k7400 SMPS ticking AGAIN [fixed] AGAIN

[lilshawn]

have a k7400 that was taken out of a somewhat working time crisis cabinet.... sometimes the monitor would stay off when powered up for about 10 mins, then decide to turn on. (couldn't tell if it was ticking or not since the cabinet is super loud) but removed it since the colors where smearing out.

replaced all the caps on the board, now cant get the board to power up (dead/Ticking),

isolated the power supply and it's still ticks

turned down the b+ a bit, a bit more, a bit more, still ticks.

switched out u101 and q101 still ticking...

not sure what else to check....thanks for the help in advance...

[MonMotha]

There will be a bunch of diodes on the output of the supply.  These can fail open, shorted or leaky (which would cause intermittent issues).  Check those out.

[lilshawn]

D106a,b,c as well as  D118 all seem to test okay, (4.456v on diode test) course it could change once it's under load.

I have replaced those diodes on another chassis, but could not obtain the exact replacement, and the NTC "replacements" failed within months. SO... if this isn't the problem, is there anything else i could check as well first, before committing to ordering the proper (BVY28-200) diodes?

[qrz]

the fact that it was intermittent suggests a poor solder connection in Hdrive/output .
suspects are the driver xistor , drive xformer, flyback connections.
any of these can kill an output xistor.

for example:
rca's ctc203 chassis had a coil on the base lead of the HOT that suffered from poor soldering .
it would cause the HOT to eventually short . which then would take out the SMPS.  parts + time , but an easy fix - $$

[lilshawn]

suspects are the driver xistor

changed it out already

drive xformer

i have another i could try, though very unlikely to fail.

flyback connections.

done and done to no avail. 

the fact that it was intermittent suggests a poor solder connection in Hdrive/output .

in this case, the problem seems possibly heat related... leaving the unit powered up (with possibly with the SMPS in overload protection couldn't tell its got 3-110v 5 inch fans going full bore) the problem went away and the chassis worked again. We had planned on just leaving it until later when we could service the machine properly when we noticed it started working. been like that for 3 months till the caps went on it and the picture smeared out on it. now i gotta fix it 

this cabinet with it's two computers and monitors gets quite warm... when taking the chassis out, you have to shut it down and give it a few minutes to cool down... the ventilation isn't the best....even with 3 5 inch fans

maybe feedback resistors?

[lilshawn]

UPDATE

iv'e managed to get the chassis working again, seems I missed a 22uf 64v cap (c117)  (tucked in behind big 400v 270uf cap) and a resistor (r117) was slightly out of spec. replaced both.

the chassis popped back to life for a while then blew the HOT

teach me to play with the B+ then not check it again.

re-replaced the HOT

tweaked the B+ and run the chassis on a test tube.

thanks to everyone who has helped.

[sarge]

Just for the record C117 will exhibit the same symptoms on a K7500 and U5000/2000.

[lilshawn]

so this thing has decided to crap out again, only this time the tick is slow. (about 3 per second) B+ pulses with the tick and is less than 1 volt so i don't think overload. Ran real good for a few days anyway.

replaced both the SMPS oscillator chip and the FET and swapped out the HOT just to be sure, to no avail.

any more ideas?!

[SirPeale]

Try manually turning the B+ down.  This chassis is really sensitive to B+.

[lilshawn]

I've tried adjusting the B+ down and up, with no change

EDIT

i've also tried adjusting the x-ray to see if that was shutting it down but no...

<rant> i'd like to smash this thing in half so that i can say it doesn't work because it's smashed in half</rant>

[castlesteve]

oh i know how you feel with these things.  I have a handful of chassis that I've decided to take the shotgun to.  Feels so much better.

[sarge]

Here's a thread from a while back with similar problems. 

http://forum.arcadecontrols.com/index.php?topic=81048.msg861217#msg861217

[lilshawn]

thanks, but i have gone through that post and a ton of others as well...

see also....http://www.2coinsperplay.com/Tech/Monitors/Monitors.html
see also....http://www.vendoramusements.com/bbs/nph-YaBB.pl?board=monitor


i may end up sending this one out if i cant fix it

[lilshawn]

well seems the HOT crapped out again, but it tested okay with a transistor tester...shows you can't trust nothin these days.

measured the b+ off the HOT and found it to be 150v where the "specs" say 152v

i turned down the brightness and contrast (to get a black screen)to find the absolute max and found it to be at 160v

turned it down to 152 and it's averaging 142 now with a moving picture.

it's on the bench overnight, lets see if it makes it till morning.

[qrz]

u might wish to replace c716 ( 1uF/200v )
used as a decoupling cap, c716 will affect the shape of the drive waveform.
as the waveform becomes distorted ( due to cap changes ) , the workload of the Houtput xistor increases and it will fail prematurely....
common tv/monitor failure (crt)

[lilshawn]

yah, that little one behind the smallest transformer, it tested okay, but i replaced it anyways when i originally fixed it.

i'm thinking maybe the HOT was just stressed out from the b+ voltage being off. the game has allot of dark scenes in the attract mode so the voltages was probably way up there a majority of the time.
 i did originally replace it with the part listed in the parts sheet c3686 but i replaced it now with a c3688.

it made it though the night on the bench and is now in the machine...we will have to see if it conks out in that nice oven of a cabinet over the weekend.

thanks to all that have 2 cents and buckets of knowledge/experience!