26 Apr
2008
26 Apr
'08
1:32 p.m.
Date: Fri, 25 Apr 2008 22:39:29 +0100 (BST)
From: (Tony Duell)
Sure, but the fan would turn slower on 50Hz mains. I doubt that would
cause the machine to overheat, but it's the only logical (as opposed to
marketing) explanaition I can think of for voiding the warranty.
Did very early machines have AC fans? All the
later ones certainly had 12V
DC ones that would be independant of mains frequency, but an AC fan would
run slower on 50Hz and could cause the machine to overheat. Was it really
that marginal (Acutally, given it's the IBM PC [1], I could beleive it
was).
Yes, they did, but the fans were rated for 50-60 Hz operation and
impedance-protected. With the factory, what 62-watt?, PSU, I don't think it was possible to stuff the thing full enough
of drives and
cards to make it overheat (remember too, that there were only 5
slots) to make it overheat. Wasn't the (black) PSU on the original
PC fitted with red tamper-telltales stuck on along the edge of the
PSU clamshell? I can't remember exactly, but I think it was.
Later PSUs had tamperproof Torx screws on them, and the IBM service info
implies the PSU should be replaced 'for safety reasons' if there's any
evidence of tampering. Needless to say I've taken IBM PSUs apart and put
them back together with the original screws...
As this was an IBM product, I fully expected that
there would have
been some provision for changing the AC input voltage. Certainly
I've only seen the UK-supplied units, but they have an internal soldered
link that is fitted for 115V nad open for 230V. Why it wasn't a rear
panel switch I don't know (it was on the PC/AT and the Portable [1] of
course)
[1] Whcih make a lot of sence given that that machine was intended to be
moved around. On the other hand, it's not as nice as the HP Integral
supply which contians a little circuit, the purpose of which is to short
out the mains (triac and low value resistor) to blow the fuse if the
machineis set for 115V and plugged into a 230V supply...
The IBM PC and XT supplies I've seen are clarely Zeniths. They are
unueual in that they have a mains-frequency transfortmer for the startup
supply and unlike many small SMPSU cannot be run from a DC input.
other personal computers of the time had the feature,
and this wasn't
supposed to be a product from "Fred's Personal Computer and Aluminum
Storm Door Company". One expected a high level of engineering from
IBM.
In the same way that I used to expect a high level of engineering from
HP, but look what they're turning out now :-( :-(
Similarly, one suspects that the printer port must
originally have
been intended as a full bidirectional design and then changed at the
last minute--on both the MDA and the printer adapter, all the
necessary circuitry was present for bidirectional operation. Cutting
and jumpering a single trace was all that was needed.
On the printer adapter (althohgh not the MDA card), there was actually a
set of solkder pads for this cut-and-jumper mod. They are not documetned
in the Techref, I have no idea why not
-tony