C650 Serial Port Modification

From: mbp@januscom.com (Marlin Prowell)
Newsgroups: comp.sys.mac.hardware
Subject: Centris 650 at 40 MHz ... *with* serial ports
Date: Fri, 01 Apr 1994 07:30:32 -0800

I have a Centris 650. I have followed with great interest the articles that Marc Schrier
(schrier@mac.com) has written about clock chipping of recent Macs, but was disappointed that Centris 650's could only be accelerated from 25 MHz to about 28 MHz before the serial ports would fail. It didn't seem worth the effort.

In September, 1993, James McPhail (jmacphai@cue.bc.ca) wrote that he had a Centris 650 running at 40 MHz. He fixed the serial port problem by adding to the motherboard a PAL that he had programmed. It is an exotic solution, and not meant for the faint-hearted.

But it was James that also suggested another avenue. He said that someone should compare the motherboards of a Quadra 800 with a Centris 650, and see if there were any differences around the IOSB (the serial I/O chip).

This is what I have done. I first checked the ROM contents and found that they are identical. The motherboards for the Q800 and C650 are identical, and have *almost* identical parts installed on them. On the underside of the board, underneath the IOSB chip, there are two differences. R151 is installed on a C650, and is missing on a Q800. R152 is missing on the C650, and is installed on the Q800.

With this information, I ordered the parts for a clock chipping kit, along with a resistor to install as R152. I first installed different oscillators without changing the motherboard. The fastest crystal I ordered was 20 MHz (to run the computer at 40 MHz), and the machine ran at 40 MHz. Of course, the serial ports did not work.

I then changed the resistors on the motherboard (details below), and tried again. Now *everything* works at 40 MHz. I have used the modem port, and printed from the printer port. I have tried both LocalTalk and the Ethernet port, and all work fine. The floppy drive also works.

I ran MacCheck 1.0.5 on the machine. When I tried MacCheck before modifying the motherboard, it hung when trying to access the modem port. After changing the motherboard, it reported that there were no problems. It also reported that I had a *Quadra* 650 running at 40 MHz.

That was a surprise. I checked the "About This Macintosh" box in the Finder, and it also reported that I had a Quadra 650. This is important to know. If you make this modification, you *must* install System Enabler 040 Version 1.1 first. This is the version meant for the Quadra 610 and 650, and is backwards compatible with the Centris 650. If you do not, your machine will refuse to boot, saying that you need to install newer software. You must install version 1.1 on every bootable disk, including all your emergency floppy disks, and your Disk Tools floppy.

It also seems a little unusual that modifying a Centris 650 motherboard so that it is identical to a Quadra 800 board instead turns it into a Quadra 650 motherboard. Only Apple knows what is going on. A Quadra 650 requires 80 ns DRAM, whereas a Quadra 800 requires 60 ns DRAM. There must be some difference in the on-board memory system that the ROM can detect.

I didn't expect to be able to accelerate the machine up to 40 MHz. I expected that memory access time would limit the modification. I can only guess that the memory system has it's own timing circuit that is not derived from the system clock, so speeding up the processor does not speed up memory access requirements.

Making the modification

I have done a little hardware work, but I am no expert at it (I'm a software guy). I'll outline the procedure that I followed, and you can modify it if you have better tools than I do.

Warning: you are making modifications that void your warranty. Also, I cannot promise that these changes will be successful for you. You could ruin your motherboard attempting these changes. The resistors are incredibly small, and changing them requires a delicate touch. If you have any doubt about your soldering and electronics skills, get someone else to make the changes. You will need one 1.2K 1/10 watt surface mount resistor (type 0805) to install as R152. I ordered mine from DigiKey when I ordered the rest of the clock chipping kit. The DigiKey part number is P1.2KABK, which is a package of 200 resistors for $6.01. This is the minimum number of resistors you can buy from them. The Panasonic part number for the resistor is ERJ-6GEYJ122. The resistor should have the number 122 on the top. You'll need a magnifying glass to read it, however.

To make the modification, I used a good quality 25 watt soldering iron with a very small tip, some desoldering braid, and a hemostat (medical locking forceps) with a small tip. You will also need a magnifying glass to check your work.

Removing R151

This is the most difficult part of the modification. Find R151 on the underside of the board. Each part, including R151, has been glued to the motherboard. R151 also has two circuit board traces running under the resistor, so prying underneath the resistor to loosen the glue is not a good idea. I first removed excess solder from the ends of the resistor with the desoldering braid. Be careful not to overheat the motherboard as you can damage the inner layers of the board. I then clamped the hemostat over the resistor, and, while applying heat to the ends, gently twisted the resistor to remove it. (Those of you with proper tools, please stop laughing. I did what I could with what I had.)

After I made the modification, James McPhail wrote:

I remove SMT resistors with two soldering irons, one in each hand. Heat up both ends and flip it aside when it is loose. I don't find the glue is much of a problem.

Adding R152

Put the 1.2K resistor in place. Hold it down with a tiny flat blade screwdriver while you solder the ends, otherwise the resistor will stick to the soldering iron.

Congratulations! You now have a Quadra 650 (running at 25 MHz). Make the clock chipping modifications that Marc Schrier has described, and you, too, can have a 40 MHz machine.

In his September 93 posting, James McPhail wrote:

I measured the temperature of the CPU heatsink and found the following (closed box, no NuBus cards).
Ambient 23 degrees C
25 MHz 42.0 degrees C
40 MHz 47.5 degrees C
So if you keep the box closed, it is clear that no additional CPU cooling is necessary.

He also mentioned to me recently that his Centris 650 is still running fine at 40 MHz. Good luck to all who make this modification!

| Marlin Prowell | There is a very thin line between ignorance and arrogance |
| mbp@januscom.com | and I have totally obliterated that line. -- Dr. Science

There are a few more bits of information that have come out since Marlin posted this file: