markc@wpi.wpi.edu (Mark B. Cohen) (03/10/89)
This morning I sat down at my PC to send out a letter, turned the
power switch on and nothing happened.
No beep, no fan, no nothing.
The monitor came on fine - blank of course, but fine.
So I checked the AC outlet - no problem.
The power supply cable was conducting fine too.
My surge protector was also normal.
So... I look around the back of the system unit for a fuse socket. Nothing.
I remove the system unit cover, and find the power supply is one single
sealed unit, as expected. (Naturally, I knew that already, from having
installed boards and drives, but one can dream, right?) There is, however,
a notice on the power supply, along with the "WARNING - No Servicable
Components Inside" label, another notice: "WARNING - Be certain to
replace fuse with exact size and rating or damage may occur" [paraphrased].
At this point I should add that RTFM mentioned nothing about fuses.
["Contact your dealer or technical support" under generic troubleshooting
section - how wonderful.]
The access screws on the power supply were placed in a way so that the
supply had to be removed from the system unit to reach them. Removing
the power supply entailed removing several expansion cards to gain access
to the system board power connectors, as well as removing the disk drives
(two hard, two floppy).
Now I had a dead power supply by itself. External inspection showed no
access points: The only openings were for cables, the switch, and the fan,
plus numerous ventilation points. Opening the power supply (yes, it had
been off for at least an hour at this point, so the caps were discharged)
involved seven screws (the system unit cover is only held on by six), which
presented me with the reverse side of a circuit board, and a view of
another circuit board. And lo and behold! There on the second board
were two fuses, one of them clearly blown.
To reach this fuses, I had to remove the first circuit board, disconnect
two multi-pin sockets which gave access to an internal divider.
This internal divider was held by three screws, which finally
allowed me to remove and change the blown fuse.
Anyway... thanks for bearing with me through this saga.
My question for the net (and the reason for this post) is this:
Why would a designer place a disposable/exchangable part
(such as a fuse) deep inside a 'sealed' assembly that has
existing external connections?
The ^&*% manual gives explicit instructions on how to
insert chips on the system board. Why can't it have a
section on how to change a fuse?
Just to kill time while having tried to figure this out, I priced the
cost of a manufacturers replacement power supply.
It's a Mitsubishi made system, marketed by the late Leading Edge, and
I paid $2500 two and a half years ago for the system with a 30Mb
drive, 1.2Mb floppy, and 1Mb RAM. The processor is a '286 @ 8MHz.
A replacment power supply (I think it's 135W, it is NOT labelled)
listed for $431.25, plus shipping, etc.
A more novice user, or someone smart enough [to not try electrocuting
themselves :-) ], might have replaced the entire unit. I don't
believe that this is good design either.
How are other systems designed? I can't recall ever having seen
any PC with a visible fuse socket - although I'm sure they MUST exist.
Frustrated, but up and running,
Mark
--
Internet: markc@wpi.wpi.edu "This is drugs...
UUCP: {backbone}!husc6!m2c!wpi!markc this is your brain...
BITnet: markc@wpi.bitnet this is your breakfast."rogers@falcon.SRC.Honeywell.COM (Brynn Rogers) (03/10/89)
I had the same problem with a Compaq 386/20e. (the small foot print)
Had to take the power supply out , open it up (after making sure there were
no 'Warrenty Void if opened' labels) and sure enough there is a fuse
SOLDERED to the board. also it was opaque so you couldn't eyeball it.
Got a trusty DVDM out and 36Kohms- sounds like a bad fuse!
Since we were still under warrenty we took it back to the dealer and got a
NEW power supply the same day. In this case I don't think fixing the
fuse would have fixed the problem because it was drawing current with the
power switch off and the fuse blown.
Our 386/20 (big mama) has a fuse carrier just above the power cord, and yes
we have blown them a couple of times.
Brynn Rogers Honeywell S&RC rogers@src.honeywell.com
also try this new address -> nic.MR.net!srcsip!rogersralf@b.gp.cs.cmu.edu (Ralf Brown) (03/10/89)
In article <1256@wpi.wpi.edu> markc@wpi.wpi.edu (Mark B. Cohen) writes: }How are other systems designed? I can't recall ever having seen }any PC with a visible fuse socket - although I'm sure they MUST exist. For that matter, I've never heard of a PClone power supply that gets the airflow right--they all blindly follow IBM in the matter. The airflow SHOULD be drawn in from the outside through a filter, then blown through the power supply and out through the chassis. Instead, the power supply uses the world's most expensive (and probably least effective) filter: the motherboard, disk drives, and expansion cards.... And the suggestion I saw recently to put a filter over the grille on the front of the machine won't help much, as it will restrict the airflow through the grille, so more air will enter through all the other openings, the largest of which are the disk drives.... -- {harvard,uunet,ucbvax}!b.gp.cs.cmu.edu!ralf -=-=- AT&T: (412)268-3053 (school) ARPA: RALF@B.GP.CS.CMU.EDU |"Tolerance means excusing the mistakes others make. FIDO: Ralf Brown at 129/31 | Tact means not noticing them." --Arthur Schnitzler BITnet: RALF%B.GP.CS.CMU.EDU@CMUCCVMA -=-=- DISCLAIMER? I claimed something? --
nobody@tekecs.GWD.TEK.COM (-for inetd server command) (03/18/89)
<long description of what it took to replace the power supply fuse deleted> > Why would a designer place a disposable/exchangable part > (such as a fuse) deep inside a 'sealed' assembly that has > existing external connections? Your description of what you did to replace the fuse did not include an explanation of why the fuse was blown in the first place. Fuses rarely blow for no apparent reason. In your description you mentioned that there was a sticker on the power supply that said that there were no user serviceable parts inside. It seems to me, you have proven that the fuse is not a user serviceable part. Sure, anyone can replace a fuse, but it takes a qualified technical person to determine why the fuse blew in the first place. It could very well be that you are now operating your system in a dangerous way because you don't know what made that fuse blow. I once restored a Dodge van from the damage done by an engine fire. In the course of replacing the wiring harness I discovered that some intelligent person bypassed some of the fuses in the fuse box! I suspect that there was probably an intermittent short that would occasionally caused some of the fuses to blow, and the owner thought that the fuses were simply a nuisance. At any rate, that's why I think burying the fuse in the power supply is probably not a bad idea. stank US Mail: Stan Kalinowski, Tektronix, Inc. Information Display Group, Interactive Technologies Division PO Box 1000, MS 61-028, Wilsonville OR 97070 Phone:(503)-685-2458 uucp: {ucbvax,decvax,allegra,uw-beaver}!tektronix!orca!stank
mcdonald@uxe.cso.uiuc.edu (03/19/89)
<long description of what it took to replace the power supply fuse deleted> > Why would a designer place a disposable/exchangable part > (such as a fuse) deep inside a 'sealed' assembly that has > existing external connections? To increase service revenue, that's why. I keep a log of repairs to the equipment in my lab ( a couple million bucks of lasers, computers, electronics (and low tech (yuck) vacuum pumps).) We have found , over the years, that 50% of fuses that blow, do so for no reason we can find - no broken parts, no short circuits, no dead cockroaches or mice in sight (this latter is not a minor problem due the fact that the wonderful state of Illinois has us in a building built in 1900). Replace the fuse and the object works fine for another year. So I am very leary of any claims that "this fuse can't blow unless something else inside the power supply dies first". Intentionally breakable parts like fuses should be user replacable.
ray@philmtl.philips.ca (Raymond Dunn) (04/01/89)
In <1256@wpi.wpi.edu> Mark B. Cohen asks: > Why would a designer place a disposable/exchangable part > (such as a fuse) deep inside a 'sealed' assembly that has > existing external connections? > The fuse in fact has exactly the same status as every other part in the power supply, it is NOT a user exchangeable part, and is there as a safety device required in order to meet FCC, FTZ etc. regulations. It is not expected to blow any more frequently than any other component in the power supply. The complaints about the generally poor design of the mechanics of PC's is however all too justified, and cannot all be explained away by the "design down to a cost" that drives most PC designs. -- Ray Dunn. | UUCP: ..!uunet!philmtl!ray Philips Electronics Ltd. | TEL : (514) 744-8200 Ext: 2347 600 Dr Frederik Philips Blvd | FAX : (514) 744-6455 St Laurent. Quebec. H4M 2S9 | TLX : 05-824090