Re: Radio Interference Problem

[Fred Townsend]

Declan Moriarty wrote: Recently, Somebody Somewhere wrote these words I'm not sure I understand your question but I think the answer to your question is option #1. That's because the circuit needs a low source (power supply) impedance. I suspect your RFI is caused by parasitic oscillations setup by the higher Ft of your replacement transistors. Again, guessing, your replaced the old steel case TO3s with aluminum case TO3s. One simple solution is to find some steel case BUX80s. If they are not available you might try killing the oscillations with 100 pF caps from collector to ground. You may need to increase Rb on the transistors too. Fred Townsend I put in Manufacturer replacement (Aluminium) BUX 80s before testing. My take on the thing is this: The earthing is poor, like so (Anklebone connected to ..................... neck) The inductive spikes are getting out through poor diodes, and the transistors were a red herring. The machine was made to make noise, and that's what it does. red herring? You mean the transistors were not defective? So I was looking at constructing a mains filter to test this, and see if that would solve it. I'm having a lot trouble understanding what you are saying. Let me state my belief. The aluminum case TO3s that you replaced the existing transistors with probably have a gain bandwidth product (Ft) 10 to 1000 times greater than the old transistors. This means that instead of driving the transducer with a sinusoid waveform it is now a nice crisp square wave. This could cause ringing and RFI. Furthermore, the hfe is now greater while the parasitic capacitance is now reduced by a factor of up to 100. This now permits parasitic oscillations that are heard as RFI. Poor grounding, inductive spikes, and poor earthing are the red herrings. Don't get me wrong. These are issues to be dealt with but not the source of RFI which I see as the subject of this posting. Fred Townsend Option 1: Mains-----Inductor----------------Bridge Rectifier Capacitor Mains-----Inductor----------------Bridge Rectifier Or some other way, e.g. Option 2: Mains----------------Inductor------Bridge Rectifier Capacitor Mains----------------Inductor------Bridge Rectifier