[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

The incredible shrinking monchrome image.

    Date: Tue, 17 Nov 1992 16:46-0000
    From: Scott_Busse@mindlink.bc.ca (Scott Busse)

    I have a monochrome console (a phillips) that has a
    progressively shrinking image (the black border is
    getting bigger). It has been suggested that it's
    probably the power supply, but I would like to get
    a second opinion (and a third and fourth :) and any
    suggestions about fixing it. It also has wavey left
    and right borders, and fuzzy top border. I have tried
    adjusting the settings inside, but nothing brings it
    back.  Thanks in advance, for any help.  Scott

There are two possibilities:- 
a) either the EHT potential (screen anode voltage) is drifting
(higher!...an increase in the EHT voltage will increase X-Ray
emissions!) if the black border is increasing proportionately in
width and height.


b) drive to the horizontal and vertical scanning coils is reduced.
(This is less likely, this usually causes a reduction in width or
height but not both, and with a charring of the plug that goes
to the scan coils on the yoke of the tube.

Try replugging all the connectors to the deflection p.c.b. If this
doesn't help, adjust the EHT voltage. This is BEST done with the
power switched off! The EHT module is a black lump with a thick
red wire wire going to the tube anode, and a 4 wire connector going
to the deflection board. Near the connector is a recess for a pot.
adjustment. Try adjusting this in 1/4 turn steps, and power on
to check the result. You must not work with the power on when
doing this adjustment; there are voltages in excess of 20kV D.C.
waiting to bite! By tweaking the EHT voltage down, the scan coils
should get a better beam spread...et voila, a wider screen!

You will probably need to readjust the focus pot. afterwards.

As for ripple, this could be a number of things. Most likely is AC
noise breakthrough from a failing smoothing capacitor on the power
supply. You can also get a "venetian blind" from a misadjustment
of the threshold and contrast pots. on the video amp board mounted
on the neck of the tube. This can give a similar ripple effect.
To test for this, try tweaking these pots and/or adjusting the
brightness of the monitor and watch the rate of the ripple. If it
is related to the threshold/contrast pots. or the overall
brightness, the ripple will appear to standstill and reverse
direction as you adjust. You should be able to improve this
condition for a selected brightness level by fine tuning of these
pots. If its noise breakthrough, then you may need another power

Another problem with these monitors is a drop in the +5v rail from
the power supply to the logic pcb. Resistive contacts on plugs and
sockets cause the +5v rail to drop on the logic pcb (as measured
across a smoothing cap or between pins Gnd and Vcc on a TTL i.c.
(pins 7 and 14 for a 14 pin i.c.)). Once the voltage drops below 4.8v you
can get keyboard/mouse/console lockup problems. These can
sometimes be temporarily cured by replugging the keyboard/mouse or
power cycling the monitor, but the real solution is to
replug/clean the power connectors from the power supply through to
the logic pcb, and to increase the +5v at the power supply if
necessary. Set the +5v as measured at the logic pcb to be +5.05V.

Hope all this helps in some way!