k-7000 low B+

paul400

Well-known member
Joined
Nov 8, 2010
Messages
2,416
Reaction score
219
Location
Hixson, Tennessee
working on this dead k-7000. replaced caps, fly HOT and VR and got it working. also put in a R96. B+ can only reach 110vdc and as the picture changes the B+ really flucuates, if I turn the screen up B+ will drop down to 80 vdc. this is causing an un stable/jittery picture. anyone ever see this happen before?
 
Bad solder joints in the power supply section, particularly around the larger resistors...

Ceramic resistors going out of spec...

Bad flyback (out of the box)...
 
this is a tough one, reflowed most of the board and checked out all the larger and meduim size resistors tried the old flyback with same results and swapped out the 220 resistor that hangs off the edge.
 
Was the voltage regulator the same model as what you pulled? The K7000 can be strapped for two different VR designs. If you install the wrong one it will cause problems.

Also, have you test the bridge? The four diodes on the input side of the VR, if one opens it will directly affect the input power to the VR.

What is the voltage reading on both sides of the large ceramic resistor when powered on?
 
Was the voltage regulator the same model as what you pulled? The K7000 can be strapped for two different VR designs. If you install the wrong one it will cause problems.

Also, have you test the bridge? The four diodes on the input side of the VR, if one opens it will directly affect the input power to the VR.

What is the voltage reading on both sides of the large ceramic resistor when powered on?

Used the correct str3123 for a 19 incher

I tested the bridge in circuit, should I pull each diode to test?

145vdc and 110 vdc on the side closer , should be 123vdc
 
Used the correct str3123 for a 19 incher

I tested the bridge in circuit, should I pull each diode to test?

145vdc and 110 vdc on the side closer , should be 123vdc


The 145 sounds low. It should be closer to 160v. The 19" chassis can accept either a STR3123 or a STR30123. The schematics will detail the different jumpers necessary for each VR.

Diodes should always be pulled, at least one leg, in order to test them.

You can also try disconnecting the degaussing coil in order to remove it from the equation.

Pull the fuse and test it for resistance. It should read as a short but I have seen the fuses partially blow creating high resistance. While you have it removed make sure the two fuse holders are snug. If they do not have a strong grip on the fuse they can create a high resistance area as well.

Resistors R101 and R103 - make sure they are not loose. with the board upside down wiggle each resistor while watching the solder points. Any movement needs to be fixed. They can look like the solder is still good when it has actually broken loose from the solder pad.
 
The 145 sounds low. It should be closer to 160v. The 19" chassis can accept either a STR3123 or a STR30123. The schematics will detail the different jumpers necessary for each VR.

Diodes should always be pulled, at least one leg, in order to test them.

You can also try disconnecting the degaussing coil in order to remove it from the equation.

Pull the fuse and test it for resistance. It should read as a short but I have seen the fuses partially blow creating high resistance. While you have it removed make sure the two fuse holders are snug. If they do not have a strong grip on the fuse they can create a high resistance area as well.

Resistors R101 and R103 - make sure they are not loose. with the board upside down wiggle each resistor while watching the solder points. Any movement needs to be fixed. They can look like the solder is still good when it has actually broken loose from the solder pad.

R101 and R103 are good and reflowed, replaced fuse but old one tested fine, holder was snug, pulled the four diodes and all tested good "open one side reading 572 other side" havent has the dequasser connected since i have been trying to get this one working right. also replaced the HV pot.
 
well this was a dismal failure. swapped neck boards from a working chassis and R30 flamed out, WTF. swapped the neck back to the woking chassis and works fine. this chassis is going in the fubar box!!!!
 

Attachments

  • 100_1877.jpg
    100_1877.jpg
    102.3 KB · Views: 37
Necro-bumping this thread because I had one just come across the bench with the exact same issue on a 25". B+ was stuck around 113vdc-116vdc. If you increase the contrast/brightness, it would dip down to around 100vdc and start going crazy. Did all the proper testing, replaced the regulator, etc. Nothing. All the rectifier diodes checked good from the top of the board but it wasn't until I carefully inspected their solder joints that I discovered the anode leg of D22 was actually out of circuit. Properly soldered it back in place and the B+ went straight to 130vdc and the chassis behaved correctly. So if anyone has this issue in the future, inspect and verify ALL the rectifier diodes are actually in circuit. If one of them (D22 in this case) is out of circuit, the B+ will behave like this.
 

Attachments

  • IMG_8300.JPG
    IMG_8300.JPG
    296.3 KB · Views: 10
Back
Top Bottom