7900A drive: 1 , Me - 0

From: Jay West <jlwest_at_tseinc.com>
Date: Wed Sep 8 16:05:44 1999

You wrote...
----->Almost all drives have an 'emergency retract' which is activated if
>power fails while the heads are loaded. Typically, this consists of a
>relay that drops out when the power fails that then connects a large
>capacitor, or more often a NiCd battery, to the voice coil. The head is
>rammed against the outer end stop, where it opens a microswitch,
>disconnecting the battery.

Yes, the power board does have a set of 3 nicads on it, and they are
definitely for retracting the heads in the event of a loss of AC power.

>Since this involves somewhat violent head movements, which are not good
>for the heads (or, indeed, for the positioner), many drives _don't_ use
>this to retract the heads on a normal unload. Instead they use the normal
>servo system. Certainly the DEC RK05 does this (I have just looked in the

I haven't checked the schematics myself definitively, but, I heard from one
repair guy that it does not use the nicads for a normal unload, just for a
power fail unload.

>If it uses the servo system, first check the power supply voltages. Loss
>of one rail to the servo amplifier may mean the heads can move in one
>direction but not in the other. Then, insert a pack, spin up, with the
>covers removed. When the heads load, try to (gently) move them. They
>should appear to be locked over track 0 -- if the heads move slightly
>off-track, the servo system should pass a current through the voice coil
>to try to move them back. If there's a problem with the servo, you might
>find that the servo can move the heads towards the spindle (i.e.
>attmepting to force them away from the spindle has no effect) but that it
>can't move them away from the spindle (and thus if _you_ try to move them
>towards the spindle, it doesn't resist).

Now THAT's handy info. Thanks! I'll give that a shot tonight or tomorrow.

Jay West
Received on Wed Sep 08 1999 - 16:05:44 BST

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