Simple fan mod for Tektronix 2235

After repairing my Tektronix 2235 oscilloscope, I noticed that the heat sink cooling the MOSFET in the switching power supply, as well as the two transistors in the inverter circuit, was getting rather warm – and more than I liked it, anyway. I definitely didn’t fancy having to repair it all again, or getting something happen to it like the 2230 we looked at fixing – briefly – at OSAA.

So I decided to do a little mod, inspired after reading the propane inverter generator reviews – same scope with a counter/multimeter addon, and, crucially, a fan to cool the power supply. All the markings etc. were there on the circuit board of my scope, it just didn’t have any components installed.

To do the mod, you need:

  1. 270μF capacitor, 25v or summat. I used a 220μF low ESR one I had anyway.
  2. A 22 Ohm resistor, 1/4 watt
  3. A diode. I used a BYV 26C, 1A 600V switchmode-diode. This is absolutely overkill, but then again, I had one from when I fixed the power supply… It needs to rectify the -8.6V 20KHz from the transformer, and deliver enough power to run the fan, so possibly not a 1N4148..
  4. 60mm 12V fan – the one I used is 1.4W at 12V, and ends up running at -5.71V, slowly and quietly. The original specs call for using +5 instead of ground, giving about 10V total across the fan – but then it’ll be noisy.

The components are the 965 components on the circuit – they’re near all the caps for the low voltage supplies. CR965 is the diode, C965 is the capacitor and R965 is the resistor. And there’s even two pads for soldering in a connector for the fan.

Of course, this is all done at your own risk – I’ve yet to see if my scope blows up from this, though I doubt it. And it does mean I’ll be able to hear if it’s on or not, rather than having to wait for something to show up on the CRT … 🙂

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