WG K7400 intermittent missing blue component

[newmanfamilyvlogs]

TL;DR - 25" WG7400 salvaged from a Golden Tee cab. No blue color. Mfg Date: Jan 04. Powers on and works, minus blue. PCB around Q506, Q507, Q508 looks like it's gotten hot; traces are lifted from board but all continuity tests good after re-flowing solder on blue transistor. Upon initial reinstall of neck board tube worked perfect, all colors intact. Subsequent power-on and blue is missing again, continuity still tests good. Salvaged a second neck board in better condition, exhibited same progression of symptoms.



I've had a long time interest in arcade cabinets, but have only recently begun indulging this interest. While I've also always had an interest in electronics, I am very new to trouble shooting the actual physical components of a chassis and the like.

I recently discovered a cache of discarded arcade cabinets and began collecting parts that were not destroyed for the purpose of eventually building a proper MAME cabinet. Among the cabs was a Golden Tee cab with a 25" K7400. Initial power up revealed a bright, clear picture, but no blue. Adjusting the bias pots on the neck board did nothing. Further inspection of the neck board showed that the traces going to the three color power transistors had lifted off the PCB, apparently due to heat of operation.

The pad connection between one of the legs of the transistor looked like it might be cracked, so I scraped back some of the traces and reflowed the solder on to them, making the component much more stable and getting good continuity between all the points. When I reconnected it after that, the blue worked and the picture was wonderful. I powered down the tube and came back to it some time later, and the blue went out while the tube was operating. All continuity tested the same as before. Very frustrating.

Upon returning to the discard cache to look for other interesting components (found a few track balls in decent condition, and what not) I noticed another K7400 neck board laying inside a cabinet (the rest was missing). It appeared to be otherwise intact, but showed similar heat marks near the transistors, though not as extensive, and more centered on the red transistor than the blue. I swapped the boards and powered the tube up again to another bright clear perfect picture, with all colors working. I powered it back down, went to get a camera to document the new board for my blog (cotmm.org, if anyone cares), but when I powered the tube back up the blue color was gone again. Tested all continuity again and it all looked good.

So, now what? Could the problem lay with the tube itself? I've got a 27" K7000 in need of a cap kit; if I swapped chassis (although I know the picture wouldn't fit) would that work well enough to rule out or confirm the chassis as the culprit? Or are they fully incompatible?

[lilshawn]

i'd check your input for proper wiring.

likely the blue signal wire is loose if swapping things around doesn't work. you may have to take a pencil eraser to the contacts of your board to get a good connection

hot transistors is a common problem with thw WG monitors, if it'san issue i put a fan to blow some air on them.

also even though a transistor checks out okay it can still be no good or intermittent once it heats up. id recommend replacing them

[newmanfamilyvlogs]

Finally got around to swapping the blue transistor in question with an assumed good one. Pulled the red transistor off of the first neckboard I had.

So far, it works. I'll let it run for a while today and see what happens.

Also, and this probably should be its own thread, but the video has flickering horizontal tearing in it. It seems to be less pronounced with I pause a game in Mame. I'm running Soft15Khz on a GeForce 2MX. Any insite on if this is monitor, mame, or card?

[SirPeale]

I'd have to see it in action, but it sounds like a minor sync issue.

[newmanfamilyvlogs]

I'll start a new thread about that issue, since it's almost certainly unrelated.

Thanks for the other help. So far it has continued to work properly.