K7400 wigs out after a few seconds

C 729 is big because it is a non polarized cap. I've seen maybe one or two bad, ever. U 701 is pretty rock solid and I doubt yours is bad. R 301 and R 303 are the resistors I would look at....they are 1.2 ohms and can not be even 2 ohms.
 
with some tolerence given for your meter and the resistors, I'd say 1.6 ohms is probably good. Have you ever used the lamp as a load for the power supply? That would isolate the problem between the power supply and the high voltage section on the chassis.
 
after your chassis wigs out....measure pin 10 on U701 for 12vdc
 
cbuschief said:
with some tolerence given for your meter and the resistors, I'd say 1.6 ohms is probably good. Have you ever used the lamp as a load for the power supply? That would isolate the problem between the power supply and the high voltage section on the chassis.
Click to expand...

I'm not sure what you mean the lamp?

I will test that pin when it's on and report back. Thanks.
 
switching mode power supplies need a load to function. By using a lamp with a 60watt light bulb and using a couple clips to attach it between chassis ground and the B+ line there by bypassing the horizontal section and high voltage section, you can run your monitors low power supply to see if it is running correctly without the error correction section of the chassis from constantly turning on and off the circuit.
 
I promise I'll look at mine again soon...I'm trying to put the finishing touches on my arcade room for Thanksgiving. We're hosting celebrations on Thursday and Saturday, and the arcade is going to get a workout. I need to be ready.

As soon as the holiday weekend finishes, I'll take a look at my chassis.
 
I think your issue is in your verticle section...possibly with the 25 volts that runs this. Since your B+ seems to be working and it looks like the screen starts losing verticle stability, that is what I would suggest. There is a supply resistor for the 25VDC that may be out of tolerance and the longer the chassis is on, it starts to flake out. I'll look for the part when I get back on line later.
 
I think the problem is in the power supply section.
The B+ is already a tad low and fluctuates even lower when the problem is happening. But it may actually be the B+ going low that is causing the issue. (if that makes sense) So what it looks like is that the screen is beginning to collapse due to a lower B+.

I would go over the entire "100" numbered series of components.
Check R108, its a .33 ohm 2watt resistor (check cold and hot)

Been a while since I've messed with one, but here is an extreme example that I farted with a couple years ago. (never did finish messing with that one)
http://www.youtube.com/watch?v=Ewba47B62f8
 
Resurrecting this thread as I just pulled this monitor out of storage, and I'm going to tackle it again, starting with the last post from Kevin.
 
So I did was Kevin suggested and tested R108. I pulled one leg, set my ohm meter to lowest ohm setting (200) and tested. It came back at 0.8

I assume that is not correct for a .33ohm resistor?
 
Thanks Cadillacman,

I put in a new flyback first thing a while back, so I know it's not those solder joints.

I just checked R111 and it's in there tight.

R108 wasn't really bad, my ohm meter tested the new the at the same. Maybe my meter can't test below 1.0 very accurately?

Anyway, I'll replace that U701 when I get one in and see what happens.
 
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