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 Post subject: Priam 3450 Head Re-alignment to Platter Tracks
PostPosted: February 3rd, 2018, 16:43 
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Joined: February 3rd, 2018, 15:25
Posts: 2
Location: Lancaster, Ca
I have a tough issue. After replacing an embedded bad servo preap chip on the servo head assembly, I'm getting a "cylinder Address Miscompare" error.
I need to realign the heads to the correct position over a specific track.

In the pic attached each head has a "eight sided" allen type alignment fitting with pass thru between heads. I'm guessing that a tool with 4 of these 8 sided fittings spaced correctly and individually adjustable while holding the other 3 stationary.Without it any attempt to adjust one set moves the other also.
I've tried to vertically align all head sets but haven't been successful.
There's a bit of play in the screw holes. Loosing the 3 phillips allows allows side movement, but little forward back movement.

Has anyone dealt with these issues after replacing heads?

TIA
Jack


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 Post subject: Re: Priam 3450 Head Re-alignment to Platter Tracks
PostPosted: February 3rd, 2018, 21:03 
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Joined: September 8th, 2009, 18:21
Posts: 10355
Location: Australia
Just FYI, I found this repository of Priam documents, photos, and EPROM dumps:

http://bitsavers.trailing-edge.com/pdf/priam/

I can't see how you could align the R/W heads to the servo head in such a design (sealed HDA). AISI, the heads are designed to be assembled without regard to physical alignment. The servo surface would be prerecorded with special dibit patterns, and each of the R/W heads would be aligned via a low level format. This is in contrast to a removable disc pack where all R/W heads must be physically aligned to the servo head to ensure pack interchangeability.

I can only relate my experience with the latter design, specifically Control Data's removable SMD drives. These had a dedicated servo surface and 19 R/W surfaces. The actuator was a linear voice coil type with the heads parking on a ramp. Each head was fastened to the headstack via a small Allen screw. There was a longitudinal adjustment slot in the side of each head with corresponding alignment holes in the casting of the headstack. A tool shaped like an old sardine can opener (ie a key), with an eccentric nipple on the end, was inserted through the hole in the headstack and its nipple engaged the adjustment slot in the head. The fastening screw was backed off to a prescribed torque, and the tool was rotated CW or CCW to drive the head either toward or away from the spindle. A special CE pack was used for this alignment procedure. The technician would observe the dibit pattern on an oscilloscope while moving the head until an appropriate "cat's eyes" signal was obtained. Then the fastening screw was torqued down.

I don't know how you would approach your problem, but in my case I was able to avoid using an expensive CE pack and disc exerciser by utilising the customer's own software pack and a machine code utility I had written. My utility would command a seek to a specific track at the centre of the disc, and would then read the ID from the target R/W read. The reported ID might be one or more tracks away from the target, in which case I would move the head in the appropriate direction while counting the required number of maxima and minima in the data pattern. The console would beep when the head was on-track.

An SMD disc exerciser is pictured here:
http://www.hddoracle.com/download/file.php?id=1491&mode=view

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 Post subject: Re: Priam 3450 Head Re-alignment to Platter Tracks
PostPosted: February 3rd, 2018, 22:46 
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Joined: February 3rd, 2018, 15:25
Posts: 2
Location: Lancaster, Ca
Thanks very much for your valued knowledge! Really!
Have collected everything Priam of this era. No schematics for this unit, have the field service manual which is decent on interfacing and some board troubleshooting but nothing HDA related. Using Priam's 15" schematic which is within the era, helps but outside of that, its a general FYA on a like circuit design.

I have a working 3450, same-o-same-o but the board revs are different. I've tried swapping servo and RW boards without any joy.

I have spun down the unit, loosened the 3 large Phillips, inserted a exact size rod thru the alignment(?) hole and moved the set of heads laterally a bit left then right. Then retightened. Rerun the confidence test still has a cylinder address mismatch but with different actuals read back. So it appears to be an alignment issue. And you are right, I haven't a clue on attempts to dynamically position the individual heads with the media spun up. And too boot there's a RW head preamp board sitting right over the carriage making access virtually impossible. I'm thinking the sucker was assembled, then the servo tracks were formatted followed by the RWs. Trying to match that in reverse might not be possible.
I was think along the lines of coding up a stepper to command to an outer track then compare waves and amplitude versus the working. I wish I had your Allen adjustments!
Trial& error having to power down then disassemble/assemble to blindly tweak sucks!
Thank again!
Jack


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 Post subject: Re: Priam 3450 Head Re-alignment to Platter Tracks
PostPosted: February 3rd, 2018, 23:15 
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Joined: September 8th, 2009, 18:21
Posts: 10355
Location: Australia
Is there any way to read the sectors in raw CHS mode? You could then concentrate on reading one surface at a time (assuming your goal is data recovery, not drive repair).

BTW, it's a pity that your drive does not support the Servo Offset Plus/Minus bits in Tag 3 (Control Select). These enable the drive to position the heads slightly off-track for error recovery purposes. :-(

Just to satisfy my curiosity, could you tell us the part number of the servo preamp IC?

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