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

if/when I do finish it, I'll have to make another version for the 110V
version of the monitor, which should be the same mobo with a few different
jumpers and parts in the mains side of the cct. shouldn't be much work to
do, but I'll need input from someone in the US comfortable with opening up
the monitor. the PCB overlay scan I did would be a good place to start,
listing any deficiencies that are screen printed on to it. or any errors,
which I think I found one only.

> 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.

I might be reading this wrong but the Horizontal Output Transistor would be
the HOT ? not the EHT XFMR. Some call the EHT the Trippler, but I could
never figure that out since it doesn't tripple the voltages :)

if the diodes are in the EHT  XFMR then the EHT XFMR is still the problem.
Can you get HV diodes for these separately, I didn't realise that ?
I don't know enough about the internals of the EHT to be able to fix one,
but I'm keen to try one if I prove mine to be faulty. I already tested /
replaced virtually all electrolytics and some others in one monitor and it
has made no difference to my monitor, still blooms, ie Screen Voltage and
Focus Voltage start creeping up over half an hour until the Focus is out of
adjustable range. ie Screen starts at 350V and the screen is black (should
be about 500-600V AFAIK) then goes up to 600V after half an hour and the
picture is OK, or if I set it to 600V cold it creeps up to over 800V and
blooms afetr half an hour. Focus Voltage starts at 3-4KV and creeps up to
8KV or more. Eventually the Focus adjustment is fully one way, the monitor
has low contrast and out of focus.

I also checked the Horizontal Transistor output on a good and faulty monitor
and the 800V peaks were pretty much the same on both. all other AC & DC
voltages around the EHT XFMR were about the same.

This is exactly the same fault I have in two monitors, and starting in a
third one I just picked up, but I have one good one left and feel confident
the EHT is the culprit. Once I swap it over I will know for sure, but
finding a few spare hours at the moment is not easy.
Another problem with my diagram is knowing what the internal config of the
EHT is, all EHT diagrams I have seen are different. Can't find anything to
help me decode this one. If anyone can help here it might make it easier to
complete my diagram. I've done continuity and low-ohms readings to come up
with my best guess. also refered to other diagrams of Mitsubishi chassis of
similar vintage to get some clues. Also I would like to know the function of
IC401, IC501 and IC601.

> 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.

agreed, but I think the faults I have are mainly the EHT since I have
replaced all electro's.

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

yes, I used that successfully recently fixing one of our 5 TVs, the fault
was a 4.7uF electro that had dropped a little to 3.9uF causing the TV to
switch off. bridged a 10uF (misread it, meant to use a 1uF), but it fixed
the TV instantly.

> (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.)

yep, agreed.