Stumped with K7000 low B+ 40VDC

menace

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Its a K7191 with a P538 board.

When I plug it in, I get 40VDC when I check the red wire on R301--R103 also heats up pretty sharply. So far:

Capkit done, flyback changed, IC4 checked=ok, R103=3.0ohm, D19-D22 checked and ok, C55 swapped with another one, reflowed a bunch of joints but they all looked ok anyways...

What else would keep my B+ down to 40V??
 
Yup--Pulled R89-ok, r97-ok, r104-ok

R103 gets really toasty really fast--I've swapped in a different c55 with no change (I can't imagine I have three bad c55's when they rarely go bad--can I???)

I'm thinking my d19-d22 diodes are testing fine and failing under load?--just guessing here..
 
what does R301 measure?

look for a bridged solder joint around the VR.

if you need parts shoot me a PM.

if you hit a brick wall, your welcome to send it in.

Peace
Buffett
 
R301 measures 177.8 (in circuit) I checked around VR9 and it looks ok.

Interesting thing--if I measure resistance across R101 you can see the value rise (as if you are charging a cap) and it peaks at 4.02Kohm. When I do the same thing on a known working chassis, it continues to rise above 5Kohm.

I'm going to get another IC4 and swap it--it seems to be the most common failure so maybe i'm just testing it wrong--other ideas are still appreciated!
 
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one thing you need to watch out for is reading in circuit because you can get readings of other components that are connected by the traces. you should lift one leg of the resistor to get a proper reading
 
one thing you need to watch out for is reading in circuit because you can get readings of other components that are connected by the traces. you should lift one leg of the resistor to get a proper reading

I've found that's not necessarily true, not with these resistors. I can test 20 of these fine out of circuit, put known good ones in, there's still a problem within the actual circuit that's dragging R101 down.

I have 2 or 3 chassis that do the same thing as OP's problem, where R101 reads substantially lower in circuit and the monitor does nothing when you apply power. I haven't been able to deduce what other part is pulling R101 down and this isn't discussed in the sticky thread at all.

I've even talked to Buffett about it, so I'm curious what the solution is.
 
Update--R301--pulled and tested ok, VR9 pulled and tested at 2K so thats ok...I swapped in a different IC4-no change..

Could a bad flyback do this? I know its brand new but maybe I got a bad one out of the box?
 
that I haven't figured out either. if you start desoldering pads on the flyback, R101 might climb to where it needs to be. I wanna say I tried a known good flyback and it did the same thing though. same with the critical safety cap. swapped a good one in, same thing once I soldered it back in.
 
I'll have to closely follow this thread. I recently ran into a problem with my 25K7191 P538 chassis as well. After turning on my cab on a humid day for the first time in about a week, something shorted on the monitor and caused the isolation transformer to go up in smoke.

Game plays fine and power supply tests perfectly, but nothing on the monitor. Tested the B+ at R301 and got 0V. Checked the fuse and it's still completely intact and perfect. So something has gone on mine and can't figure out what. (Testing things while the monitor is in the cab in a Dynamo HS-5 is a bit of a pain. Can't take monitor out of cabinet to test though I might do that and create an extension connection for the video and power and ground connection). Looking at the chassis, I see nothing wrong with it. No burned up areas, no irregular looking caps. I'm stumped as well.
 
I'll have to closely follow this thread. I recently ran into a problem with my 25K7191 P538 chassis as well. After turning on my cab on a humid day for the first time in about a week, something shorted on the monitor and caused the isolation transformer to go up in smoke.

Game plays fine and power supply tests perfectly, but nothing on the monitor. Tested the B+ at R301 and got 0V. Checked the fuse and it's still completely intact and perfect. So something has gone on mine and can't figure out what. (Testing things while the monitor is in the cab in a Dynamo HS-5 is a bit of a pain. Can't take monitor out of cabinet to test though I might do that and create an extension connection for the video and power and ground connection). Looking at the chassis, I see nothing wrong with it. No burned up areas, no irregular looking caps. I'm stumped as well.

there's a lot of resistors that drive the power supply that people don't bother touching a soldering iron to. they get stressed enough to weaken the solder joints. any of those go open or the pads sizzle you'll lose B+ entirely.
 
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