[sebhc] H89 BOOT SUCCESS

[Paul A. Pennington]

    Barry said:

> The power supplies in all Zenith products prior to the Z-100 series were
> linear power supplies, while the power supplies in all PCs are switching
> power supplies.

    Not true:  the H/Z-89/90 had the mother of all switching power supplies 
inches away from the disk drive in the form of a CRT assembly.  I glossed 
over that detail because I thought everybody on the list already knew that.

> There is no issue with a linear supply being "open" vs. in
> a metal box.  In fact, the reason for the metal box is that switching 
> power
> supplies have lethal voltages in them, while linear power supplies do not
> (except on the primary side of the main power transformer).

    My experience on that has been different.  While protection is one 
reason to enclose switchers, a bigger benefit, in my opinion, is less 
radiated noise from the switching circuitry.  You don't have to take my word 
for it:  bring an oscilloscope probe near an open switcher (or CRT).  You'll 
see some pretty wild voltages floating around.

    My statement about Heath vs. PC's was based on swapping several Tandon 
disk drives between a Z-100 (non-CRT model) and an original IBM PC.  To me, 
this pins the noise down to the switching power supply:  some drives that 
worked in the PC would not work in the Z-100.  I observed the same thing 
with Kaypro computers (with open switcher) vs. the PC.

    That's my story, and I'm sticking to it :-)  On what do you base your 
opinion that radiated noise from switching power supplies is "not relevant"?

    Paul Pennington
    Augusta, Georgia

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