WG D9400 Red slowly got brighter, now full brightness despite red gain set to 0

[yatzr]

After doing a cap kit and replacing IC801 (LM1269), I had a perfect picture and played a game for several hours with no problems. Then I switched to some other games. During one of those, I noticed a black screen was showing some red. Over the next 2 minutes or so, the red slowly got brighter and even starting showing red retrace lines. I adjusted the red gain down to 0, but the red brightness stayed.

I'm assuming I have a bad transistor in the red circuit on the neck board, but that's really just a guess (and maybe it's the reason my original LM1269's red output was busted). I tried doing a diode test on all of the transistors in the red circuit and compared them to the transistors in the blue and green circuits but didn't any inconsistencies. I'm not sure if testing without removing the transistors invalidates the test or not.

Anyone have experience with this particular problem, or have ideas of other things I could try checking? Thanks!

[lilshawn]

most likely bad transistor. it's not uncommon for them to get weird once they heat up.

generally if you replace the video amp IC, you should replace the transistor too as well. it's possible for the transistor to short in a way that dumps high voltage into the IC killing the output.

luckily the transistors used are common and are cheap

[yatzr]

most likely bad transistor. it's not uncommon for them to get weird once they heat up.

generally if you replace the video amp IC, you should replace the transistor too as well. it's possible for the transistor to short in a way that dumps high voltage into the IC killing the output.

luckily the transistors used are common and are cheap

Which transistor is it most likely to be? The Q812 SC2682 is the one with the big heatsink, but the Q811 ph2369 is the little guy connected to the ic output. Or maybe I should just replace both?
Also, where are some good places I should be looking to buy these? I was struggling yesterday cause the shops I normally use either had them as special request or very large minimum orders. I ended up getting the video amp ic off ebay, but there's gotta be a better option than that.

[lilshawn]

in this case i'd replace the pre-driver and the driver transistor. might as well.

mouser has good onesies and twosies prices. (of course the more you order the cheaper it is.) shipping is variable depending where you are. if you need a pile of stuff (over 100 bucks) shipping is free... it would be a good time to pick up some soldering stuff or a power supply or microcontroller boards or something to pad out the order.

if you don't need anything else, even if you pay shipping, it's super fast. i get stuff in like 2 days here in canada. it's not going to be a week or anything.

[yatzr]

Interesting, mouser was actually one of the first places I looked. Is there a strategy for finding newer equivalent transistors? When I search for SC2682, I only get non-stocked or obsolete results. Similar story with PH2369.

[lilshawn]

it is basically an obsoleted part since you'd really only use something like that for CRT's.

off the top of my head i don't recall the last place we ordered from.

EDIT: just had a lookup in a cross reference, if you can't find a 2SC2682, the 2SC2690 looks to be basically the same transistor but it can carry 2 amps instead of 1 amp which should be okay. it shouldn't change the operating characteristics much... if any.  mouser has some on semiconductor branded KSC2690AYSTU parts that SHOULD work okay. it's the same family of transistors as the original, just higher current rating. i'd double check the datasheets between the original and the proposed replacement to be sure it's switching speed is sufficient and the turn on voltage is the same or similar.

the 2sc4001 might work as well. hard to tell without really digging into it.

[yatzr]

I ended up getting some exact replacements from utsource.net. I replaced Q812 SC682 and Q811 PH2369 and still had the super bright red. I also tried replacing the LM1269 again (I had put a socket there last time, just in case) and no change. Then I went ahead and replaced Q813 SD1609, Q814 SB1109, and Q815 BF423 since I had ordered replacements of those as well (simply forgot to order BF422 for Q816). All that, and still have the super bright red. Not sure where to start looking next...

[yatzr]

I started poking around with an oscilloscope on the neck board to compare signal levels at various spots between red and the other two. Eventually figured out that R836 had failed open . I replaced it and voila, perfect picture again!

[jayleone]

I started poking around with an oscilloscope on the neck board to compare signal levels at various spots between red and the other two. Eventually figured out that R836 had failed open . I replaced it and voila, perfect picture again!

I had the exact same issue with my D9400.  Thanks for following through and posting the solution; you saved me countless hours of time!