RCA/ProScan - PS27121 Strange White Screen

[Nubly]

Were dealing with an early 90's Proscan TV.

About two months ago my TV started to display a solid white background
with no retrace lines. The sound works fine along with all menus and tv
features. Ex: I can change the channels and the channel number will pop
up in the upper right hand corner in full color or the menu system will
display in the spectrum of colors as intended. Sometimes the tv will
lose the white screen and display the correct image, however this is
rare(It works for minutes, days, or weeks at a time on rare occasions).
PIP works well and it displays the seperate images PIP is supposed to
look, however when PIP comes on the background other than PIP turns
from white to a medium gray. The Svideo(90) and other inputs are also
white.

The TV is less likely to display full color when its been sitting and
its cold. There is no smell or visible trace of burnt components.

Anyone have any ideas?

 

[David Naylor]

Were dealing with an early 90's Proscan TV.

About two months ago my TV started to display a solid white background
with no retrace lines. The sound works fine along with all menus and tv
features. Ex: I can change the channels and the channel number will pop
up in the upper right hand corner in full color or the menu system will
display in the spectrum of colors as intended. Sometimes the tv will
lose the white screen and display the correct image, however this is
rare(It works for minutes, days, or weeks at a time on rare occasions).
PIP works well and it displays the seperate images PIP is supposed to
look, however when PIP comes on the background other than PIP turns
from white to a medium gray. The Svideo(90) and other inputs are also
white.

The TV is less likely to display full color when its been sitting and
its cold. There is no smell or visible trace of burnt components.

Anyone have any ideas?

When I first started reading your post I thought video amp on CRT board.
But reading the rest, it sound as though your jungle ic has gone south
or going that way

 

[Nubly]

Anyone have any ideas?When I first started reading your post I thought video amp on CRT board.

But reading the rest, it sound as though your jungle ic has gone south
or going that way

Does this sound about right?

"Other than power supply problems, this is the second most common
circuit to
give people problems on our sets made in the last 5 years. If you have
high
voltage and vertical deflection (as you indicated) the unit is
probably in
what we term as 'IK blanking'.

The jungle IC sends a 60us pulse in the vertical blanking area in
order to
monitor CRT cathode current, thus maintaining white balance. Each
color drive
output has its own pulse in this area (Red=line 17, Green=line 18, and
Blue=line 19. These pulses are them matrixed and returned to the
jungle IC to
be monitored. If one or more of the pulses does not return (or is very
low)
the jungle IC will keep the video off. Bad video drivers, weak CRT
cathodes or
faulty IK pulse return circuits will cause this symptom".

Here's how to find the cause: Even though the video is off, the Jungle
IC is
sending the pulses to the CRT board. Measure each of the cathodes with
a
scope. At vertical rate you should see a 60us pulse at each cathode
about
20-50 volts in amplitude. What we're looking for is one of the
cathodes to
either have an extremely large pulse (in comparison to the other two)
at about
150 Vp-p or having no pulse at all".

If one cathode has no pulse, troubleshoot the driver circuitry for
that color.

If one cathode has a very large pulse, the cathode is weak or the
return
pulse is not making it back to the Jungle IC. To tell if the CRT is
bad, turn
up the screen control and see if you get a raster with one of the
primary
colors missing. It should be the color that you saw the large pulse
on. If the
raster is white or is slightly dominant in one of the primary colors,
the CRT
is probably OK. Troubleshoot the IK return buffer transistor and
associated
components for that driver.

If all 3 cathodes have a large pulse (150VPP) Then check the
following:

1. The IK buffer transistor which is sending the combined pulses out
of the
CRT board (all three must be present here and at least 700mvpp).

2. The screen control is turned too low.

3. G1-G2 short or leakage. this will be evident by G2 voltage dropping
drastically while the CRT board is mounted on the tube and raising
when
removed."

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