K7201 Dead - Thoughts?

modessitt

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Brought in a K7201 that "worked fine for a few months, then died."

Installed all new caps. Caps have been verified as installed properly, and no bad solder on them. HOT and VR have been tested as good, and the flyback appears okay. No cold solder joints found, and all parts listed in the WG Troubleshooting Chart for the K7201 have been checked.

Monitor is still dead.

There are no signs of life - no ticking, no neckglow, nada. Power has been verified into the monitor, and can be measured on the output of BD001.

I guess I should check the output (and input) of T3 next? Where's the best place to measure the B+ and what should it be - 126v? Is there a way to disable the shutdown circuit to determine if the issue is in the power supply or the HV section - maybe D10? Is the B+ and PS troubleshooting for this similar to the regular K7000?

Thanks for any ideas...
 
Brought the chassis home and tested the flyback with my ring tester, and it's fine.

T3 also seems to be fine, but T2 is giving me some iffy readings, so that might be my problem. I'm going to pull D10 first to see if it is in shutdown, and maybe swap out T2....
 
if this is a regular k7000 then the voltage regulator could be bad...

they usually short causing b+ to climb too high on powerup and they consequently quickly shut down.. however ive had ones in t he mast that metered goos but still did not work properly.. so it may be wirth your time to swap it out with a known good one...

ive also seen broken or dirty shutdown pots cause shutdown on these..

and re-replace the caps next to the shutdown pot or at least verify that they are the proper uf rating.. ive seen those caps be bad and cause swhutdown more then once..

the old r101 thing usually wont make em shutdown but should be checked while you got it apart...

good luck!!!!!!
 
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check R103. the big sand resistor by the big cap. just found one of them open.

you probably already cheacked C36, C38 for shorts ?

fuse is ok right? wiggle fuse holders as if they arent gripping tight monitor wont power up. just ran into a fuse holder fall apart in my hand. same symptom.
 
no neck glow and replaced all in the flow charts. am thinking there must be a trace with a cut in it that test ok for continuity but with a load, no go.

i wonder how much flex does it take to rip a trace.

wish i had a working oscilliscope so i could see the waves...
 
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Tsk. :rolleyes: For the last time, this is a 19K7201 monitor. While it has many similarities to a regular K7000, it is NOT the exact same chassis. I may be able to use K7000 troubleshooting techniques to work on it, as parts of the PS and HV section are very much alike. I also have the schematic already, as well as the proper troubleshooting list. But telling me what worked on a K7000 may not be relevant here.

K7201

mon53.jpg


K7000

mon3a.jpg


Ok, now that we've gotten that out of the way, I will peruse this info tomorrow and see if anything may help....
 
Tsk. :rolleyes: For the last time, this is a 19K7201 monitor.

Excuse me.... I was referring to a 7201.
Hadn't seen a chart specific to the 7201, so I was looking for something in between.
Aside from that I was just curious if it was a 13" version.

You NEVER mentioned it was a 19". ;)
 
Have you checked the output of the switching transformer...T3? Any voltage there at all?

Edward
 
Okay, here's an update so far.

I have +177 measured at the PS input side of T3, but have no voltage at all on the output side of D101 on the output side of T3.

I have another working chassis handy, and I measure +162 on the input of T3 and +125.3 on the output side of D101.

So, unless the +177 is so high that it doesn't rectify voltage to the other side, there is something wrong on the non-PS part of the chassis. Even if the +177 caused a high B+ that caused shutdown, I should still be able to read that high B+, and I get nothing.

Also, pulling D10 to disable the shutdown circuit does not let the monitor power up.

I have swapped the following parts from the working chassis to the non -working chassis in an attempt to identify the fault:

BD001 (VR)
T3
D101
C101
L002
R101

I have also pulled andtested the values of C36 and C37, and have tested numerous resistors, diodes, and transistors. The working chassis has a cap at C69, and the non-working chassis doesn't, but the non-working chassis never had a part there, and I've seen it work that way before it died.

I'm thinking I either have a shorted-to-ground part pulling the B+ to zero, or an open part somewhere that isn't allowing the circuit to complete.

Thoughts? It's frustrating not having ANY voltage, so i can't trace through looking for where it drops off...
 
Have you checked the output of the switching transformer...T3? Any voltage there at all?

Edward

I didn't see this before I typed the above post. T3 has only two leads connected for the output. One is tied to ground, and the other goes to D101. The reference voltage chart shows I shouldn't read anything on the side of D101 connected to T3 (and I don't), but that I should have +127 on the other side of D101. On the working chassis I have voltage, and on the non-working chassis I don't. Swapping T3 didn't help....
 
I didn't see this before I typed the above post. T3 has only two leads connected for the output. One is tied to ground, and the other goes to D101. The reference voltage chart shows I shouldn't read anything on the side of D101 connected to T3 (and I don't), but that I should have +127 on the other side of D101. On the working chassis I have voltage, and on the non-working chassis I don't. Swapping T3 didn't help....

Unsolder and lift up one leg of L002...the side that connects to R101....NOT the side connected to D101....with an alligator clip, connect this side to a 120V light bulb. I use my shop light for this. Alligator clip from L002 to AC hot side of my work light plug......another alligator clip to the AC neutral side of my work light plug (and connect this to ground...chassis frame). Fire it up and now check your voltage (also, the light should light). If you still have no voltage...something's up with the power supply side of the monitor/switching transformer.

Edward
 
Well, assuming I hooked it up right, the bulb didn't light. I'm assuming the bottom of the light bulb is the AC Hot and the threaded part is the neutral? I have a desk lamp that I just connected the wires to the prongs on the plug and it didn't light. So - unless I have my Hot and Neutral backwards....
 
Assuming you are kidding, it's a DC Voltage you are putting on the bulb and light bulbs will light regardless of which way you wire them. I usually use the video ground connector for my ground but either way this will isolate the supply. If you are not getting your 126VDC after isolating the supply, one other thing to consider changing is your PWM chip, IC001, which is a standard 3842 chip.
 
That might be my next move. I just don't understand why i have voltage into T3, but nothing on the other side. I'd think SOME voltage would be induced....
 
I have a ring tester for testing flybacks and transformers, and T3 measured okay. Besides, I swapped the one off of the working chassis "just to be sure" and it still didn't work, but the working one still did with that other T3, so I know the transformer wasn't bad.

I'll be doing more testing of the PS tomorrow...
 
Assuming you are kidding, it's a DC Voltage you are putting on the bulb and light bulbs will light regardless of which way you wire them. I usually use the video ground connector for my ground but either way this will isolate the supply. If you are not getting your 126VDC after isolating the supply, one other thing to consider changing is your PWM chip, IC001, which is a standard 3842 chip.

I agree....start checking stuff in that "feedback" circuit. IC001, as stated....also look at D001, D003, D004......and any electrolytic caps in this area (if you haven't already changed them).

Edward
 
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