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Cleaning a 7200/90 motherboard

zuiko21

Well-known member
Hi all,

I purchased one of these -- with the sole intention of housing my maxed-out 7600 mobo ;)

However the original board and PSU work just fine and I could keep them for experiments... but now that I'm used to those beautiful, sparkling boards, this one looks filthy to me :disapprove:

I'd love to wash it, just like any other vintage board I have, but I don't know how to handle that Peltier device over the PPC601... especially since the service manual warns about not disconnecting it from the board! Yes, this 7200 does have the extra fan on top of it, besides the PSU frame -- I'm afraid I should remove it in order to fit the 7600 board.

So far, I think I should remove the heatsink and the Peltier, clean off any dry thermal paste residue and wash as usual. But then, for reassembling, should I put a bit of new thermal paste between the CPU and Peltier? Between peltier and heatsink? On both sides? None?

Thanks a lot in advance!

 

trag

Well-known member
If it is like all the ones I've ever seen, the Peltier is glued to the heat sink, so you do not need to worry about that interface.

For the rest, yes, just unplug the peltier from its power supply, then unclip the peltier/heat sink assembly from the logic board. Be gentle; uneven forces here could, conceivably crack the CPU, then clean the old heat sink grease (probably powder at this point) with alcohol, apply a dab of new grease and carefully clip the peltier/heat sink back on.

The caution about not removing it really should be a caution about operating the board without it attached.

And if you need another one, for some reason...

http://www.shrevesystems.com/misc.html

 

zuiko21

Well-known member
Thanks! With the board turned upside-down, after loosing the clips, the heatsink (+Peltier) fell off... several IPA-soaked Q-tips later, both the CPU and Peltier are clean from dry thermal paste.

Before:



After:



The much-better-looking board is now being rinsed :cool:

(yes, I'm aware that I must clean off that paste residue from the CPU pins!)

Thanks again!

 

techknight

Well-known member
this brings up a rather stupid question for me. Why the peltier? why did apple use that? its only a 90mhz CPU. all the peltier would do is just add heat to the heatsink.

 

mcdermd

Well-known member
I like how fancy the processor package is for those 601s. When I pulled the heatsink off of my 7100 I was a little surprised to see that big blue under there.

 

trag

Well-known member
this brings up a rather stupid question for me. Why the peltier? why did apple use that? its only a 90mhz CPU. all the peltier would do is just add heat to the heatsink.
In theory, the Peltier will create some temperature gradient between the heat sink and the CPU surface, let's say 20 degrees C. If the energy added by the Peltier does not increase the heat sink temperature by 20 degrees, then that should be a net reduction in CPU temperature. I'm sure you realized that already.

I agree, that in practice, it seems like the Peltier doesn't really gain one much. I've tried these Peltier/heat sink combos on our chips at work a few years ago (bought a few from the link above) and found that cooling wasn't any better than just using a similar heat sink with a 60 mm fan blowing on it. And a fan uses a lot less power than the Peltier.

Example for two paragraphs above: For a certain energy (heat) input, and a fixed ambient (air) temperature, a heat sink will reach some equilibrium temperature, at which it radiates/conducts/convects (I'll use "radiate" to mean all three) away heat at the same rate as it is input from the CPU and (if present) Peltier element.

This works, because the hotter the heat sink, the faster it radiates heat. So, increase the heat input to the heat sink and the heat sink's temperature will rise, as the temperature rises, it radiates heat faster, and eventually, it gets hot enough to radiate away heat at the same rate as the heat is coming in.

The downside, of course, is that as the heat sink's temperature rises, so, too, does the temperature of the CPU.

So, if you burn more power in your CPU, the heat sink will still radiate it away, but it will do so at a higher temperature than if the CPU was using less power, and thus, your CPU will experience higher temperatures as its energy use increases.

Enter the Peltier. The Peltier sits between the CPU and heat sink and creates a temperature difference between them. IIRC, the temperature difference is based on the voltage you feed the Peltier. So, if before you had a heat sink/CPU combination which operated at 50C, now, in a perfect world, with the Peltier in between them, you'd have the heat sink operating at 50C, and the CPU would be cooled to 30C.

The catch is that the Peltier also consumes energy, which gets radiated away as heat. So, let's say that we started with an energy input to the heat sink which could be radiated away at 50C. So, CPU and heat sink were at 50C. Now we add a Peltier running off of 12V power. The Peltier uses enough electricity, which it converts to heat, to increase the heat sink temperature to 60C. The Peltier also creates a temperature gradiant of 20C. So the CPU is now experiencing 60C - 20C = 40C which is 10C lower than before.

That's how it should work.

In practice, it seems like the Peltier often increases the heat sink temperature by 20C and creates a gradiant of 20C so that it is a total wash, and a waste of power.

It should be possible to choose the heat sink and air flow so that the temperature increase caused by the Peltier is less than the temperature gradient created by the Peltier. Perhaps Apple did. I haven't had great luck with them though.

Anyway, IBM had some spec. that said that for speeds over 100MHz, the PPC601 needed to be cooled below some temperature and that's why Apple and Power Computing used the Peltiers on PPC601s which ran at 100MHz or faster. I don't know why the 90MHz 7200 has one.

 

Charlieman

Well-known member
Trag asked:

Anyway, IBM had some spec. that said that for speeds over 100MHz, the PPC601 needed to be cooled below some temperature and that's why Apple and Power Computing used the Peltiers on PPC601s which ran at 100MHz or faster. I don't know why the 90MHz 7200 has one.
That board design was used in some markets with a PPC 601 running at 120MHz in the PowerMac 8200.

 
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