1570 drunken stepper

From: Pasi Ojala (albert_at_cs.tut.fi)
Date: 2006-12-27 00:07:43


Just thinking aloud here..

While making inventory of my Commodore stuff I took a look into a
non-working 1570 drive. It seems to load a directory if the head
is positioned correctly, but seeks seem to go anywhere but to the
desired direction.

The waveforms measured from the stepper signals T0-T3 show the
problem: some of them just don't work like they should. This
1570 uses the gate array to generate the signals (instead of
using 7406 and 7407 ports and the gate array) and the R/W hybrid
Y0-Y3 seem to get clean pulses (around 1Vpp).

The stepper motor should be okay, as I measure 80 Ohms from it.

So, the problem should be in the R/W hybrid, right?

If the hybrid internals are like the discrete transistor+diode
thing that is in the 1541, there should be a diode from +12V to
T0-T3, but I measure 1.2MOhm (and 3.0MOhm in the other direction)
from all phase signals. So the diode probably does not exist.

From the phase signals it would seem that two phases do not give
enough current (the waveform drops, unlike in a working 1570,
although that one is built on a 1571 motherboard), so it could
point to the read/write hybrid transistors.

Or is this just a coincidence?

(Yes, I temporarily gave up repairing the non-working C128.
 Two working ones should be enough.)
"One from three leaves one."
	-- Marcus in Babylon 5:"Exogenesis"

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