There are a number of possibilities including incorrect screen (G2) or bias
(G1) voltages, or a problem in the video or blanking circuitry. Any of these
could be the result of bad connections as well. A short in the CRT can also
result in these symptoms.
|NotTaR of Television Sets : Excessive brightness and/or washed out p..
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- Excessive brightness/washed out picture is often an indication of a
problem with the screen (G2) supply to the CRT. May be a bad capacitor
or resistor divider often in the flyback transformer assembly or on
the board on the neck of the CRT.
- If the excessive brightness just developed over time, then a simple
adjustment of the screen or background brightness controls may keep
it (and you) happy for a long time.
When good, a typical value would be in the 200 to 600 VDC at the CRT. The
screen (it may also be called master brightness, bias, or background) control
should vary this voltage. However, it may be difficult to measure as the
resistors in the voltage divider network may be quite large - hundreds of M
ohms. If your unit has an external screen control (less likely these days)
and it has no effect, trace out the circuitry in the immediate vicinity and
check the resistors and potentiometer for opens, look for bad connections,
etc. If it is built into the flyback transformer and is sealed, the entire
flyback will need to be replaced unless the actual problem turns out to be a
bad connection or bad component external to the flyback.
- Where the brightness control has no effect, suspect a missing bias supply to
the G1 (control grid) electrodes of the CRT. This is usually derived from
the flyback with a simple rectifier/filter capacitor power supply. Parts
may have failed (though not likely the flyback itself). Adjusting the user
brightness control should vary this voltage over a typical range of 0 to -50
V with respect to signal ground.
- It could also be a problem with biasing of the video output transistors.
There may individual controls for background brightness on the little board
on the neck of the CRT. However, we are looking for a common problem since
all colors are wrong in the same way. This is likely to be a missing voltage
from a secondary supply from the flyback.
- A short between electrodes inside the CRT can result in brightness problems.
It may be possible to check this with an ohmmeter with the power off and the
CRT socket removed. Test between G1, G2, and F where all colors are
affected though a short between F and G2 will result in the focus control
changing brightness and vice-versa - a classic symptom.
However, in some cases, it only shows up when operating and one must deduce
the presense and location of the short from its affect on voltages and bias
See the section: Rescuing a shorted CRT and other
First, check for bad connections/cold solder joints by gently prodding
with an insulating stick. Check voltages and bias levels.
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