Why is this resolution "inaccessible"?

[Gene Heskett]

On Saturday 21 January 2006 17:48, Alan Hourihane wrote:
>On Sat, 2006-01-21 at 22:08 +0100, Mikael Eriksson wrote:
>> On Fri, Jan 20, 2006 at 07:59:34AM -0800, Stuart Kreitman wrote:
>> > try stuffing 320x240 into mode 30, just to see if it might pick it
>> > up.
>>
>> No change.
>>
>> > Very strange that the chipset name is unknown. what does the rest
>> > of Xorg.0.log say about the graphics?
>>
>> Attaching the log.
>
>The 320x240 mode requires a hsync value of 15kHz yet you've got the
>lowest to be 28kHz in your config file.
>
>You'll need to try and lower it.

WARNING

This is VERY VERY UNSAFE for the monitor, and will have a high 
probability of letting all the smoke out.  Many of the low dollar types 
are so closely engineered that even 28khz is overheating them.

Check the nameplate rating of YOUR monitor before attempting this edit.
And then add a couple of khz just for insurance.

The problem is this:  When the transistor that drives the scan 
transformer is turned on as the scan passes the midpoint on the screen, 
good overall current control is performed by the inductance of the 
transformer and its load.

This inductance is magnetically reset to about 0 at the turnoff, and the 
damping recovery cycle that represents the left approximately half of 
the screen.

The current then rises in a linear fashion until the end of the scan OR 
the core in the transformer becomes magnetically saturated.  At this 
saturation point, the inductance disappears and the current then rises 
at many amps per microsecond, achieveing destructive values in only a 
microsecond or so.  The circuit may be able to sustain this for a few 
seconds but the transistor is getting ever hotter, and harder to turn 
off.  At some point the i2r losses in the transistor during the turnoff 
will destroy it and often many of the parts around it.  Its not 
uncommon to open up such a monitor and find the epoxy blown off the top 
of the scan transistor.

Lowering the operating frequency makes more time for the current to 
rise, or for the core to saturate.  Generally speaking, it is not a 
Good Thing(TM). OTOH, going somewhat above the rating is not normally 
destructive as the main effect is a dim picture as the high voltage 
falls below optimum.

Who am I?  A C.E.T. with 55 years of chasing electrons for a living.  I 
have been there, and done that.  Sometimes more than once.

-- 
Cheers, Gene
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