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| PowerMac G4 MDD unstable after bus speed mod |
Posted by: croissantking on 2026-06-06 07:36:22
I've installed a Dual 1GHz CPU in my MDD (formerly Dual 867), and bumped the bus speed to 166MHz by removing R676 on the underside of the logic board.
However, it now has RAM-related issues with stability that it didn't have before. Depending on configuration, it either fails to boot or kernels panics/freezes later on. The only configuration that seems to work is a single 1GB module in the slot nearest the CPU. If I add my other 1GB module back in, things become unstable again. FYI, both modules are PC3200.
Are the 166MHz bus MDDs usually this picky about RAM, or could it be that 133 boards don't always like being overclocked? |
Posted by: alectrona6400 on 2026-06-06 07:46:46
I've had a G4 MDD that had issues at a higher bus speed, so I underclocked it back to 133MHz. A good amount of early G4 MDDs are like that in my experience. A good idea would be to get a motherboard that is already clocked at 167MHz if you're unsuccessful with the bus overclock. |
Posted by: croissantking on 2026-06-06 07:48:58
That would suggest there is some other difference between a stock 133 and stock 166 board. Perhaps there is another option resistor that needs to be set (voltage?), or maybe the bill of materials is different and there is a chip inline with the memory controller that's rated to a slower speed. |
Posted by: alectrona6400 on 2026-06-06 07:50:23
Perhaps... I haven't looked into it in years. |
Posted by: croissantking on 2026-06-06 16:44:14
Does anyone have a high-res photo or scan of the underside of the MDD board (166MHz, FW400)? |
Posted by: joevt on 2026-06-07 01:51:42
However, it now has RAM-related issues with stability that it didn't have before. Depending on configuration, it either fails to boot or kernels panics/freezes later on. The only configuration that seems to work is a single 1GB module in the slot nearest the CPU. If I add my other 1GB module back in, things become unstable again. FYI, both modules are PC3200. Can you grab the SPD of your 1 GB DIMMs to compare with these two at
https://68kmla.org/bb/threads/g4-mdd-ram-recognised-at-wrong-size.52427/post-592172 |
Posted by: croissantking on 2026-06-07 05:18:33
Can you grab the SPD of your 1 GB DIMMs to compare with these two at
https://68kmla.org/bb/threads/g4-mdd-ram-recognised-at-wrong-size.52427/post-592172
How do I get into OF from another machine without a serial port on the MDD?
Or is there a way to save OF output to disk? |
Posted by: s_pupp on 2026-06-07 05:43:43
Does anyone have a high-res photo or scan of the underside of the MDD board (166MHz, FW400)? |
Posted by: croissantking on 2026-06-07 06:00:15
Nice one, @s_pupp and that's definitely a stock 166MHz board (not a modified 133?) |
Posted by: croissantking on 2026-06-07 06:33:53
Would you be able to share the top side as well? |
Posted by: joevt on 2026-06-07 14:18:20
How do I get into OF from another machine without a serial port on the MDD? Connect the MDD to your network via Ethernet. Boot into OS X to get it's DHCP assigned IP address. Boot into Open Firmware and use that IP address in the io command to change the input/output device of Open Firmware from keyboard/screen to telnet.
In the first post at
https://forums.macrumors.com/thread...l-work-in-a-beige-power-macintosh-g3.2303689/
search for telnet and click on the links to read posts discussing the topic.
example command:
" enet:telnet,192.168.0.121" io |
Posted by: s_pupp on 2026-06-07 14:20:23
Would you be able to share the top side as well?
The previous picture was the back side of the 166Mhz 820-1476-A board from a dual 1.25GHz G4 I purchased when it was brand new.
The front side of that board has been worked on, so I've attached a picture of the front side of another 166MHz 820-1476-A board in my possession. The "820-1476-A" model number can be seen near the bottom right corner of the picture. |
Posted by: croissantking on 2026-06-07 16:51:18
The previous picture was the back side of the 166Mhz 820-1476-A board from a dual 1.25GHz G4 I purchased when it was brand new.
The front side of that board has been worked on, so I've attached a picture of the front side of another 166MHz 820-1476-A board in my possession. The "820-1476-A" model number can be seen near the bottom right corner of the picture. Thank you for this. |
Posted by: s_pupp on 2026-06-07 17:26:28
Thank you for this. You are very welcome. |
Posted by: croissantking on 2026-06-08 04:46:52
I've had a G4 MDD that had issues at a higher bus speed, so I underclocked it back to 133MHz. A good amount of early G4 MDDs are like that in my experience. A good idea would be to get a motherboard that is already clocked at 167MHz if you're unsuccessful with the bus overclock.
Yeah, I’m starting to accept that maybe my board just isn’t capable of stable 166MHz operation. What’s good, is that once R676 is removed from underneath the board, you can manage the setting from the CPU card itself.
Connect the MDD to your network via Ethernet. Boot into OS X to get it's DHCP assigned IP address. Boot into Open Firmware and use that IP address in the io command to change the input/output device of Open Firmware from keyboard/screen to telnet.
In the first post at
https://forums.macrumors.com/thread...l-work-in-a-beige-power-macintosh-g3.2303689/
search for telnet and click on the links to read posts discussing the topic.
example command:
" enet:telnet,192.168.0.121" io
I’ll get to it.
What would the SPD info help us establish? |
Posted by: joevt on 2026-06-08 14:37:58
What would the SPD info help us establish? I want to see if your working 1 GiB DIMMs have a similar layout to the DIMMs that @DanaTheElf has which are being detected as only 512 MiB. |
Posted by: indibil on 2026-06-08 23:50:51
Have you tried lowering the CPU to 933MHz? |
Posted by: croissantking on 2026-06-09 05:53:06
Have you tried lowering the CPU to 933MHz?
Well, no, they're 1GHz rated 🙂 |
Posted by: indibil on 2026-06-09 11:57:55
Well, no, they're 1GHz rated 🙂
Sometimes the Vcore isn't enough. If you don't want to drop to 933, try increasing the Vcore by 0.05V or 0.1V. What's your current Vcore? Is it 7450 or 7455? |
Posted by: croissantking on 2026-06-09 12:01:11
Sometimes the Vcore isn't enough. If you don't want to drop to 933, try increasing the Vcore by 0.05V or 0.1V. What's your current Vcore? Is it 7450 or 7455?
I haven't checked the Vcore, but the chips are 7455A. It should be at the stock setting. I'm a bit unsure as to why it would need a boost if it's being run as designed; this card was originally sold with a 166MHz-bus speed system. The issue is between the Memory Controller and RAM. |
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