Hi Franc.
sorry to barge into your mailbox, but i couldnt upload pics to sci
electronic repair.
Attached please find the pics of the plasma problem i was describing.
The problem is more acute with a higher contrast setting.
The history so far:
I have 2 of these units for repair , lets call them unit R and unit S.
Unit R came in with no picture no screen. Just the blue power led
would come on flash at .5 sec intervals and then nothing. I could turn
on and turn off power but no picture or light.
I opened her up and found a shorted diode on the Y drive pcb D5010.
Thought the manual (as usual) has no schematic for the Y pcb , i did
some tracing of the section and found it to be a rectifier at the
output of a transormer based sub power supply driven by a 5M0380R (5
pin device) on the primary side. A 7815 and an optocoupler driven by a
H431 (three terminal reg) lay in the feedback loops (4 windings on the
transformer). The diode on replacement would heat up indicating a
short ahead. I located this short in the Y buffer pcb which receives
over a 100 volts from this diode. The y buffer pcb has 8 SN755870
panel drivers on it. My short circuit measured 47 ohms. I used a trick
here, keep ohm meter connected, heat each of the 8 SN755870's with my
smd hot air blower. i noticed the short 47 ohm reading diminishing
slightly down to 38 ohms when i hovered over 1 particular ic out of
the 8. I desoldered the suspect ic and voila, sure enough my short
disappeared.
On powring up now, i'd see a picture with the symptom illustrated in
the pics. I've tried swapping all the boards from the other set "S" to
no avail, maybe cos the other set has the same problem. I'm lost here.
Unit S came in with a picture that was continuously varying in
brightness. Sort of like white flashes. Couldnt really tell if this
contour/rainbow effect was present then. I noticed that i could'nt
vary VSCAN coming in from a DC to DC power supply board. The sticker
on the chassis listed Vscan at -185 volts but i was measuring
variations between -80 and -300 volts. The V scan preset wouldnt let
me set voltage to the desired -185. Hooking up a scope to the vscan
line showed that the voltage varied in sync with the white flashes. I
opened the DC to DC convertor , poked around a bit and i found an open
180K ohm resistor in series with the preset. Repalced it and mt white
flashes were solved
. This is when i noticed the contour/rainbow
effect. In trying to move the powered set around 2 IGBT's RJH3077 blew
along with a driver ic IR2106S. I'm now left with 1 working Y drive
board.
The other set "R" has 1 SN755870 missing but when i plug that in i get
the same contour/rainbow effect, of course with a horizontal black
band in the region of the missing buffer.
I have the service manual and i can get into servce mode. The manual
has no schematcs for Y section.
Any clues?
Your help will be really appreciated.
Jango
Woops, can't post pics here (how dumb of me). I guess i'll mail u
direct , hope that's ok with u !
