I've recently been messing around with a Sapphire 3850 AGP in one of my retro machines. While the card is quite fast for the old Netburst-based computer, it's also insanely loud and runs very hot.

Being blessed with an overabundance of old hardware, I happened to have a reference 3870 laying around that was ready to donate it's cooler to the cause.

I couldn't actually find any good reference images for the Sapphire 3850 AGP PCB so I'll start by posting that here.

So nothing about the 3870 cooler fits the 3850 AGP at all. However, much of it seemed like it was close enough to attempt to modify the cooler to fit. The first problem was the the 3850 VRM heatsink is much too talk to fit with the 3870 cooler shroud. Additionally the RAM heatsinks for the 3850 can't be transferred because they're part of the original cooler, which is one piece. To additionally complicate things, several capacitors on the 3850 interfere with just transferring the 3870 VRM/RAM heatsinks.

But they're pretty close, so I thought some light Dremeling might do the trick.

30 minutes and 1 grinding wheel later and I think we've got it. Also, I discovered that the heatsinks are actually aluminum with some kind of cooper colored laminate. They did feel a bit lite.

Test fit

Adding some thermal pads. I went with 1MM Arctic TP3 pads. I'd probably use .5MM pads on the VRMs if I was doing it again. Either way the VRMs are a mess because they're actually much farther apart then the reference 3870 and the 3870 VRM heatsink is not as wide.

The next issues is that there are several components that are just too tall on the 3850 AGP board, including the PCI bracket.

I thought about using the Dremel to solve all of my problems but I think it makes sense to try a less destructive solution first. I grabbed a copper shim I had lying around (1.8MM) and sandwiched it between the die and heatsink with some Arctic MX-6.

Next problem - The 3870 rear x-bracket wont fit with the PCI-E>AGP bridge chip. Just barely in the wrong position. I decided to just reuse the the mounting screws from the original 3850 heatsink and see how it goes before trying to engineer a new solution.

Finally, the 3850 AGP fan uses a mini 2-pin header which doesn't feature PWM control. The 3870 fan features a mini 4-pin which is PWM but can't socket into the 3850 board. I could have just wired the 4-pin to the board but I wanted to try and maintain PWM functionality. Although my Tyan Tiger doesn't have any PWM fan headers I figure if the fan at max is too loud I can always just buy a PWM controller.

Complete build mounted and running on my test bench.

The new fan at full RPM is probably close to as loud as the original 3850 fan, so a PWM controller is definitely in my future. However, the cooling results are fantastic.

Original Heatsink
Idle: 40-45c
Full Load 74-79c

3870 Reference Heatsink
Idle: 30-32c
Load: 48-55c