7900A drive: 1 , Me - 0

From: James Willing <jimw_at_agora.rdrop.com>
Date: Tue Sep 7 22:53:23 1999

At 10:15 PM 09/07/1999 -0500, you wrote:
>Here's another installment in the ongoing HP7900A drive saga (yes, it's a
>plea for advice and/or sympathy <grin>)

>Hit unload, and after about 20 seconds heard very soft HDI. This time I left
>the cartridge in, powered everything off, removed the covers, and this time
>the heads weren't retracted at all. They were sitting right on the media.
>Strange thing is, there's no marks on the media, the lower heads I replaced
>look pristine, and the upper heads have only the very slightest indication
>of oxide.

>Gee - perhaps this is why the drive originally had a head crash before I got
>it? I've been through the "theory of operation" text and flowcharts. Several
>things confuse me. First, I thought that hitting the unload switch was
>pretty much of an immediate voice coil retraction - I didn't think this went
>through all the normal logic circuits. Second, the coil can't be completely
>shot I wouldn't think, because as I understand it the drive ready light
>won't even come on unless the positioner moves out to cyl 0 correctly.
>Before I replaced the heads and media, I don't THINK I had this problem, I
>think I would have noticed if the heads weren't retracting before. But, I
>can't imagine that anything I did related to replacing the lower head set
>and the lower platter would have anything to do with this. Input anyone?????

Well... not having (personally) worked on the HP drives, but having done a
fair deal of time on CDC cartridge drives which sound functionally similar,
I'll offer a couple of thoughts...

You are basically correct in your observation that 'head retract' does not
usually go through the 'normal logic'. In the CDC drives at least, the
head retract circuit is the same as the 'emergency' retract circuit (i.e.
get the heads OFF in the event of a power failure).

In brief, the circuit is this:

Keeping in mind that it takes more force on the positioner to pull the
heads back up the load/unload ramp, the standard drive circuit does not do it.

When the drive is powered up, a relay is engaged which opens a circuit to
the positional coil, allowing the normal drive circuit to function.

When you hit unload, or the power fails, this relay drops out which
connects a large(ish) capacitor (which has been charging all this time) to
the coil, and WHOMP! You unload the heads! (the discharge rate of the
'cap also ensures that the coil does not bounce back off of the stops)

Really! It is designed this way!

So... (if the HP drive uses a similar system) you look for: An oddly out
of place looking (heavier than the norm) cable coming to the positioner
coil, a large(ish) relay with good sized contacts, and a large electrolytic
(metal can) capacitor!

These should all be connected together in a fairly logical fashion...

A frequent cause of 'retract failure', especially after the unit has been
aligned/serviced/ heads replaced, is the failure of the technician to
reconnect the unload system!

Also... burned relay contacts, bad relay, bad capacitor, failed charging

It is actually part of the stated proceedure to disable the unload system
when you are working on the unit, since there are a number of conditions
that can trigger it (spindle speed fault, positioner overrun, off-track
fault, etc...), and you only have to have your hand behind the head
carriage ONCE (especially a large one like a CDC Phoenix) when that system
triggers to ensure that you will NEVER do it again! (BTDT!)



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Received on Tue Sep 07 1999 - 22:53:23 BST

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