Macintosh SE/30 replace processor MC68030RC16B to MC68030RC30C

[Plottem]

Good morning, I am restoring a Macintosh SE/30 that I received as a gift. The case is the following, the motherboard did not have the Motorola MC68030rc16 shock type processor (someone removed it to sell the gold) I bought the same one but the seller sent me a MC68030RC30C, instead of the MC68030rc16B, I assumed that there would be no problem in that in instead of 16mhz I had 30mhz. I recap the board and got it working. The equipment works but the resistor R29 (12 Ohm, 2w) of the analog board gets very hot!!! and I'm afraid it might burn. Do you think that the increase in temperature in this resistor could be due to the difference in MHz between the original processor and the one that I have installed? Greetings

 

[Phipli]

Good morning, I am restoring a Macintosh SE/30 that I received as a gift. The case is the following, the motherboard did not have the Motorola MC68030rc16 shock type processor (someone removed it to sell the gold) I bought the same one but the seller sent me a MC68030RC30C, instead of the MC68030rc16B, I assumed that there would be no problem in that in instead of 16mhz I had 30mhz. I recap the board and got it working. The equipment works but the resistor R29 (12 Ohm, 2w) of the analog board gets very hot!!! and I'm afraid it might burn. Do you think that the increase in temperature in this resistor could be due to the difference in MHz between the original processor and the one that I have installed? Greetings

You should be fine with your 30MHz processor, if anything, it will use less power and run cooler.

I suspect the resistor is meant to run that hot.

 

[Plottem]

You should be fine with your 30MHz processor, if anything, it will use less power and run cooler.

I suspect the resistor is meant to run that hot.

Thanks for your answer, the temperature gets too high, you have undone the silicone that holds the fan wires, and these are getting shiny, and it smells like burning.

 

[Phipli]

Thanks for your answer, the temperature gets too high, you have undone the silicone that holds the fan wires, and these are getting shiny, and it smells like burning.

I suspect it isn't because of your CPU, unless the CPU is faulty. Try powering on without the CPU installed and see if the resistor still gets hot. If it does, search near the resistor (for things electrically connected) for a part that has shorted.

Can you desolder one side of the resistor and measure if it has the correct resistance?

 

[Plottem]

I suspect it isn't because of your CPU, unless the CPU is faulty. Try powering on without the CPU installed and see if the resistor still gets hot. If it does, search near the resistor (for things electrically connected) for a part that has shorted.

Can you desolder one side of the resistor and measure if it has the correct resistance?

Good idea!! I'll do the test! thanks!

 

[Melkhior]

I bought the same one but the seller sent me a MC68030RC30C, instead of the MC68030rc16B, I assumed that there would be no problem in that in instead of 16mhz I had 30mhz

You probably know that already, but for the sake of completeness: your new CPU is _capable_ of 30 MHz operation, vs. the original one that would only be rated for 16 MHz for whatever reasons. However, in the SE/30, this CPU will still _run_ at the 16 MHz, as dictated by the CPU clock. This is totally safe from the CPU point of view (as mentioned by @Phipli, if anything it will save power/energy as the 30-MHz rated CPU is likely running on less power/cooler than the 16-MHz rated one), but it doesn't help with performance.