K7000 in HV Shut Down (Oh how I loathe thee)

EagleTG

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How does anyone work on these things? Every time I touch one, things go badly. These chassis are the devil, Bobby Boucher. THE... DEVIL...

Flash back a few months. Trying to get a resistor on a K7000 chassis (Medium res, Super Sprint) to stop desoldering itself. Give up, swap another K7000 chassis into the cabinet. (Click here for link to thread of dispair.)

New chassis was working like a champ. Started showing signs of flyback failure (cracks in the flyback, white knobs, would lose focus as it heated up). (Link to me being here recently on another monitor.)

Order new flyback, install it. Check all the caps while it's out, looks like it's been recently capped, nothing shows bad on ESR. Decide to rejuve it while it's out of the cabinet.

Back in the cabinet, B+ is reading ~160v. I go over the board (been there a thousand times), find a bunch more bad solder/traces. Replace traces/reflow solder. Same situation. Repeat at least 6 times.

Following the flowchart, remove D10. Monitor comes up with image. Tube must still be good, right? I didn't kill it on the rejuve. (Which in hindsight, should have been done as a separate step. I should have confirmed the monitor was working post flyback replacement first. I know for next time.)

Re-attach D10, replace C57 (it was testing marginal anyway). Find a few more (@&#^$%@) bad solder connections. Been jumpering them as I go, thought I had them all resolved.

Caps next to HV shutdown pot test very good on ESR, have been recently replaced anyway.

I swapped in another (known good) voltage regulator as that's the next step in the flowchart. Still does the same thing.

At this point, I've been over the chassis solder and traces with a fine toothed comb, I don't suspect any additional problems (but who really knows with these chassis). Yet, this thing is STILL in HV shutdown. Still reading ~160v on the resistor, still powers up when D10 is disconnected.

My question is, can a potentially defective new Flyback cause HV Shutdown? Looking at the schematic, I'd believe it could. Any suggestions on what else to try at this point? I kept the old fuzzy flyback for the possibility of throwing it back in to see what happens. I've read a few things about bad flybacks right out of the box, this one was from ArcadeCup (obviously a trusted flyback source).

And I've reviewed the sticky on K7000's repeatedly, but maybe I missed something.

Thanks in advance for any help on this one. If it's not obvious, I'm beginning to really, really hate the older K7000 monitors. They make the quirky connectors on the K4600 seem like child's play. (And with that, for some reason the K4600 is still one of my favorite monitors to work on... not sure why.)
 
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What is the voltage going INTO the voltage regulator?
One side of that big resistor is the voltage input, the other side will be your B+ (output).

19", 25", 27" ?
 
Whoops. This is a 19" model.

Voltages were around 169 on one side, 171 on the other. Typically over 160, and within 10 volts of each other from side to side.

Thanks.
 
According to those measurement, the input voltage is too high, which will obviously increase your output voltage. (causing shutdown)
Should be 130v-ish DC input....123v-ish DC output.

I would pull the voltage regulator out (eliminate it from the circuit) and then measure the input voltage again.
If it is still too high I would backtrack into the power supply side of things. (rectifier diodes, caps, etc, etc)

Check the resistance of R301 (the big one) to make sure it's ok too while your at it.
 

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BTW - K7000's are easy - for me. If you give up, I fix them for $50 + parts + shipping...
 
what are the numbers on c36????

i dont know how the med res ones are. if this is the same fly as a regular k7000, i seem to remember something(a post by mod???) about some of these used a flyback that ran with a 4100pf cap at c36 instead of a standard 6100pf cap...


with the new fly, the chassis would shut down. ended up bieng caused by the 4100pf cap at c36. the cap was swapped with a 6100pf cap and tada, runs again.


the b+ reading is with it in shutdown correct? with it running , what does it read?
 
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Finally working on this again.

Replaced c36 with the new Bob Roberts one. It's a 6100pf now instead of the 4200pf. Still in shutdown.

With D10 removed, I get 125 and 161 volts on the resistor. Screen comes up, is distorted (naturally).

What should I try next?
 
To answer another question in the thread. R301 is a 180 ohm on this chassis, not the 220 that is in the schematic. It is reading correctly.
 
usually if it comes up with D10 lifted it is a bad VR.
test it and see what you get. also make sure you don't have cold solder joints around it.

replace that if its bad and it should be good to go.
make sure you reinstall the mica insulator and have good solder on the legs.

Peace
Buffett
 
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