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

From: Tothwolf <>
Date: Wed Jul 24 00:20:00 2002

On Tue, 23 Jul 2002, Tony Duell wrote:

> > 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
> I thought some/all of the resistors were formed as thick film components
> on the ceramic substrate. Which makes it a sort-of hybrid circuit.

You're right. I wasn't thinking about those.

> > 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?
> The original transistor array was 2 NPN transistor and 2 PNP
> transistors. A 14 pin DIL package, I think
> Those 8 pin chips sound like single/dual op-amps (of which there are
> many in the servo circuit). Are there also discrete SMD transistors on
> this board?

The 8 pin chips are marked 4558. They probably are op-amps, but I haven't
looked them up yet. There are indeed 4 SMT transistors on A103. I
overlooked them the last time I had the board off the drive.

> Desolder them, and at least use a diode check (on your DMM?) to check
> the base-emitter and base-collector junctions. And make sure that
> collector-emitter tests as open-circuit both ways round.
> If you have a better transistor tester (I use a Tekky 575, but then I
> would!), you can use that, but then you don't need me to tell you how.

I don't have a really nice transistor checker yet, but I do have a couple
of pocket types that work well. I desoldered and tested all 4 1N4001s and
all 4 transistors. I checked all of them with a one of the small testers,
and then checked them all again manually with my DMM. None of those 8
parts appear to be bad.

> > would spin down if it had trouble reading only some of the servo tracks?
> Possibly? But then again, I'd expect it to attempt to move the heads
> under such conditions. A drive where the heads don't move is either
> locked on-track, or has a servo amplifier failure, at least most of the
> time.

I certainly don't hear the heads moving, but I'm not sure how noisy these
drives are anyway.

Received on Wed Jul 24 2002 - 00:20:00 BST

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