Crown XLS-202-402-602 Fan Issues
Having worked on a few 1st generation Crown XLS power amplifiers, it seems that fan problems are common. Either the fan noise is unbearably loud or they stop working altogether. This compelled me to take a closer look at the cause.
Crown XLS Power Amplifier Fan Schematic
Fig 1 is the circuit of the fan control. It operates from -24V supply rail. The two 12V (0.17A) fans are wired in parallel, followed by two resistors to 0V.
When the amplifier is powered up, the fans will be drawing a current of 0.34A. The same current passes through the two resistors resulting in a 12.24V across them. Subtracting 12.24V from 24V will result in approximately 12V across the fans.
During hard use, transistors Q1 and Q2 switch on when the heatsinks get hot. The lower 24Ω resistor is then bypassed. Now there is a 4.08V across the upper 12Ω resistor only. Add 1.2V voltage drop for the two transistors, that’s a total of 5.28V. Subtract this from 24V results in 18.72V. I will round this off to 19V for easy reference.
Cause and Effect
So the big picture is this. When you switch on the XLS, the two fans work at what they are designed to do, that is run at 12V. When the heatsinks are hot, they now run at 19V. At this higher voltage, they will spin faster. Good for cooling. The problem is when the fans are over-run like this, they will sound like jet engines. Eventually, the fan motor overheats and seizes up.
The simplest solution is to re-wire the two fans in Series (Fig 2). On powering up, the fans will be about 18V. When the heatsinks temperature rises, the fan speed increases because their working voltage has gone up to 21V. This is below the 24V so there won’t be any loud fan noise. Under-running the fans will also extend the life of the fans.
It is interesting to note that in the 2nd generation of XLS amplifiers, Crown made the case taller (3u) and installed a single 24V fan at the rear. Those models should not have fan issues.
October 24, 2018Amplifiers