What's the model number of the fan that's on it now? You will need that info to determine the flow and static pressure, the voltage and current/power. All of these factor into how the fan works. Since these amps tend to run hot, and because it's important.
There were 3 different models of this amp, the first didn't have a fan and had problems driving 4 ohm loads in bridge mode, The second version built after Sept 1993 (may have been labeled 400S) used a 24V fan (I'm pretty sure but not positive which is why you need to double-check) wired 1/2-wave to a lower voltage tap on the transformer's main rail coil, some just switched on and off but some were low speed/high speed. The third version, which was during the Fender era (labeled Corona, CA) and were generally SM-500's BUT there may have been some with this power module released as SM-400, using a 24V fan driven by a proportional speed control circuit. These are the trickiest of all to match, since the control circuit and the fan's performance curves need to match up fairly closely.
Note also that some original SM-400's were factory modified with a fan being added, these would have been done differently because there wasn't the lower voltage tap present.
First step... get the fan model that's on there.
