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- Path: merle.acns.nwu.edu!judd
- From: judd@merle.acns.nwu.edu (Stephen Judd)
- Newsgroups: comp.sys.cbm
- Subject: Re: 1541 Problem!
- Date: 25 Jan 1996 04:05:18 GMT
- Organization: Northwestern University, Evanston IL
- Message-ID: <4e6vhu$1f0@news.acns.nwu.edu>
- References: <Pine.SV4.3.91.960122005303.14233A-100000@pb.net> <4e6tok$oo@news.acns.nwu.edu>
- Reply-To: sjudd@nwu.edu (Stephen Judd)
- NNTP-Posting-Host: merle.acns.nwu.edu
-
- Oh, what the heck...
-
- In article <4e6tok$oo@news.acns.nwu.edu>, Stephen Judd <sjudd@nwu.edu> wrote:
- >In article <Pine.SV4.3.91.960122005303.14233A-100000@pb.net>,
- >
- >That, sir, is a blown rectifier. If you open up the 1541 there are two of
- >them, towards the back of the drive; they look somewhat like this:
- >
- >
- >That is, a notched square with four pins coming out of the bottom. These
- >guys convert AC current to DC current. One of them is smaller than the
- >other, and it is also probably hotter than blazes.
-
- Just for the sake of reference...
-
- What is happening to your drive: as I stated, a full-wave rectifier converts
- alternating current to direct current (actually I said AC current, which is
- redundant ;). As you know, the current looks like a cosine wave in time,
- with a period of about 1/60 of a second. Over half of this period the
- amplitude is positive, and over the other half it is negative.
-
- What a half-wave rectifier does is to filter out one half of the wave, either
- the positive or negative portion. Bridge two of them together and you get
- a full-wave rectifier, so now your current now looks like the absolute value
- of cos(t). This is then run into a large capacitor -- there are two big
- metal cylinders in the disk drive, and each one connects to one rectifier.
- I think one controlls the motherboard and the other, larger one controlls
- the drive motor. All a capacitor does is store charge, and it's purpose
- here is to smooth out this wave the rectifier is sending out.
-
- (Imagine that you have a dam, with a small pipe at the bottom. You can start
- adding water or taking it out from upstream the dam, but you will have a pretty
- steady flow of water out of the pipe, as long as the water doesn't get too
- low).
-
- What is happening is that part of your rectifier is failing, so that only
- half of it works; instead of a full-wave rectifier you now only have a
- half-wave rectifier. The capacitor is now only being charged half as
- often as it was, which means it has lots of time to discharge.
-
- If you were to hook up an oscilloscope across that capacitor when everything
- is functioning correctly, you would see an almost constant line, meaning
- roughly constant current (or voltage, depending on how you want to look
- at things). With half of the rectifier gone, it will look more like a
- sawtooth/triangle wave. The net effect of this is that instead of getting
- the 9 (I think... this is what I remember for some reason) volts the
- board is expecting you are only getting 3 or 4 volts (there is some root
- mean square stuff I am forgetting about, I think).
-
- So, your lights dim, your drive controller freaks out and starts spinning
- the drive, and other things start acting weird.
-
- How's that for some dimestore electrical engineering? ;)
-
- evetS-
-
-
-