Replacing failed powersupplies on qbus PDP-11s

Gordon JC Pearce MM3YEQ wrote: That's a bit of a sweeping statement! I agree badly done simulation is worse than useless, but I have to use simulation all the time in my job (power station planning). Some of the things I frequently have to tell both trainees and clients are worth mentioning here, though: 1. The model is not the system. "The model predicts it" is not an explanation for anything. Any prediction of the model has to represent a physical process on the system you are studying, and you need to understand what that process is and why it happens [1]. (Or, for that matter, why it doesn't behave as the model predicts) 2. No one model is good for everything. You need different models for different studies. Before you use a model, it is important to understand what it is for, and what it can do. This often involves quite an in-depth look at how the simulation software actually uses the data. [2] 3. You can't make a model more accurate by throwing a bigger computer at it. (So many clients don't appreciate this!) A model stands or falls by the data that goes in it. It doesn't matter how sophisticated the simulation software, if you haven't got good data you can't do an accurate model. GIGO. Take those three things into account, and simulation is a useful tool. But it's no more than a tool, and some of the best simulations are very simple models that run on that ubiquitous platform, the human brain... Philip. [1] I remember long arguments with the suppliers of some simulation software on this subject. They insisted that a second harmonic oscillation should die away so quickly they didn't need to model it. I maintained that it was a result of dc flowing in the stator of an ac generator (the Hammond Organ effect), and should die away no faster than the dc that caused it. [2] Which is why the engineer at the network company (I'll not name them) couldn't reproduce my results. She was trying to cheat by putting her calculated time values of impedance in the simulation software's initial value, and didn't realise what else it would affect. To give her the credit that's her due, she realised what was wrong as soon as I pointed out how the software calculated one of the impedances. But I shouldn't have had to - and it was just luck we both had the same software...