I often work on systems with power on. With PCs it's no big
deal in terms of my safety. The SE, however, is an incredible
pain to repeatedly re/disassemble if my fix doesn't work, so I would
like to work on it cover off/plugged in. I'm only dealing with the
logic board anyway. That's why I originally asked...
By the way, how do people work on machines like the PS/2 Model 25,
in which one can only get to the MB when it's not plugged into the
PSU?
>> > But not everyone has an EHT meter. It's probably OK to use a 10M
resistor
>> > (prefereably a high-voltage one) and a well-insulated screwdriver
to
>> > discharge the CRT. Connect one end of the resistor (use clip leads,
etc)
>> > to the CRT earth and the other one to the screwdriver blade. Then
push
>> > the screwdriver under the edge of the anode cap and hold it in
contact
>> > with the metal contact for about 30s.
>>
>> Most resistors are rated for a 500V or 1kV rather than EHT, so there
is a
>
>That's why I said 'preferably a high-voltage one'. Farnell and Maplin
>both sell the Philips HV resistors good to about 10kV. A couple of
those
>in series.
>
>> small risk that they'll break down or flash over. Better to use two
or
>> more in series if you can.
>>
>
>I'd not trust my life to it, but most resistors break down and go to a
low
>resistance when they flash over, not open.
>
>> It's also worth pointing out that even 1mA at 15kV is a fair wattage
--
>> another reason to use more than one resistor. A large colour CRT
might
>
>Yes, but the energy stored in a CRT is not that great. Resistors burn
out
>because they overheat, and if there's not enough energy to heat them
up,
>then they'll not fail.
>
>> have a 25kV supply. Of course, as Tony points out, usually the CRT
will
>> discharge when switched off. But if it doesn't, that's just when you
don't
>> want to find out the limitations of ordinary resistors. (I once
proved
>> this theory on a Commodore PET).
>
>'Nice' monitors have EHT bleeders anyway, often as part of the EHT
>reguation circuit (the feedback loop consists of a potential divider
>across the EHT supply). So the EHT should discharge at switch-off. I
>check it anyway with the meter, of course.
>
>As this is classiccmp, I think I should mention that some vector
>displays, like the DEC VR12, VR14, VR17, etc have a mains-derived EHT.
>There's a sealed (oil-filled?) can containing a transformer and a
voltage
>doubler - a schematic is printed on the can. These things can supply
>considerable current for short periods - easily enough to kill you.
>They're nasty.
>
>>
>> Perhaps this is a good place to mention the "hand in pocket" rule:
if
>> you're working on a high-voltage system, and one hand is in your
pocket, it
>> can't be touching a good earth (unless you're even odder than I am
:-)) so
>> there's a reduced chance of current crossing your body. You used to
often
>> see TV engineers with one hand in a pocket and an EHT probe in the
other.
>
>Nowadays you see computer preservationists with one hand in their
pocket.
>I do it whenever I work on HV stuff with the power applied.
>
>The idea is to prevent the current taking a path through your heart.
Down
>one leg is possibly OK, arm-arm is not.
>
>I didn't mention it, because the original poster didn't say he was
>planning to do work with the power on. If you are, there's one other
rule
>that I'll state - Always have somebody nearby who knows how to turn off
>the power if anything goes wrong. Never work alone.
>
>As a practical point, it helps if that person is not either of your
>parents, wife/husband, or girl/boyfriend. The reason is that they tend
to
>cut the power on the slightest problem - like a minor flashover to a
>screwdriver, which is very annoying!
>
>> Pete Peter Turnbull
>
>-tony
>
>
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<Wait, are ST-506 drives the ones with a wide ribbon connector
<and a narrow one? If so, they are pretty common here. I have a bunch
Yes and so is EDSI!
<in my desk drawer. BTW, why is MFM so much more popular than RLL?
RLL tended to push the drive and if the bit jitter was high there would
be lots of bad blocks. Drives of that time using non servo positioners
were doing their best but they were on the way out for their own reasons
like being slow and tending toward higer error rates from wear. This
combined with arriving late on the scene, and EDSI gaining popularity
and other details sorta made it's life short. While RLL did die for
drives using seperate controllers it is nearly standard for IDE and SCSI
drives (we cant see it).
Disk and controller technology were playing catchup with each other
through the 80s.
Allison
On 4 Jun 98 at 9:07, SUPRDAVE(a)aol.com wrote:
> they were all 32bit from the get-go, but i think the memory adressing was
> 24bit which means my IIcx can only goto 16meg maximum. there was also some
> issue about some applications wouldnt run because of the 24-32 bit
> discrepancy, and a program called mode32 was created to circumvent the issue.
> i got my copy of mode32 from connectix, who presumably made it.
The IIX and IICX and SE/30 ROMs contained the 24bit addressing code.
For System 7.0 --> 7.5.5, Connectix released the Mode 32 fix which
permits the Macs with "dirty ROMs" to successfully address up to 128M
of physical RAM and to use virtual memory as well.
For System 6, you were stuck with 8M physical RAM on these machines.
Apple did not support virtual memory at that time, but Connectix sold
a VM kit. The kit also supported a Mac II fitted with a PMMU.
Can anyone clarify the memory situation for A/UX?
Phil
**************************************************************
Phil Beesley -- Computer Officer -- Distributed Systems Suppport
University of Leicester
Tel (0)116 252-2231
E-Mail pb14(a)le.ac.uk
<"The federal government is in dire need of technical resources to fix th
<Year 2000 problem-so dire, in fact, that there is speculation about a
<possible draft of Cobol programmers."
<
<
<I wonder what bootcamp is like?
Confusing, oblique, boring, overated and longwinded. Other than that
likely I'd be the first draft with a minimum age! ;)
Allison
So then, I'm a C-64 either way. Kinda discomforting. Maybe I have one
of those upgrade cards. I hope I'm running GEOS ;)
>> A silly suggestion. Computer-astrology. Your way of thinking is
>> determined by the computers that were in production at the time of
your
>> birth. No, I don't believe it either :-)
>
>More likely, your thinking is determined by the computer which you
first
>heavily used.
>
>--
> _______ KB7PWD @ KC7Y.AZ.US.NOAM
ecloud(a)goodnet.com
> (_ | |_) Shawn T. Rutledge on the web:
http://www.goodnet.com/~ecloud
> __) | |
\__________________________________________________________________
>* VRML * electronics * Gravis Ultrasound * Khoros * emusic * X window *
>
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Wait, are ST-506 drives the ones with a wide ribbon connector
and a narrow one? If so, they are pretty common here. I have a bunch
in my desk drawer. BTW, why is MFM so much more popular than RLL?
>> > Okay gang. I just can't take it, I gotta help this guy become an
atarian. :)
>> > Okay, this is what; you need. You need an ATSCI to SCSI converter
>>
>> OK, but what to do with the system that takes an ST-506 interface
drive? I just
>
>This is a problem that I've been thinking about rather a lot - not for
>Ataris, but for old workstations (PERQ, Xerox, etc) that have an ST506
>controller built in to the I/O board.
>
>I've been thinking of designing an interface that'll link a SCSI or IDE
>drive to an ST506 controller. The problem is I'd have to know quite a
bit
>about the low-level format of the drive so that I could simulate it on
>the interface. And that data is not easy to come by.
>
>But I am worried that ST506 drives are becoming less common every year,
>and there's no easy way to repair them.
>
>-tony
>
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Hi there!
Just a word to say that you can go and see my 8-bits devoted site. It's
called the 8-bits Rendez-Vous! Actually there's only 50 computers
featured but I'm adding a new one nearly every-day. So go there to see
if you like and come-back often. There is a lot of infos and pictures
and I also present my own collection.
Please give me feedback!
8-Bits Rendez-Vous
http://perso.club-internet.fr/oboissea/
(best viewed with last versions of IE and Netscape)
BOISSEAU Olivier
oboissea(a)club-internet.fr
At 08:56 AM 6/4/98 -0700, you wrote:
>>Year 2000 problem-so dire, in fact, that there is speculation about a
>>possible draft of Cobol programmers."
>
>draft agency would have to be Y2K compliant so they can track down and
>punish draft dodgers...
No need to skip the country... Just tell 'em you were born in *18xx*, not
19xx, so you're too old. "Wish I could help ya with these new fangled
computerthings, young feller, but if you ever need a good musket loader,
you give me a call, y'hear?"
--------------------------------------------------------------------- O-
Uncle Roger "There is pleasure pure in being mad
roger(a)sinasohn.com that none but madmen know."
Roger Louis Sinasohn & Associates
San Francisco, California http://www.sinasohn.com/
<I have wanted to ask this for a while. What is the actual risk of me
<blowing myself up if I service a machine with a built-in monitor?
<An example is the Mac SE. Is there any risk involved (as long as I
<don't short capacitors with my fingers) in taking it apart? What are
if your careful and aquaint yourself with where the HV is...and make
very sure you don't put your fingers there you should be ok. Also
remember that 17,000 volts can jump a good distance and find you
fingers.
To make a point though... I got blasted fixing a VR201 monochrome monitor
yesterday. So you know this monitor is powered off 12v in the pro350.
Well, I kept clear of the flyback and crt anode and got nailed by 70-100v
down on the logic board. I wasn't watching for the focus cap having
anything big voltage wise on it. Many monitors have points other than
the CRT where higher than supply voltaged can be found and some can really
sting!
<the parts I should watch out for? How long do they hold a charge?
<Could I discharge them?
How long... amazingly long and should be treated as charged until proven
otherwise. You can discharge them, make sure the "ground" you use is
a real one and simply not the case or some other floating point.
Allison
From: Tony Duell <ard(a)p850ug1.demon.co.uk>
>
>The stories I mentioend didn't refer to the BCM, but to a couple of major
>collections in the UK.
Where are there any decent public displays in the UK. There doesn't seem to
be much at the science museum in London.
Regards
Pete
