On Nov 17, 2003, at 2:14 AM, Steve Bell wrote:
On 14/11/03 8:31 pm, "iMac List" <[EMAIL PROTECTED]> wrote:
Subject: Re: Dead Rev A bondi Date: Thu, 13 Nov 2003 11:50:26 -0500 From: chris <[EMAIL PROTECTED]> Message-Id: <[EMAIL PROTECTED]>
The pinout on that cable is not VGA. It won't work by just plugging in
a monitor.
I was under the impression from searching around that the pinout is standard RGB, and that any multisync monitor capable of RGB would work fine (like the Apple MultiSync 15AV).
I have previously run an Apple Multiple Scan 20 directly off the tray loading iMacs, and other PC monitors via a Viewsonic Mac adapter.
You have a dead analog board. Part costs about $150. I do them all day
long in the shop.
Would this stop the internal video connector (which is connected to the
logic board) from sending a video signal? All the "hack" directions out
there for moving a Rev A into another case don't seem to indicate that
you need to bring the Analog board along in the conversion.
The most common failure of the Analog board (What monitor manufacturers
normally call the Deflection and Video boards, Apple include as one and name
the Analog board) is the flyback transformer. The most common failure mode
in the iMac circuitry is leakage of a HV capacitor within the flyback.
Normally there would be infinity resistance from the EHT cap to common, but
defective flybacks usually show around 35K to 80K ohms. This resistance
presents a very low load to around 25KV EHT and the increased current trips
the beam current sensing circuit and shuts down the iMac.
I think what I am trying to determine is, what role does the Analog board
play in the operation of the iMac?
The Analog board provides the CRTs EHT, G2, focus voltage and negative
voltage the the g1 control, also vertical deflection, horizontal deflection,
sound amp, video amp, and contains a small microcontroller.
By removing the iMacs video plug and thus cutting off the video signals, it
also removes the H&V syncs. Without these syncs the monitor section goes
into power saving. Power saving cuts off the B+ voltage to the flyback, so
if faulty it will not operate and trip the power supply. Note that other
components could fail that would not be effected by pulling the video
connector, and could shut down or stop the iMac from starting.
Any idea what part commonly dies on the analog board? I have no
intentions of replacing the entire board at a cost of $150. If however
there is a cheaper dead component of the board, I would consider
replacing it instead (such as a $5.00 capacitor... although a $70 Flyback
transformer probably exceeds my cost threshold).
The most common Analog board failure item is the flyback transformer. To
find any other problems you would need component level fault finding
experience, which you don't have or you wouldn't be asking these questions.
If you only want to invest very little capital into the repair and you don't
have swap out modules to assist in fault diagnosis you will have problems
repairing.
Steve Bell
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