RK05 Help

[Tony Duell]

> a cartridge to be put in, and once a cart is inserted, allows it to be spun
> up, but
> the heads will never load and the drive will never go READY without the
> controller
> attached.

Are you sure? It's been a long time since I've done this, but I thought 
an '05 would load the heads without a controller connecteed, provided 
there's a pack installed, power is good, etc.
> The safety relay does pull in as expected when he unit is powered up.  

OK, so far, so good.

> However, the blower motor will not run.  I've checked
> the relay contacts...they are fine.  120VAC is present on two of the
> three wires that go into the blower motor.  There is a 5uf motor start
> capacitor that, from what I can tell, seems fine.  But...the motor
> doesn't spin.  The funny part is that both RK05's act this way.
> I'm puzzled.  The motors aren't seized -- they spin very freely.
> I've put an ohmmeter across the windings of the motor and there
> are no shorts or opens.  
> 
> I'm really stumped!  Could the start capacitor be bad?  I don't know

Could well be. It's actually a 'run' capacitor I think (there's no 
starting relay for the blower motor). 

Baiscally, the motor has 2 windings. One is directly across the 120V AC 
input. The other has the capacitor in series, 120V AC across the lot. The 
result is that the currents in the 2 windings are not in phase with each 
other (due to the capacitor in series with one of them). These windings 
magnetise alternate poles in the motors stator, so you get 2 out-of-phase 
magnetic fields in the stator, which appears a a 'revolving' magnetic 
field. In effect the rotor is dragged round by this field (yes, this is 
a simplification, but it should give an idea as to what's going on).


> much about how these types of AC motors work.  I swapped the start
> capacitor with one from another "parts hulk" drive that I have, and
> it didn't make any difference.
> 
> Any ideas, anyone?

What I would do now : 

Remove a blower (disconnect the 3 leads at the barrier strip, undo the 4 
allen-head screws hoding it in place, take it out.

Strip it down. THe ring round the intake comes off by some self-tapping 
screws under the foam. Then the impeller comes out by undoing the 
setscrew. Then a coupleof nuts to hold the motor to tbe blower housing.

The motor also comes aaprt (4 little metal clips). Pull out the rotor and 
bearings.

Look at the windings. Damaged insulation is, alas, common on these 
motors. Shorted turns will mess things up, alas.

It's then put the motor back together and wire it up on the bench with 
the staring capapcitor. Try powering it up. Measure the voltages across 
the 2 windgs (one should be 120V, the other somewhat less due to the 
capacitor, but certainly not zero). 

Try a new capacitor if you can get one. Capacitors do go off, even when 
not in use.

-tony