7900A drive: 1 , Me - 0

From: Merle K. Peirce <at258_at_osfn.org>
Date: Wed Sep 8 01:58:22 1999

Jim, thanks for those comments. I have a Phoenix drive that has crashed
and I've been thinking about trying to take a look at it. I've been
following the 7900 thread especially to get an idea of what I might be in

On Tue, 7 Sep 1999, James Willing wrote:

> 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
> circuit...
> 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!)
> FWIW...
> -jim
> ---
> jimw_at_computergarage.org
> The Computer Garage - http://www.computergarage.org
> Computer Garage Fax - (503) 646-0174

M. K. Peirce
Rhode Island Computer Museum, Inc.
215 Shady Lea Road,
North Kingstown, RI 02852

"Casta est qui nemo rogavit."
              - Ovid
Received on Wed Sep 08 1999 - 01:58:22 BST

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