Mark Cummings wrote:

>I tried to get one of these a couple of years ago.
>Apple's own repair centre in Sydney didn't have one and the chassis
>manufacture, Mitsubishi didn't either. Consequently I was forced to draw my
>own. As it stands it's on about 4x A3 pages and is like a rats nest. So far
>I have not found the time to draw it up properly but intend to one day. not
>sure what the best format will be for the internet, but probably .bmp or
>.gif will be the most useful if I can convert it from Protel. If I can't
>convert it I'll probably have to print it on A4 and scan it back in.
>As far as parts go, the EHT transformer cannot be purchased anywhere in
>Australia either, and since I think that's where most monitors fail (ie.
>blooming) I don't hold much hope for repairing them anyway. For other faults
>however they may be fixable and I don't intend throwing out any broken ones
>yet.

This sounds like a great project.  I'm sure it will be increasingly useful as
more of these monitors begin to fail.

In my experience, blooming does not indicate failure of the Horizontal
Output transformer itself, but rather the failure of the high voltage rectifier
diode string which is embedded in modern HOT's.  If it is possible to fain
access to the outer winding of the high-voltage winding, another HV
diode can be connected and will probably fix the problem.

Another common problem with all monitors is the deterioration of
electrolytic capacitors.  As they dry out and lose capacitance and
increase in Equivalent Series Resistance, the circuits they are meant
to decouple and the power busses they are meant to filter begin to
suffer the effects--usually decreases in gain, and drops in voltage.
As the circuit gradually becomes marginal, a host of symptoms
are manifested.  And simply bridging the defective capacitor with
a good one (often even one of much larger capacitance) will
immediately restore correct operation.

The bridging technique is a time-honored trick of TV service
technicians which works beautifully, as long as:


1. You don't contact the wrong points,
2. You don't _personally_ contact anything,  ;-)
3. You don't get the polarity wrong, and
4. You remember to discharge the capacitor by shorting
    its terminals after bridging, and before bridging any other
    capacitor!

(Of course, if you haven't navigated around exposed high voltage
circuits before, this is dangerous practice.  But if you have worked
on TV's or transmitters, then you know how to do it safely.)

-michael

Check out 8-bit Apple sound that will amaze you on my
Home page:  http://members.aol.com/MJMahon/