k7400 troubleshooting

[lilshawn]

have a K7400 that started with a dead condition. Unit was recapped with new 'lytics ~1 year ago.

Unit would fire up when cold, run for an unknown amount of time and go into shut down.

Found the flyback to be very hot to the touch.

swapped out the flyback from a decommissioned K7500 unit. (99% sure this flyback is okay) found flyback part number(s) listed for both boards so i believe it's compatible. Tested the HOT and it seems to test okay.

reinstalled board and the monitor comes on completely white then gets bright bright white and starts flickering.

Problem with fixing this unit is... the tube is still in the machine a ways away and not in the shop... and i have no tube here i can test this chassis with.

Just looking as to what i should be looking at.

original flyback # "C53X0651" replaced with "C53X0671" various websites listed both of these #'s as replacements for this series of chassis.

[grantspain]

what voltage do you have on the video b+,read at r310

[lilshawn]

i'm going to pull the tube from the game and bring it in tomorrow

...this is getting ridiculous trying to fix it and then take it back out to the game to test...
 
(it's a big ass time crisis 3 cabinet)

[lilshawn]

whilst searching for R310, I found D310 (hiding under R310) blown out and cracked in 2 pieces.

I replaced it with a pull from a scrap board and doublechecked the components leading and folwing the diode

D810 C812
R310 C310

all still within spec.

on my way out to test it now, if it's still blotzed i'm dragging the tube and all back to the shop.

[lilshawn]

started out okay with the test. picture came up. a little dark as i'd turned down the flyback.

was going to turn up the flyback to get the brightness up and before i could the picture started expanding (horizontally and vertically). something still not quite right.

I shut it down and just brought the assembly back to the shop. i'll measure up the B+ in a bit here.

[lilshawn]

B+ at TP202 is 118.5vdc

vidB+ at TP203 is 145.4vdc

vidb+ at R310 as suggested is twitching out, my meter can't lock on to a voltage there.

[grantspain]

vid b+ is ok
b+ is ok

maybe the flyback is faulty

with most monitors i have seen an unstable b+ tends to affect one axis and the brightness will change,i have seen the flybacks cause zoom issues on kortek 2914f monitors

can you post a video

[lilshawn]

i seem to have it for the most part...i'm going to run it out for a bit and see if it's going to stay up.

I changed out the caps on a board that sticks out of the board. (not sure what it is or what it does. couple pots, transistors, few caps and resistors) and changed a resistor that seemed like it had drifted out of spec (all the lines where black or gray) R715? 780? something like that.

I also noticed there are some high voltage caps that didn't get changed (we usually mark the tops of caps that we've changed out with a marker and these ones aren't marked) i'll have to see if i have some to change those out.

there is still a bit of a horizontal twitching... when the screen is full of colors the horizontal pulls in a bit and the vertical stretches out a bit. stuff like space invaders or the like where the screen is mostly black, doesn't seem to exhibit this behavior. i figure it might be one of these high voltage caps that didn't get changed out causing it. the main B+ cap was changed (C105) but there are 2 or 3 other smaller value C808/C812 that seem to have got missed.

it's always possible this flyback is also on it's way out. It does have a small hairline crack in the case near the focus control... like they all pretty much do... maybe i'll see about ordering another.

this tube is still in great shape...nice and bright, hardly any burn in. could be around for a while longer yet.

[grantspain]

are you running a multigame board on this,if so you will get a small amount of blooming on bright screens-these boards do it to some degree with any monitor

[lilshawn]

I tested it in shop with a multi in 1 board and a golden tee 97 board. The original game is a time crisis 2 cabinet.

It survived a few hours last night without blowing up. I'll swap out those caps just to future proof it a bit and get it back in the machine.