WG 25K7401 low B+

[debaser138]

I've got a 25K7401 chassis that I did a cap kit on and replaced the flyback. It powered up the first time and was on for a minute or two and then I turned it off. Went to turn it back on again and now it's not working.

When powering it up, I hear the high voltage trying to come up but it's almost like it keeps turning on and off very quickly. The B+ is fluctuating between 20v to 40v very quickly. I've reflowed the headers and any joints that looked suspect. I checked the traces coming off the flyback pins and they are good.

Any ideas what I should be looking for?

 

[mecha]

check R100, 4.7 ohm resistor

 

[debaser138]

check R100, 4.7 ohm resistor

My 7401 chassis has a jumper at R100, not a resistor. Looks factory as well.

 

[mecha]

flowchart also calls to check C107 and C155. I didn't think those were electrolytics but I went back and the one might be one of those green disc caps. zenomorp had something with either a K7400 or K7500 years ago and one of those were shorted or something.

you can give me U5000 to fight with over these any day. when these break they become quite nightmarish.

 

[Ohmerone]

My 7401 chassis has a jumper at R100, not a resistor. Looks factory as well.

Need to Divide the circuits when you Can.
To find what is working as well.

Lift the jumper For R100 and measure the B+
at L100 in power supply next to it
see if the B+ goes up on the B+ supply???

If not the Problem is in the Power supply!

Check C119 and C121 for a short
if good we can check others in PS
that could fail, I know R104 can get loose
sometimes, handling the chassis

If dc goes up
you can put jumper back in
then unplug chassis
then with Ohms Check at TP202 for a short to ground?
It should be high in ohms on
the B+ TP202

Does it read short to ground?

If so then de-solder the center pin of the HOT
this will take the High voltage out of circuit

Each test depends on the results to know which
way to go next



For me When C119 or C121 shorts it reads

a obvious short across the part in or out of circuit.

The Green Disc cap's mention

Check it out

 

[debaser138]

Thanks for the great info Ohmerone. I'll be digging into it again this weekend.

 

[debaser138]

I'm finally getting back around to this monitor as I got busy with other projects. If I remove the jumper at R100 and measure the B+ at L100 it's a steady 115v so it looks like the power supply section is good. HOT is good and there is no short from TP202 to ground.

When I reinstall the R100 jumper and fire it up, the B+ jumps around from 25v to 45v and I can hear the high voltage try to kick in every second. I'm not very familiar with this monitor. Any ideas what else I should look at?

 

[mecha]

measure R100, should be 4.7 ohms. these monitors like to eat resistors.

 

[debaser138]

R100 is a jumper on this chassis.

 

[mecha]

oh right... that was a revision at some point I guess. follow that chain in the schematics then, the juice at the power supply is fine, it's where it gets out that's the issue. that U5000 I worked on recently wouldn't even tick and the B+ was like 177V LOL and that's cause all the resistors that take the power out of the power supply were bad.

 

[Ohmerone]

Tip. B+ pot needs to be replaced
when service in general, if never replaced
because on the Epoxy resin
on it, over time. Or remove it and add a 390 ohm
resistor to Keep the B+ fixed voltage during
Trouble shooting and can get the Pot when ready if needed.

May need to lift one end of z701 to force drive to flyback
so it doesn't go in to shut down if its trying to.
see what happens to the B+ and High voltage
LUK

 

[debaser138]

I replaced the B+ pot and no change. I also lifted one end of Z701 and no change. B+ still fluctuates between 20-40v DC.

 

[Ohmerone]

Need to disconnect the hot
Start dividing the circuit when you can
B+ Should adjust over and under the 120vs dc
For good regulation with the hot out and dc meter
And bulb during this test on tp202
Can you make sure the ohms on the b+
trace is high ohms. Not a continuity check to ground
May be a short on the b+ trace
Luk the ohms at tp202 with the hot out

 

[pinballdirec]

Rodney, Did you ever figure this issue out? I have a couple doing the same thing.