Shugart 851 power problem

From: Richard Erlacher <edick_at_idcomm.com>
Date: Tue Apr 25 15:25:07 2000

It seems to me that the problem involves the two power supplies, and
probably not any of the S-100 hardware.

If the enclosures are connected with a substantial protective ground, that's
probably OK if the protective ground subsequently goes to a genuine "earth"
ground. If you examine the ground lines on the FD cable, and there are
plenty of them, they should all be connected to your system ground at both
ends and bypassed/AC-coupled to frame ground. If your drives have signal
and frame ground connected together yet not connected together at the FDC,
i.e. in the Imsai box, that could be contributing to the problem. If I were
in your position, I'd hook the 'scope up to the ground line with the
reference ground at the S-100 bus, not at its power supply. Then you will
probably see an oscillation. If that's the case, disconnect the drives and
any of your S-100 hardware from frame ground, paying particular attention to
the serial I/O boards, where it's common to connect pins 1 and 7 of the
DB25's together. If you don't want to interrupt that connection, replacing
it with a 0.1uF bypass cap, then I'd recommend you connect all your supply
grounds to frame, using a 1"-wide copper braid, and connect that braid to a
ground like a water pipe. If you have plastic plumbing, tear it all out and
replace it with iron (snicker!), otherwise use a gas pipe or some other
adequate ground attached with a really low-impedance connection. Since your
ground isn't a single point, you need a really low resistance between them
all. That's why it's actually better to isolate frame and power supply
(signal) ground.

The comparison with the TM-100's doesn't really show anything because it
uses a different supply, not to mention that it uses a small fraction of the
current.

There's probably just a single ground line floating somewhere between the
drives and their supply ground. It may come up stabile enough, but suddenly
jump into oscillation as soon as a surge occurs on the +24 Vdc supply.

 When you hook the 'scope up, look at it while the system tries to move the
heads. If the regulators for the FDD's remain solid, it's got to be the
connection between the FDD box and the Imsai box. If not, then look for
inadequate grounding or inadequate bypass at the regulator. This is
particularly critical if you have a pass-transistor regulator, since there's
a phase shift there that often causes oscillation. There should be a solid
frame ground, though both supplies should be connected to the "earth" ground
at their frames regardless of whether they're well connected to one another.

----- Original Message -----
From: Bill Sudbrink <bill_at_chipware.com>
To: <classiccmp_at_classiccmp.org>
Sent: Tuesday, April 25, 2000 1:19 PM
Subject: Shugart 851 power problem


> Hi,
>
> I have a pair of 851s in a nice case with power supply.
> They are attached to a Cromemco 16FDC in an IMSAI box
> with a Cromemco ZPU and an Expandoram 64K board. I'm
> using the ROM enable on the 16FDC to drive the phantom
> line. There is a reason for these details...
>
> Here is the problem:
>
> #1 Power up the drives by themselves:
> At the drives:
> 5 volt line reads 4.9
> -5 volt line reads -4.9
> 24 volt line reads 23.9
>
> #2 Power up the IMSAI box:
> At the drives:
> 5 volt line reads 1.2
> -5 volt line reads -0.9
> 24 volt line reads 2.6
> and (needless to say) the drives don't work. The Cromemco
> ROM monitor gives all sorts of errors trying to access them.
>
> #3 Power off the IMSAI box:
> (Same as #1)
>
> #4 Power on the IMSAI box again:
> At the drives:
> 5 volt line reads 5.0
> -5 volt line reads -5.0
> 24 volt line reads 24.0
> and the drives work fine.
>
> What gives? Does this make sense to anybody out there?
>
Received on Tue Apr 25 2000 - 15:25:07 BST