As I was testing the new switch on my second AX-990 (with the cover removed) I happened to touch the heatsink and was surprised to find it was very hot. It's a piece of hardware literally designed to absorb and re-radiate heat, so perhaps I shouldn't have been too shocked at this. But it was very hot — almost untouchably so. That got me thinking: is it possible to replace the linear regulators with a modern equivalent that emits less heat? After all less heat is better, right? And electricity turned into heat is basically wasted energy.
I've poked at the power supply in this machine before — the electrolytic capacitor just after the rectifier had burst and needed replacing — so I knew that about 12V was output from the transformer.
The linear regulators were L7805CV parts capable of supplying up to 1 amp of current.
That means that, worst case, each regulator was dropping
(12V - 5V) = 7V.
At one amp that's seven watts of heat.
No wonder the heatsink was a bit toasty!
Of course normal operation isn't anywhere near worst case: the Mega Drive 2 power supply is only rated at 850ma which provides a good guide for the power consumption of this clone system. But even so it's still a lot of wasted energy and the heat certainly won't do the case above it any good. So I got hold of some TR10S05 switching DC regulator modules from Farnell. These have the same pinout as the linear regulators but use a miniature switching regulator to supply the same load with no heatsink needed. Perfect! The only downside I could see was that they apparently have more noise in the output (from the switcher I assume).
Removing the existing regulators was a bit of a pain — I had to remove the output smoothing capacitors before I could unscrew them from the heatsink. The heatsink itself was attached to the PCB with two rivets that had gone through the wave solder along with the through-hole components. I ended up having to bend the heatsink fins slightly so I could slide it off the pins.
The new regulators fit neatly inside the footprint of the original components and worked (almost) flawlessly. The one problem I had was that the regulator at the rear of the machine would only output 3.5V until I removed the electrolytic capacitor from across it's output. The other regulator had no problems so that was a bit of a mystery. With that out the way the machine is running cooler and I'm happier! I've kept the original parts in case I ever need to refer to them…