All modifications, even though there may be no visible sign that you modified your Mac, will void the warranty on the Mac. As Apple states:
"This warranty does not apply if the product has been damaged by accident, abuse, misuse, or misapplication; if the product has been modified without the written permission of Apple; or if any Apple serial number has been removed or defaced."
Many Macintoshes can run at higher speeds than Apple ships them. See the file Mac Crystal Oscillator Speedup History 2.5 by Marc Schrier (firstname.lastname@example.org) for details.
The Duos were not among those described in the speedup history. Since the Duo 210 resembles the Duo 230 in many ways, it had to be possible to run the 210 at 230 speeds. There's only one way to find out: give it a try. So I did.
The Duo's keyboard can be flipped open. That leaves the PCB visible. The test clip people use to speed up a desktop Mac can be fitted on top of the 50 MHz crystal oscillator (which clocks the CPU and many other parts). I have added the test clip containing a 64 MHz oscillator (since I could get that one cheap), kept my fingers crossed and pressed the power-on button. Boing. It works!
A more permanent solution
The trouble with this setup is of course that there is no room in the Duo for the standard sized 3M test clip. And since I did not want to solder on the surface mounted oscillator, I had to find another way to install a new oscillator.
I think there are three ways of speeding up the Duo.
I went for option 3.
This document describes the clip I made. It is fairly easy to construct, cheap, and very small.
The Duo is very compact as you will have noticed already. The headroom above the surface mounted oscillator is about 2.5 mm (1/10"). The other problem is that there is also very little room for a standard metal can replacement oscillator anywhere in the computer. The modem bay, if empty, is just high enough to accommodate the replacement. If the modem bay is occupied, then you will have to find a surface mounted oscillator, which presumably just fits next to the original oscillator.
(note the drawings that follow are not to scale!).
For the clip you will need:
It is the kind of connector that is used on SCSI drives to connect to the flat cable in the computer, it has two rows of 32 pins.
Molex makes them (part no. ,,,,,). Use a gold plated one. The should cost somewhere near $ 3.
To get an impression what it looks like from the top consider the next picture.
The (medium) grey part is the oscillator on the PCB. The light gray part is the actual clip. The yellow parts are the pins sticking out of the clip. The black rectangles are the pins of the onboard oscillator. The blue part is the notch in the onboard oscillator indicating pin 1 (left of the notch). Left and right of the clip are the two springs pulling the clip together.
As I presume the previous drawing is not completely clear, let's look at the clip from the sides.
The black box is the surface mounted crystal oscillator. The gray thing is the clip. There are wires running from the clip to the replacement oscillator in the modem bay.
Notice the height of the clip: just under 2.5 mm (less than 1/10").
Cut a segment with 2x5 pins from your 2x36 strip.
Viewed from the top you get this:
Next carefully pull out the 10 pins using the pincers. You will now have 5 large and 5 small pieces of gold plated metal:
Cut the plastic part along the long side in two. You should now have two equal sized pieces of plastic, each with 5 holes in it.
We will number the holes from 1 to 10. Carefully file some of the plastic from both top and bottom (the black on drawing below).
These two plastics will become the body of the clip.
Using the small drill, widen holes 1, 5 and 8. They will be used to slide the guiding pins (coming from holes 6, 10 and 3 respectively) through.
We are now ready to put back selected pins. Take one of the longer pins and push it through hole 6. Do the same with another pin and hole 10. Note that the pin should be inserted with the bends facing left and right respectively. The long part of the pin is inserted in the holes. Put some glue on the pins to secure them to the plastic. Look at the drawing below, which gives a side view.
Insert two short pins in holes 2 and 4 also with the bends outwards. You now have the `handles' to which to attach the springs. Cut the pins in holes 2 and 4 on the inside of the clip and glue them to the plastic.
Next insert the pins that will make contact to the onboard oscillator. You use 1 long pin which you stick through hole 3. The long side is inserted in hole 3. The short side is facing down.
Now take two other short pins and insert them in holes 7 and 9, with the bend facing down. Cut the pins where they come out of the plastic (see drawing).
Last bit is to solder a piece cut from one of the spare pins on the pin in hole 3 to form an T upside down. This piece should be extending from somewhat left of pin 4 to somewhat right of pin 2. It will be used to pull Output Enable of the onboard oscillator to ground, thereby disabling it.
Now bend the `flaps' (or handles) somewhat downward (see drawing). The springs will not touch the keyboard but will be next to the onboard oscillator.
Put a small amount of glue on either side of the plastic at holes 2, 3, 4, 6, 7, 9, 10.
Put the two pieces together and attach the springs. The gap between the two plastic parts should now be variable. The springs take care of pulling the two body parts together.
Solder leads (about 10 cm) from the pin in holes 3, 7 and 8. Connect them to the replacement oscillator.
All crystal oscillators have 4 pins. Some are numbered 1,2,3,4 and others 1,7,8,14. Pin 1 is always the pin next to the pointed edge (the others are rounded), with the dot, or next to the indentation on the newer CMOS, or surface mount crystal oscillators. With the pins facing down, put the dot, or indentation to your left, and the pin on the left, closest to you is pin 1. Going counter clockwise, pin 2 (or 7, depending on what numbering scheme) is to the right, Pin 3(8) right side and further away, and 4(14) left side, and further away.
Connect pin 3 of your clip to pin 2 or 7 of the replacement oscillator (depending on numbering scheme). Connect pin 7 of your clip to pin 3 of 8 of the replacement oscillator. Finally connect pin 9 to pin 4 or pin 14 or the replacement oscillator.
You can now carefully put the clip on the onboard oscillator. Flip open the keyboard. You will find the oscillator located approximately under the F key. It has 50.00 printed on it. The T part of your clip (coming from pin 3) should cover pins 1 and 2 (or 1 and 7) of the onboard oscillator. You will find them left of the notch on the oscillator. Put the replacement oscillator in the modem bay (maybe using some double sided tape), taking care that no metal parts are going to touch any metal of the Duo (just in case).
Cut a small piece of plastic of wood somewhat longer than the height of the clip + onboard oscillator. This will serve as a support for the keyboard. Another option is to use a small piece of cardboard (say 2.5 mm) and put that on top of the other (16.67 MHz) oscillator.
The support prevents the keyboard from touching the clip.
Check everything carefully and reassemble the Duo.
Keep your fingers crossed and press the power-on button. The familiar Boing should sound. If it does not then your clip is not making contact. Adjust the clip a bit so that it does.
If your Duo freezes during operation the clip is not fully making contact. Adjust the clip so that it does.
I have had this modification in operation for a week now and it works to my satisfaction. Probably you can use an oscillator that runs at 66 Mhz or higher frequencies, though I have not tried.
The internal temperature in my Duo is in the 31 degrees Celsius area (room temp somewhat under 28 degrees! Summer in Holland). I think those are normal temperatures for Duo 230's.
Good Luck, and let me hear from you.