[Tech] LOPT

[LukeWells]

[ukvac] [Tech] LOPT
ok, i've head people talk of the LOPT being faulty or weak

afaik the LOPT is a transformer, and a transformer either works or it doesnt
(caused by an open or short somewhere in the coils)

is it true that a lopt can be bad? but still work, i.e. the picture keeps
dying or the focus is shite? what causes it to be bad other then being open
or short

Luke



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[P-Man]

Re: [ukvac] [Tech] LOPT
the LOPT is often a multi-tapped transformer.

The LOPT not only has windings for the main screen brightness and focus
grids, but can also be used as a low-voltage power supply just like a mains
transformer.

Cracks/crud/age etc can make the windings weak and unreliable causing
problems in any of the windings. Small shorts can occur inside the lopt,
bypassing certain windings, therefore changing the actual make-up of the
transformer.

You could have a great low voltage psu running the oscillators and RGB amps
fine, but a shit brightness/focus problem.

Likewise you could have a good brightness winding and crap focus winding.

Any kind of major screen problems aren't usually labelled as their
individual problems, you simply say 'the LOPT is cack' if it has a crap
picture. when in fact its only one winding of the LOPT that is bad. Nothing
you can fix, just repalce the LOPT.

The high voltage parts of a LOPT rely on proper 'harmonics' (is that the
right word to use?) if the resistance, inductance, capacitance is off, the
rest of the circuit will be out too, causing crap brightness, washed out
picture or other related probs.

You can often measure the output of (the HV winding of) a LOPT and find that
the voltage is well under what it should be. This could be because the B+ is
too low. alternatively if the B+ checks out ok, somethign is wrong with the
actual transformer if turning up the 'screen' control does nothing. Measure
the output of (the HV winding of) a LOPT and find it is too high doesn't
often occur because this by now, would probably have sterilised you from
x-rays. All monitors have built-in x-ray cutoff's, this will shutdown the
LOPT (by routing the B+ to ground) if its measuring a HV over over a certain
amount (20kv rings a bell, but can't remember and it mayt vary depending on
the type of tube used). This is the point at which the electron stream is
hitting the front of the tube so hard, it is managing to get past the
lead-glass.. anyhow, a HIGH HV output is rare because other circuits should
have kicked in in the moniotr to save your family jewels.

Ever wondered why the manuals state 'only replace the CRT glass tube
assembly with an exact model replacement.' ? because if you used a
differently made tube, with a weaker lead shield in the glass, the moniotr
chassis could be pumping out 18KV as per normal, where as the tube is only
rated at 15kv or somethign silly, the xray cutoff wouldn't work in this
scenario, because it was only designed to cutoff at 18kv, as far as its
aware, there's nowt wrong. You only realise your error when your hair starts
falling out.

Or something like that..

Andy

 

[P-Man]

Re: [ukvac] [Tech] LOPT
> Measure
> the output of (the HV winding of) a LOPT and find it is too high doesn't
> often occur because this by now, would probably have sterilised you from
> x-rays. All monitors have built-in x-ray cutoff's, this will shutdown the
> LOPT (by routing the B+ to ground) if its measuring a HV over over a
certain
> amount (20kv rings a bell, but can't remember and it mayt vary depending
on
> the type of tube used). This is the point at which the electron stream is
> hitting the front of the tube so hard, it is managing to get past the
> lead-glass.. anyhow, a HIGH HV output is rare because other circuits
should
> have kicked in in the moniotr to save your family jewels.

just to add a bit here i missed..

common problem.. Nintendo monitors (sanyo) are very intolerant of slightly
out of range B+ voltages, they are 100v moniotrs, if you plug them into a
110v psu, the xray cutoff will kick in and you are left wondering why it
works perfectly in one machine, but not the next, or (as when i was in the
states) if you plug a sanyo monitor right into the US mains outlet (110v)
they will refuse to work as little as a 10v difference on the input
voltage can upset the xray cutoff, making a monitor appear totlaly dead.

Andy



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