Oscillator Switch for C/Q 650s with PPC PDS Cards

Submitted 12/15/94 by James Lockman (lockjaw8y9@aya.yale.edu).

After I clock-chipped my Centris/Quadra 650 to 40 Mhz, I acquired a PPC PDS card and discovered that it did not run well at 80 Mhz. After some tinkering and much swapping of oscillators, I found that in PPC mode, it ran best at 76.8 Mhz, which corresponds to a 19.2 Mhz oscillator. Unfortunately, this corresponds to a reduced speed for the '040. Since I often switch between PPC and '040 and demand maximum performance in both modes, I wanted to be able to do so without having to take the cover off of my machine. As a service to other users out there who are in a similar predicament, I wrote up this page to describe my solution.

I have built a switch for my Centris/Quadra 650 with PPC PDS card that allows me to switch between a 20 Mhz oscillator for '040 use and a 19.2 Mhz oscillator for the PPC Card. It has gone through two incarnations in as many days, and I am including both incarnations here. Aside from some slightly increased TV interference, it seems to be working fine. I also see no reason why this could not be extended to other models of Macs for which the PPC PDS card is an option.

WARNING! Be sure not to switch this switch while your computer is running! You will be very disappointed. It is not intended for "on the fly" switching. You must shut down, switch, then restart.

Here's the schematic for the Mark I Clock Chip Oscillator Switch, which you can build any way you want. I simply extended my home-brew zippy clip that I built last year using Marc Schrier's fantastic instructions, which are available via the Clock Chipping Home Page. A pre-made "Zippy Clip" can be purchased through Output Enablers. If you have not already built one and you want to do so, go read his instructions and then come back here.

On Figure A, the clip is A, and the other oscillators' sockets are B and C. The dots are the reference dots on the oscillators. I highly recommend sockets for the oscillators so that you can play with the combinations that maximize your system's performance. As stated above, I found that my Centris/Quadra 650 runs best at 40 Mhz, while the PPC card runs comfortably at 76.8 Mhz. I also added a fan to both the '040 and to the PPC card, using the info found in Dan Gezelter's instructional page.

Figure A

What I originally did was to simply switch which oscillator's output gets sent to the motherboard. These TTL oscillators have a pinout, counter-clockwise from the dot with the dot first: n/c or Output Enable (depending on the type of oscillator you have), Ground, Output, +Vcc. At first, I tried to disable the oscillators by grounding the OE pin. In my case, it did not work. If you can get OE oscillators, do it and switch the grounding of the OE pin. That way, there is no signal moving through any long wires. More on that later. I next tried to just switch the +Vcc pins, but I found that I could not boot with both oscillators socketed. I thought about building a three-line bus switch, but they are bulky and more expensive. In the end, although there are two oscillators running at the same time, the output is switched between them as necessary. Do not switch on the fly, however, as you will be very unhappy. Since both B and C are active all the time, I suspect that is the source of my TV interference. If it's a problem, re-orient your antenna or get cable.

The switch is a double pole single throw micro toggle which selects the output of the additional oscillators. I drilled a hole in the slot cover in line with the PPC card and mounted the switch there. I wrote a P and a Q on the slot cover on either side of the switch to indicate which mode I was running. For added security (see the big warning above), you can use a locking toggle or a slide switch, but round holes are easy to make and non-locking toggles are cheap.

Figure B

As you can see in Figure B, I have the extended clip not cover up either of the free NUBUS slots. I have no other cards in the machine, but many people have additional video cards, so be sure to make the wires long enough to avoid conflict with your existing cards. Unfortunately, long wires make good antennae, so the longer the wires are, the worse your TV interference will be and also the greater the chance that you will get interference from the outside. On the C/Q650 it is not so much of a problem since the case is metal and grounded, but in plastic boxes it might be problematic. Although the diagram is misleading in that the B and C sockets are actually above the clip A, you will find that room is tight for two sockets and two NUBUS cards. Be careful where everything goes, and you'll be fine.

So off to bed and be satisfied, right? Well, Jonathan Jacobs (jacobs@cwbiol2.biol.cwru.edu) wrote to me that he "... would recommend switching for both [Output and +Vcc]. [He] found that results can be unreliable when [he] only switched the output. Switching both Vcc and Output fixed the problem. True, it does add three more wires, but [he] just got a DPDT switch instead. [He's] been operating with the switch for about five weeks now and have never had any problem with it."

I took his advice and built the Mark II Clock Chip Oscillator Switch. It has been running for a little over an hour now, and I seem to be having a better time of it. I was getting spurious video errors under the Mark I, but I figured that it was due to the Oscillator. Now, I am having no video problems. You decide what you want to build. I used a locking DPDT switch that I got at an electronics warehouse nearby. If you, too, would like to make a Mark II instead, then make the switching circuit as follows:

Figure C

In Figure C, the switch is cut in half for clarity. The switch is really two switches stuck together, so one half switches the output and the other switches the power. Be sure that you get the power and output from each to be going in the right places as follows:

Figure D

There you have it, folks. Once again, I urge you to read the Clock Chip History before making this mod to the clip. As I do not have one of the commercial models from Output Enablers, I cannot say how easy it is to modify a genuine "Zippy Clip" to be switchable. Nevertheless, for those of you with the PDS card on either a Quadra or Centris 650 or any other machine that can take the PPC PDS card, I highly recommend this mod to your clock chip clip.