K7401 HV Shutdown ?

[Kevin Mullins]

Ok... need some suggestions here. I've got two of these 7401's with similar yet different issues.

Basically one will randomly just shutdown. Power cycle the machine and it usually comes right back on and will remain on for yet another random period of time. (typically a fair length of on time) It's the better of these two issues.

The other one seems to be on the verge of shutdown but kicks back on at a very fast rate.
(kinda hard to describe) Almost as if you were flipping the main power switch on and off really fast. I tried to take a cell video of it happening, but it's not that great.
When you first turn the machine on everything seems happy, let it warm up a bit and it'll start doing this at random intervals. And sometimes even stay on fine for a good length of time and then suddenly starts wiggin' out again.

B+ is good, newly capped, several other things changed and tested, but I kinda wanted to hear some suggestions on this particular model before I got into much more of that.

I ran BOTH of these chassis here at home on my test tube setup for more than 24hrs and NEITHER of them even flickered a problem. Put them back in the machine......same problems. We swapped chassis around and the problem DID move with the chassis.
We've even swapped isolation transformers (it used to have K7101's in it), we've swapped game pcb's, you name it we've probably swapped it or measured it just to make sure we needed to still go back to looking at the monitor chassis and such some more

I'm scratching my head here, so throw it at me.

[youtube]http://www.youtube.com/watch?v=Ewba47B62f8[/youtube]

[Kevin Mullins]

Can anyone verify the B+ on this chassis ?

I may have been wrong on it and need to go back and double check it.
One doc says it should be 117v and another doc says these may shutdown at 120v and these are both running @122-123v.

I know darned well I'm gonna bust the pots all to hell if I try and get the glue/epoxy they used off. 
 

[SirPeale]

I can't give you an exact figure, but I can tell you these are particularly sensitive to B+.  I'd turn it down.

[Ken Layton]

I've seen more and more of the xray shutdown pots and B+ pots go bad from the glue Wells-Gardner drenches them with. Replace the pots.

[ZEN]

KEN,

not to hijack the thread, but I have the same chassis clicking on me with a replaced HOT and flyback, I dont know how to check the B+ but there is so much glue on there, could that be what is sending my board into protection mode?

[Kevin Mullins]

I dont know how to check the B+

There is a little post labeled TP202 right by the bigger capacitor I circled. (Test Point 202)
One meter lead there and the other on Ground. (I use the heatsink I also circled)

Measure for DC voltage.

[ZEN]

good stuff Kevin, thanks

[sarge]

Your B+ is a DC voltage.  It should be around 118VDC.

[Kevin Mullins]

I can't give you an exact figure, but I can tell you these are particularly sensitive to B+.  I'd turn it down.

I've seen more and more of the xray shutdown pots and B+ pots go bad from the glue Wells-Gardner drenches them with. Replace the pots.

I was leaning more and more towards the pots myself....or at least trying it to see. So the suggestions really help decide. I just new if I had tried to adjust them it would turn into a mess because of that crap glue they used. And "drenched" in it is about right on these two. I had also pondered if this was the type that became corrosive or conductive with age.
I'll replace them and readjust everything and let ya know how it goes.
Probably be a week or so before i get back on those.

Just weird I couldn't get either chassis to mess up here at the house on my test tube/rig.
And both were bought new in early September 2005, so only 4 years old.

[Kevin Mullins]

Your B+ is a DC voltage.  It should be around 118VDC.

Crap.... what the heck did I say AC for ?!?
(going to fix it now)   

[ZEN]

Mine is a 2005 rig too Kevin, also, it is rather minty fresh with little route use at all, and now of course light home usage, but I have some K4900s that still look good from 83 with lots of burn but little work on them......of course they dont have 9 ounces of glue on certain pots either........

[qrz]

fyi guys ....,
when a crt type tv or monitor goes into HV shutdown, it generally will need to idle unpowered
for a few ( 5-15) seconds to allow the various voltages to bleed off.

i.e.   the Hosc WILL NOT START if shutdown is active .....

[castlesteve]

Ive also seen this happen (flakey power cycles) due to cheap China sourced HOTs.  Try with another brand if possible.