BBC Alive
Video Help
Yellow lines on monitor
Connecting IBM green screen monitor
Colour from BNC socket
Colour on my TV set often vanishes although TV
shows are fine
Getting NTSC output
Q. After about 2 hours my monitor screen gets
yellow lines across it, making it difficult to continue using. Is there any
cure?
A. It depends whether the lines are moving or static.
If they are static then they are locked to the video signal and so the problem
is likely to be in the area of the memory circuitry. However, if they are
flickering it is likely there is an intermittent connection in the blue video
line. This is because white minus blue equals yellow. Some component is heat
sensitive as the problem only occurs after a while. The first thing to find out
is whether the problem is in the monitor or computer. When you see the lines
turn off one but not the other for half an hour to let it cool and turn it back
on. If the lines have gone your problem is with the device you turned off. If
it's the computer then you can use freez spray to track down the overheating
component A good starting point is IC6.
Q. Can I connect an IBM green screen monitor to my
B+ 128k?
A. Yes. Two different types of monitor are available:
those that take the RGB signal and mix it and those that require a composite
video signal. You will need a sync separator for either to provide horizontal
and vertical syncs. If it's the latter connect the Video Out at the back
of the computer to the Video In of the monitor.
Q. How do I get colour from the BNC socket on the
Master?
A. Normally, the machine can't produce composite
video output but if you connect a 470pF capacitor between the left of R137 and
the left of R153 which are to the left of the modulator and you will get colour.
There is no need to remove the board; you can solder directly to the compnent
side.
Q. I use the UHF output to a TV. The colour
on the screen often vanishes although it is fine when used to watch TV
programmes. A different TV makes no difference. I can recover the colour by
repeatedly inserting the TV lead into the computer or retuning the TV. Replacing
the lead doesn't help.
A. This is normally caused by the receiver going out
of tune and losing the lock on the colour sub-carrier. Owing to the way the
colour is encoded on your computer this is more critical than for broadcast TV.
It is normally a problem with the stability of the TV set rather than the
computer. However, you can try adjusting the trimmer capacitor on the
sub-carrier oscillator (labelled VC1, situated in the top right hand corner of
the board).
Q. Is it possible to gain an NTSC output by
altering one side of link 15 on the Master 128? I gather disconnecting an IC pin
may also work provided I change a crystal.
A. Removing link LK15 will stop the alternate line
signal reaching the modulator. This produces an NTSC output. But the colour
sub-carrier frequency is still needed for the UK 625 line system. It is doubtful
whether an NTSC set would lock on to it. The Master Service Manual suggests
changing the UM123/E36 modulator for a VHF version as the circuit board can
accommodate both.
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