Re: Some tests with my Macintosh Centris 650
On Wed, 21 Jan 2015, John Paul Adrian Glaubitz wrote:
> Hi!
>
> I have recently managed to bring my Macintosh Centris 650 back to live
> after it didn't want to boot anymore after replacing the PRAM battery.
> Just powering the Mac on with the battery removed helped to fix the
> issue and afterwards I could just put the battery back in and it would
> still work fine.
>
> I had some issues with the SCSI bus with kernel constantly resetting the
> SCSI bus during boot and the CD-ROM drive showing up multiple times in
> MacOS. I managed to fix this by just detaching the CD-ROM drive from the
> SCSI bus. Since the CD-ROM drive and the HDD worked fine with the exact
> same setup, I just assume those are some problems with the SCSI cable
> used or the contacts since I had to detach the SCSI cable to disassemble
> the Mac to access the PRAM battery. Anyway, it works for the time being
> and I will probably just replace the SCSI cable at some point.
I agree, the cable is probably flakey.
>
> As for booting Linux. The Mac boots fine with Linux 3.14 but just seems
> to crash when trying the most recent kernel, 3.16.0-4. I haven't done
> any debugging yet, but I'd guess it's either a regression in the kernel
> or just an issue with the kernel image itself. I haven't played so much
> with the Penguin boot loader yet, so I don't know what to try to debug
> the issue (serial port?). In any case, I guess that problem should be
> fixable as it can be reliably reproduced and is not some sort of
> Heisenbug :).
There is a known bug in Penguin that might affect your 3.16.0-4 kernel, if
it is large and gzipped. The work-around for this bug is to unzip the
kernel binary before using it with Penguin. (If that works, you probably
want to switch to the EMILE bootloader. It used to work on my Q650 when I
tried it years ago.)
Failing that, can you send me a photo of the crash? The serial console log
would be a lot better than a photo but you'd need a suitable cable. (I use
an Apple StyleWriter cable with a Keyspan USB-to-mini-DIN8 adapter.)
>
> As for the performance: The Mac currently has a 68040 running at 25 MHz.
> Reading a bit on what people from the past have to say about
> possibilities to improve the performance on these machines, it turns
> out, you can actually overclock most 68k Macs without any risk of
> permanent damage. The easiest way is to replace the on-board quartz
> crystal oscillator of 12.5 MHz with 20-MHz-type which would increase the
> CPU clock from 25 MHz to 40 MHz [1].
You might want to order a 16.666 MHz crystal as well, in case of
instability at 40Mhz.
>
> Measurements have shown that the overclocking the 68040 to 40 MHz will
> increase the core temperature of the CPU from around 42 C to around 47
> C, thus the CPU stays rather cool. The only drawback is that the serial
> and parallel ports will stop working, but even this can be fixed by
> modifying a jumper setting on the mainboard which changes the machine ID
> of the Centris 650 into the one of a Quadra 650 which actually never
> existed [2].
The Quadra 650 (33 MHz) has a big heat sink on the CPU; I don't recall
seeing one on the slower machines.
>
> Personally, I think the serial and parallel ports aren't that essential
Maybe. But you may find that AppleTalk is enabled when the PRAM is reset,
which would bring the serial chip into play...
(There is no parallel port BTW.)
> and I will just use a testclip to attach a 20 MHz quartz oscillator
> crystal and ignore the ports [3]. This way, I can just remove the
> overclock modification at any time and can restore the original
> configuration of the Mac without any traces of the modification. I have
> already ordered such a testclip from Taiwan and now I am just waiting
> for it to arraive.
For reliability, I think it would be prudent to do the whole conversion
(resistors and crystal, soldered) just so that MacOS knows what clock is
fitted, because it probably needs to know which timing loops to use. It
seems likely that drivers besides serial will benefit. Did you find any
reports of working Ethernet with a partial conversion?
--
>
> I guess the Centris 650 with a 68040@40 with 136 MB RAM and a fast
> IDE harddisk will make for a usable buildd machine.
>
> Adrian
>
> > [1] http://www.applefool.com/clockchipping/
> > [2] http://www.applefool.com/clockchipping/c650.html
> > [3} http://www.applefool.com/clockchipping/mod3.html
>
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