[ALUG] Silly Hardware Qs

[Wayne Stallwood]

The old AT style PSU's like these (with a mechanical on off switch) have
no problem having the power disconnected/reconnected by pulling out the
mains plug. All the mains switch is on these is a double pole switch in
the mains supply anyway.

What I would recommend is that you disconnect power by switching off at
the wall socket or by pulling the plug out of the wall rather than
pulling the IEC (kettle) lead out of the back of the PSU. Those
connectors have a bit of a tendency to arc under load and this could
shorten the life of the PSU.

> 
> 2. If a TFT/LCD screen simply goes all white (every pixel
> bright white) when it's switched on, regardless of whether
> there's any input signel, does this indicate any particular
> type of internal problem which it might be worth probing
> for (by taking the back off and looking for, e.g., a loose
> chip/connection)?
> 

It is either a dud panel driver board (in which case it is probably
beyond economic repair I am afraid) or if you are lucky it is a dodgy
connection between the LCD and the driver board. 

Make sure the power is off the screen and has been for at least an
hour..only idiots and experts mess around with live equipment.

Inside the panel you should see a largish circuit board with a wide
ribbon cable (of many pins) going to the actual panel itself. The board
may be covered with screening plates which will need to be removed
first.

Briefly ground yourself if you can..LCD's and the associated drivers are
super static sensitive.

Check the security of the connections at both ends on this cable,
usually the connectors are of the zero insertion force type and will
have some sort of clip mechanism that retains the cable. Usually this is
in one of the following forms.

The whole top of the connector (where the ribbon cable goes in) is a
frame (usually of a different colour) that can be gently (and very
little force is required, don't yank it) be prised up by a couple of
mm..the cable will then slide out freely...reinsert cable as far as it
will naturally go and push the frame down to lock.

The other type is more of a side entry type. These have a bar across the
length of the connector where the cable goes in that hinges up to unclip
the cable. same procedure as before..gently flick the "bar" up where the
cable goes in and then remove.  

It is not unusual for the release mechanisms of these connectors to be
additionally secured with tape..do not try to cut the tape with a knife
or by pulling at the cable...carefully peel it off. The cables are very
fragile. Inspect the cable while you are doing this to check that it has
not been snagged or otherwise visibly damaged.

Also there usually be a smaller PCB (or sometimes it is part of the main
one) that has two thick'ish wires that also go to the panel. You can
stay away from this bit as that is the drive for the backlight..and we
know that works or the screen would remain black.

If the screen is the type that has mains going straight into it rather
than being powered by a PSU brick then stay well away from the
components near the mains socket even if the power is disconnected.
there are capacitors there charged up with over 300VDC and they will
bite you even after the unit has been unplugged for a while. 

Sometimes you will encounter a multicore cable rather than a ribbon..you
will in this case just have to carefully examine both ends to see how it
is connected. In other cases the main board actually connects directly
to a connector on the back of the panel, in this case simply carefully
detach the board and re-attach it making sure all retaining screws are
put back correctly.

Hope that helps
Wayne