FD400 drive troubleshooting

Tony Duell ard at p850ug1.demon.co.uk
Thu Nov 22 17:14:06 CST 2007


> 
> That's interesting -- who?
> 
> The way that the service manual tells you to check the speed is the pulse
> width on a specific test point on the PCB. I was just going to use a laser
> tach on the spindle...but I have to get a tach first :-)

You can of course determine the spindle speed by looking at the timing of 
the index pulses (one per revolution on a soft-sectored disk, so 5Hz for 
a 300 rpm disk, etc).

But there is one ;gotcha' in doing this. Don't simply hook up a frequency 
counter and measure the freqwency. If you have a 1-second gate time, 
you'll get 5 pulses through for quite a wide range of speeds. It's just 
not accurate enough. It's better to use the period maode of your counter 
and see how many (say) 1MHz clock pulses you can count between index 
pulses from the drive. 

Alternatively make a stroboscope disk (either copy the one off a TM100 
pulley, or work out howm many segments you need and write a trivial bit 
of postscript to draw it), stick that on the spindle pulley or 
direct-drive motor rotor and view it under amins-powered lamp. 

-tony



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