Strange Polo Problem

[PhilT]

Anyone seen this issue before, green haze appearing when there is nothing to display, but picture shows fine when its being sent a full screen signal. Below are stills from a block drop pattern from a signal generator. And it only starts after its warmed up a bit. Its a Polo 33, 15khz. don't get how this particular fault could occur.
If i switch the signal generator off once the fault has appeared, entire screen goes green. I just like to know how its occurring, rather than just shotgunning everything . ty

 

[Georgian2]

Low voltage on B+ would be my guess. Can you measure it whem cold and after it starts to get green?

 

[gunblade]

thats very odd, looks like the generator is causing it

 

[tb2000]

I'd say it's something in the green circuit, i'd start at the green transistor on the neckboard and work backwards through the circuit to the chassis. There's also a common problem i've found with Polo 1's where the brightness changes with the image (black background goes grey but could go green if green is turned up more than the red and blue) but i've never got to the bottom of what causes it (it's normally always full screen though, not like your problem), normally I just swap out the neckboard for a working one as the fault is always neckboard based.

 

[PhilT]

got to bottom of this eventually.
one of the drive transistors was measuring different, although it was for blue not green. Reflowed the solder on all 3 and the problem went from green to white, so started looking for things that would affect all 3 guns.
socketed LM1203 and tried another, no change.
C21 had bad solder joints, this is to do with the black voltage reference circuit in the LM1203 so was hopeful it would be the fix but replacing this didn't fix the issue, but then some very light jail bars could be seen if looking carefully.
R36 was open, replacing this did fix the issue.

still have no idea why the problem only happened after things warmed up though. Its not like R36 value changed when it got hot, its OL all the time. still, its fixed so i'll take that.