Multibus ST-506/QIC-02 (was: Re: SGI IRIS 1400 ...)

From: Tothwolf <tothwolf_at_concentric.net>
Date: Mon Jul 22 21:44:00 2002

On Tue, 23 Jul 2002, Tony Duell wrote:

> > Thats where my board differers. The 4 transistors on this board are
> > connected to one of the hybrid circuits (A103 on this board).
>
> Ah, so the transistor array, etc are somehow inside this hybrid circuit.
> Pity...

I guess its not exactly a hybrid circuit. Some of those small SMT boards
are coated with the same material most ceramic caps are coated with. Most
of the boards are not coated, and the standard SMT parts are somewhat
accessible. I think A103 contains 3 small 8 pin chips which control the 4
amplifier transistors. How many transistors made up the original
transistor array?

> > Well, I'm not so sure any of it is at fault after thinking about it...
>
> I would still check them. I've had hard drive servo amplifiers fail
> before.

What would be the easiest way for me to check them?

> > When I connected a meter from ground to each of the servo connector pins
> > while the servo coil was connected, I got just under 11VDC. When I checked
>
> I would have expected them to be close to 6V if the servo amplifier was
> working correctly. Having them both so close to the +ve supply makes me
> suspect the NPN transistors or the sense resistors.

The sense resistors read very close to their printed value. How else
should I test them?

> > the voltage across the two pins, the 0.4VDC was just the voltage
> > differential. When I checked the voltage across the servo connector with
> > the servo coil disconnected, it was near 12VDC because it was trying to
> > move the coil. It seems like that part of the circuit may be working fine,
> > because the H-bridge controller chips on that hybrid package are switching
> > those 4 transistors on and off to position the coil.
>
> So you think the servo may be locking on-track? Possible, but then why
> does it spin down? And why are the 2 positioner coil pins both close to
> the +ve supply?

I'm not so sure its locking on-track either... Do you think the drive
would spin down if it had trouble reading only some of the servo tracks?
The drive doesn't sound like it has any head/platter damage, and spins up
smoothly. Should the positioner coil pins be right about half the +ve
supply for most servo circuits? I certainly don't hear the clicking noises
I'm used to hearing when heads are seeking.

-Toth
Received on Mon Jul 22 2002 - 21:44:00 BST