25" K7000 Black Screen, Low voltage at R301

whoamonga

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Hey Guys,

Had another thread going but there's been developments that rendered the title unhelpful...

I hear high voltage static, but the screen stays completely dark, no neck glow. So I checked voltage at resistor R301 and got 47 vdc on the blue leg and i think 60 on the other side.

According to the flow chart, I should replace the large filter cap, C55, but these are supposed to rarely go out, right? I've also adjusted all black levels, screen, and the such...

Any suggestions on what else I could check? I've done cap kit and flyback replacement, reflowed the suggested areas in the sticky.

Thanks in advance!
 
I have seen something similar when the fuse was partially burnt, what is the actual resistance of the fuse? While there make sure the fuse holder is nice and tight as well.
 
fuse holder is tight. took some effort to get the fuse out, and the fuse physically looked fine, no burn marks or anything on the fuse.

I've never measured resistance before, but I put the multimeter on the Omega symbol and set it to auto. On the range options of 4 40 400 and 4000, it went to 400 and displayed measurements mostly between 2.8 and 3.5, though sometimes dipped to 2.5 or peaked to 4.2.

Is that helpful? Thanks for your reply!
 
That is helpful. The value should be close to zero on the '4' range. It looks to me like the fuse may be partially burned, and they almost never show physical signs of damage when partially burned. If you have another fuse try testing it and if the readings are substantially different install it and reapply power.
 
For the ground on the resistor reading I used the metal frame that the PCB sits on. That frame has continuity between ground on the neck board and ground on the line filter. Felt that was okay?


I checked resistance with the monitor turned on. Should I check it without power? Also, that's a 250v fuse? what wattage, do you know?
 
Man... I was really hoping it was just a fuse...

I think my multimeter was wigging out with the Auto setting. I manually set it to read at 4 (instead of the Auto reading 400) then took a measurement and it read .02, then dropped to a steady .000

The amp that was in there was 7amps though! I swapped it out with another 2 amp fuse that read .000 resistance, but nothing changed... B+ voltage at R301 was still the same...

Any other suggestions while I save up for a C55?
 
For the ground on the resistor reading I used the metal frame that the PCB sits on. That frame has continuity between ground on the neck board and ground on the line filter. Felt that was okay?

That is earth ground, but not necessarily "chassis" (circuit) ground.
Just to double check your voltage readings... use any of the standup heatsinks as a ground. (including the one that resistor is mounted to)
 
Wow. I'll be dang... I took a voltage reading from the heat sink (chassis ground) and the voltage was closer to being correct! 130v on one end and 164 on the other! Blew my mind. However, the flow chart is looking for either 123v, 135v, or 160v. I kinda split the difference between all of them.

Just for grins, I checked resistance between R103, D19, D20, D21, and D22. None were open. Checked the other couple of ceramic resistors as well, all checked out.

Do you guys suggest I look at R213 on the neck board or desolder D10 to check out the voltage regulator?

Thanks again! I'm learning so much!
 
If you have a 25" K7000, then you should have +130vdc as a B+. If you have that, then you're in business. If your monitor still doesn't work, check the voltages going into the neckboard at P202 and see what's missing (if anything). And set your meter to AC and measure across the neck socket pins labeled H and see if you get about 5-6vac. If they all look good, turn up the brightness on the flyback and the chassis and see if you get anything...
 
I'd suspect a bad solder joint if they have not been gone over real well.
(look over the entire chassis)

Heater Glow -
R213 is known to come loose on the neckboard.
Also on the main chassis in the triangular area where R97, R101 and R89 meet.

Check All LARGE resistors which tend to get hot.
 
reflowed almost all the neckboard, which included R213. Reflowed questionable solders on the main chassis, none that were an obviously cold joint. When I get home tonight I can actually test the voltages.

A question though... does the neckboard need to be grounded to the chassis ground, or to earth ground, cause it's ground wire terminates on the monitor frame as well. Should I install another ground wire to connect to the chassis ground?
 
A question though... does the neckboard need to be grounded to the chassis ground, or to earth ground, cause it's ground wire terminates on the monitor frame as well. Should I install another ground wire to connect to the chassis ground?

No. The one that goes to the monitor frame is the dag wire and MUST be connected on a K7000 or you'll get big time problems.

The chassis ground is connected to the neckboard via the two blue/white cables...
 
Man... I was pretty hopeful tonight, but to no avail. The neckboard measured around 18vac between the H solder points, and P202 measured -81vdc.

I had to measure P202 from the earth ground, because I couldn't get any reading at all when attempting from a chassis ground (heat sink). That normal?

I'll go ahead and check resistance at all the large resistors to make sure none are open.

Thanks again for all ya'll's help. As frustrating as this monitor is, it'd be impossible without ya'll!
 
Try measuring from these points. Mind the differences between AC and DC voltages.

AC voltage:
Heater voltage directly at neck pins -
Heater voltage at circuit ground and P202 -
Heater voltage at earth ground and P202 -
Heater voltage between each ground and each end of R213 -

Might as well measure your DC voltages too while you're there.

Remove neckboard and check continuity between the two heater pins on the tube itself.
 

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Removed neckboard and there is no continuity between the two pins labeled H.

75.8vac across the two heater pins

Chassis ground and P202 measured 2vac for the most part, yet sometimes bounced up to 8 or 10vac and fell back down. (admittedly, this coulda just been my coffee-less shakey hands, though I managed consistency between the other measurements...)

Earth ground and P202 measured 76vac.

Chassis ground and top of R213: 2vac
Earth ground and top of R213: 11.8
Chassis ground and bottom of R213: 8vac
Earth ground and bottom of R213: 8.6

2vac is the "heater voltage before R213" measure taken from the two circled pins in the image.

DC voltages measured 12.7 and 130 on the two pins illustrated in the image.

Whattya think?
 
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crap, i checked continuity on the neckboard solder points. I'll check it on the tube.

Did those voltages look good?
 
crap, i checked continuity on the neckboard solder points. I'll check it on the tube.

Phew... let us know....otherwise I was fixin' to tell you you were SOL.

Your DC voltages look fine....but your AC voltages are all over the place. Some are close, and some are waaaay off and should be the same as other points.

75.8vac across the two heater pins
That's not even remotely close. Should be @ 6.3vAC, but may vary a bit when using a multimeter, but should be "close".

Chassis ground and top of R213: 2vac
Earth ground and top of R213: 11.8
Chassis ground and bottom of R213: 8vac
Earth ground and bottom of R213: 8.6
These look much better.

Pull one leg of R213 and measure it's resistance. (Ohms / Omega looking symbol)
Should measure about .68 Ohms

Was the flyback changed ?
All solder points on it look good ?
 
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