How to build a $20 47A power supply

[BrianG]

If you have a multimeter that has a frequency display, you could use that to get an idea of the frequency; sometimes frequency measurement is a mode of AC voltage measurement. This won't tell you the duty cycle though, for that it will just take some experimentation. It would be much faster to borrow a oscope if at all possible.

I'd start by trying to nail down the frequency by starting with fan rpm - 66Hz for a 4000rpm fan. 4000 rpm (rev/min) is 66.67 rev/sec (Hz=rpm/60), and then double it if that doesn't work, and so on. Once you get the frequency, start with a low duty cycle (very narrow pulse) and work your way up to 50% duty cycle.

The 555 timer will allow adjustments in both frequency and DC% (duty cycle). However, there are a few things to consider. IIRC, a 555 timer will have limits on DC%, if it won't get as narrow (or wide) as you need, you may need to use a transistor configured as an emitter-follower which will basically "invert" the output. Also, adjusting the resistances tends to adjust both freq and DC% at the same time, so sometimes it is easier to use two 555's in series, the first one as the astable oscillator to generate the right frequency, the second as a monostable oscillator to adjust the DC%. More complex but much easier to control freq and DC% independently. The 556 timer is just two 555 timers in one IC case, so that might be a better way.

I know you want to replace both fans, but how loud is just one fan by itself? Because like I said, you could tap into the tach signal of one fan and feed it to the other input and then use whatever fan you want as the second one if you need it.

If you still want to go with building your own oscillator, here are a couple datasheets for the 555 and 556 timers. Each has an applications section at the end with circuit examples: 555 Timer, 556 Timer.