the LCIII is support for stereo sound, I'm pretty sure I saw a mod for the Color Classic analog board where you can "fill-in" missing components and add second internal speaker to get a Stereo Color Classic!

JDW · Nov 29, 2021

Fizzbinn said:
I'm pretty sure I saw a mod for the Color Classic analog board where you can "fill-in" missing components and add second internal speaker to get a Stereo Color Classic!

Actually, the Stereo Mod is on the agenda for my Color Classic video series, and I've already acquired the second speaker. But I intend to do the 640x480 and overclocking videos first. Stay tuned!

Fizzbinn · Nov 29, 2021

JDW said:
Actually, the Stereo Mod is on the agenda for my Color Classic video series, and I've already acquired the second speaker. But I intend to do the 640x480 and overclocking videos first. Stay tuned!

Oh great, can't wait!!

JDW · Oct 1, 2025

To @Drake and whomever else might know...

Does the resistor overclock mod increase the clock speed of the CPU and the FPU too? And does it boost the BUS SPEED?

I'm just curious if the resistor mod transforms the LC520 board into an LC550 board, which of course means CPU, FPU and BUS Speed.

Thanks!

alxlab · Oct 3, 2025

JDW said:
To @Drake and whomever else might know...

Does the resistor overclock mod increase the clock speed of the CPU and the FPU too? And does it boost the BUS SPEED?

I'm just curious if the resistor mod transforms the LC520 board into an LC550 board, which of course means CPU, FPU and BUS Speed.

Thanks!

According to the original modder's docs this mod should make the LC520 board identical to the lC550 board.

https://web.archive.org/web/20160420202025/https://www.thinkclassic.org/viewtopic.php?id=600

JDW · Oct 4, 2025

@alxlab
Thank you so much for that link!

Uniserver was a significant contributor of helpful info pertaining to our hobby in the past, so I’m pleased to see it was he who provided that crystal clear answer. I’ve actually tried to contact him through the years, but he sadly has become unreachable. I do hope he is doing well.

Anyway, thank you. My question is answered.

JDW · 2025-10-13T15:47:33+0100

I just learned something new. Even though @Drake and Uniserver both mention the need to remove the 3 resistors R67/75/79, the Japanese book Doping Mac (published in Japan in 1997) says that in addition to those 3 resistors, R80 also needs to be removed...

I only have an LC520 motherboard and not the LC550. I sadly cannot find any photo online that shows the solder side of the LC550 board in complete hi-resolution detail. However, it seems that Doping Mac based its article on the fact that when you compare the 520 and 550 boards on the solder side, 4 (not 3) resistors are missing on the 550.

So if anyone out there with an LC550 board (Color Classic II motherboard) could either post a hi-rez photo or otherwise confirm if all 4 resistors are missing, then such would provide further evidence to show what we should do.

The problem is, we don't really know what each of those resistors does, so even though the removal of 3 of the 4 "appears to work" for those like Drake and Uniserver, we don't know what R80 does.

Fizzbinn · 2025-10-13T17:20:24+0100

Here you go:

High Res (as I can do) front and back of my LC 550 logic board. RC67, R75, R79 and R80 not populated:

For similar resolution reference my LC 520 logic board:

Another thought, there are at least two versions of the LC 550/CC II logic board, one with the 3.6 Volt 1/2 AA battery holder and one with the 4.5 Volt PINs and velcro. I've also read somewhere that the CC II version had less built-in VRAM than the LC 550. Perhaps R80 is related to one of those things?

kefkafloyd · 2025-10-13T18:31:36+0100

Fizzbinn said:
Here you go:

High Res (as I can do) front and back of my LC 550 logic board. RC67, R75, R79 and R80 not populated:

View attachment 23758 View attachment 23759

For similar resolution reference my LC 520 logic board:

View attachment 23760 View attachment 23761

Another thought, there are at least two versions of the LC 550/CC II logic board, one with the 3.6 Volt 1/2 AA battery holder and one with the 4.5 Volt PINs and velcro. I've also read somewhere that the CC II version had less built-in VRAM than the LC 550. Perhaps R80 is related to one of those things?

I can also confirm that R80 is missing on my Rev A LC550 logic board for another data point. No idea if R80 is populated on a Rev B board.

Re: LC550/CC II, the LC550 and CC II logic boards are the same logic board. There's no separate part; they're one and the same. All CC IIs had 512K VRAM on board. Apple's official spec guides had a misprint. I don't know if Rev B boards were ever shipped in a CC II before its discontinuation in spring 1994.

How Much Video Memory Does A Mac Color Classic II Have, Anyway? — Userlandia

Have Apple's spec pages and service manual for the Color Classic II been wrong about how much VRAM is on its logic board? Let's find out!

www.userlandia.com

Fizzbinn · 2025-10-13T21:24:07+0100

kefkafloyd said:
I can also confirm that R80 is missing on my Rev A LC550 logic board for another data point. No idea if R80 is populated on a Rev B board.

Re: LC550/CC II, the LC550 and CC II logic boards are the same logic board. There's no separate part; they're one and the same. All CC IIs had 512K VRAM on board. Apple's official spec guides had a misprint. I don't know if Rev B boards were ever shipped in a CC II before its discontinuation in spring 1994.

How Much Video Memory Does A Mac Color Classic II Have, Anyway? — Userlandia

Have Apple's spec pages and service manual for the Color Classic II been wrong about how much VRAM is on its logic board? Let's find out!

www.userlandia.com

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?

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.

JDW · 2025-10-13T22:57:30+0100

@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…

LC520 Overclock

TinkerDifferent Board Vice-President 2023

Administrator

TinkerDifferent Board Vice-President 2023

Administrator

Tinkerer

TinkerDifferent Board Vice-President 2023

Tinkerer

Active Tinkerer

Tinkerer

Tinkerer

Administrator

Tinkerer

Administrator

Active Tinkerer

Administrator

Administrator

Tinkerer

New Tinkerer

Tinkerer

Administrator