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

From: Tothwolf <>
Date: Mon Jul 22 20:45:01 2002

On Tue, 23 Jul 2002, Tony Duell wrote:

> > This board uses 2 MJE200 transistors, 2 MJE210 transistors, and 4 1N4001
> > diodes that appear to form some sort of H-bridge.
> As does the V150...

I think that much is identical to the V150.

> The emitters of the MJE201s (PNP) go to +12V. The emitters of the
> MJE200s (NPN) go to the 0V rail via 0.499 ohm resistors (current
> sensing). The collectors of each pair of transistors are connected
> together and go to one side of the positioner coil connector. There's a
> diode (1N4001?) across each truansistor, C-E, to protect it against
> back-emf spikes.

I also just finished sketching out the schematic for that part of the
board. The four diodes are indeed 1N4001.

> The base circuit involves several resistors and the 4 sections of an
> MPQ6002 transistor array (U39 on my board).

Thats where my board differers. The 4 transistors on this board are
connected to one of the hybrid circuits (A103 on this board).

> But I would start by testing all 4 power transistors and the 2 current
> sense resistors. Those are the most likely things to fail.

Well, I'm not so sure any of it is at fault after thinking about it...

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
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.

Does that sound right?

Sigh. If I had a pair of probes for one of my scopes, I could verify this

Received on Mon Jul 22 2002 - 20:45:01 BST

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