CTC187 picture too big

[James Sweet]

Found a shorted diode in the pincushion circuit, ordered an exact
replacement part and installed it. Pin is now fine however it appears
the image is a bit too large, I'm not sure yet how large but when I
change channels the very edge of the text runs past the edge of the
screen. Pic looks good other than this fault, nice and bright with good
color.

Is there a common cause of this? I almost wonder if the EEPROM got
slightly corrupted though with everything else working fine I'm worried
that a fault may still be present so I hesitate to adjust anything.

 

[Dani]

If you found CR4403 bad, did you check C4407 with a cap checker?
Check the other big diodes, & blue caps in the area. Also, is the B+
too high? Dani.

 

[James Sweet]

If you found CR4403 bad, did you check C4407 with a cap checker?
Check the other big diodes, & blue caps in the area. Also, is the B+
too high? Dani.

I checked the capacitor both for value with my multimeter and for
leakage by charging it up to rated voltage with a variable HV PSU,
didn't think to check ESR on it though. I'll check the other caps and
diodes, what should B+ be on this set? 180V?

 

[Dani]

Standby B+ can be about 145 volts, & about 140 running....approx.
You didn't say if the pix is too high also...B+ could be too high. I
don't think the EEPROM is corrupt, but if it's just too wide, & not
keystoned, it MAY need a tweak. Dani.

 

[James Sweet]

Standby B+ can be about 145 volts, & about 140 running....approx.
You didn't say if the pix is too high also...B+ could be too high. I
don't think the EEPROM is corrupt, but if it's just too wide, & not
keystoned, it MAY need a tweak. Dani.

It's pretty hard to tell really, when I get the chance I'll drag the DVD
player down there so I can put a test pattern on it.

 

[Bill S.]

Found a shorted diode in the pincushion circuit, ordered an exact
replacement part and installed it. Pin is now fine however it appears
the image is a bit too large, I'm not sure yet how large but when I
change channels the very edge of the text runs past the edge of the
screen. Pic looks good other than this fault, nice and bright with good
color.

Is there a common cause of this? I almost wonder if the EEPROM got
slightly corrupted though with everything else working fine I'm worried
that a fault may still be present so I hesitate to adjust anything.

Don't have schematic handy for this chassis, but a lot of these have
a set of ceramic caps that form an AC voltage divider across the
pincushion diodes. Failure of one of the capacitors commonly causes
diode to overheat, and of course any problems in the pin circuitry
affects picture size. My guess would be look for open ceramic cap
connected to the diode that failed.