WG K7000 repair help needed: Neck glow but no one home

[TheBasement]

I tested the voltages at IC1, IC2 and IC3. Most seemed with tolerance. Other than being a few decimal points off, the only ones that seemed slightly off are the following.

IC2 Pin 13 11v manual says 12.0v
Pin 14 12.3v manual says 13.7v

IC3 Pin2 8.4v manual says 12.2v
Pin3 22v manual says 23.6v
Pin6 21.6v manual says 24.0v

For all I know, these might be within tolerable range. I am just posting just in case.

It looks like Pin2 of IC3 is the Vertical Deflection output. I think that output jumpers across the board and then goes across to the yoke windings. With a 4V drop, would that be part of the problem? Instead of just replacing IC3, is there something else I should look for in that circuit? I also measure the vertical yoke winding with it unplugged and its 10.8 ohms, within specs. Wouldn't though I get some sort of horizontal flat line, even if the vertical wasn't working properly?

Any input someone could give would be great.

 

[cbuschief]

k7000

Do you have 12 volts ....also can you check R89 and R91...if these are good I'd question Q10...pre drive to the horizontal output.

 

[TheBasement]

Do you have 12 volts ....also can you check R89 and R91...if these are good I'd question Q10...pre drive to the horizontal output.

R89 Measures out good to 3.9K and R91 measures out good to 1.2. I took a reading on Q10 and the collector voltage measures +38V. The manual says +40.6V so that seems within tolerance.

The +12V measurement that gets rectified through D14 measures to +10.8V. There are new caps through that section also. I double checked and reflowed solder on those caps. It looks like the datasheet on the LA7823 says min. VCC supply voltage is 11.8V and max voltage is 13.2V.

Also note that as mentioned previously, the 24V supply to pin6 of IC3 was reading lower at 21.6V. Since this is an effective doubling of 10.8V, then that makes sense. So if the vertical and/or horizontal IC's aren't firing properly over a low +12V and/or low +24v, then what should I look at next? Aren't these voltages derived from the windings in the flyback? If so, then my B+ wasn't reading 123V, but a lower 112V as mentioned in the beginning. I suppose that would explain a lower +12 and +24. However, I have replaced IC4 the B+ regulator twice now so I'm not sure what the next step would be on what is causing that B+ to be lower. Seems to me if I get that B+ at or near 123V everything else might start firing properly with a more proper +12v and +24v. Someone can correct me if I am wrong.

 

[cbuschief]

When you did the cap kit ....did you replace cap 204 on the neck board as well? Can you check your 130v on pin3 of J202?

 

[TheBasement]

When you did the cap kit ....did you replace cap 204 on the neck board as well? Can you check your 130v on pin3 of J202?

I did replace C204 with the electolytic instead of the old ceramic. I did make sure the negative was soldered to ground even though it isn't marked.

It is a 19inch monitor, so the B+ should be 123v. My B+ has been reading 112V at the large white resistor. At pin3 of J202 it is reading the same of 112V.

Additional: I also replaced the critical safety cap with a 6000pF capacitor and did the jumper wire solution from Bob Roberts when removing the original cap.

Looking at the schematics it appears that R301, the big white resistor across the B+ regulator is 220 ohms, and may have a 1.8 kohm resistor in parallel for a K7900 series (what I have) which would lower the rating to 200 ohms. I looked at mine, and it was rated at 180 ohms, nothing in parallel that I could tell. Thinking that was the culprit I swapped in a 220 ohm resistor. I turn the game on, measure B+ and still get exactly 112v at B+. At the 12V power supply line, I still get 10.8V. I just know if I could figure out why these voltages are slightly lower it would cause IC2 and IC3 to turn on with a proper VCC.

 

[cbuschief]

Please test R102...47ohms at 1 watt, R103....2.7ohms at 7 watts, and R104....15ohms at 5 watts. When resistors fail, they always increase in resistance, thus reducing your B+ voltage. Good luck

 

[TheBasement]

Please test R102...47ohms at 1 watt, R103....2.7ohms at 7 watts, and R104....15ohms at 5 watts. When resistors fail, they always increase in resistance, thus reducing your B+ voltage. Good luck

I test R102, R103, and R104. R102 and R103 tested good. R104 was completely open. Looking at the schematic it looks like part of the degauss circuit, and runs through a thermistor across the AC lines. I pull R104 and R105 both leaving that circuit open. B+ still measures 112V. Does that circuit need to complete through R104 for proper B+ operation? R103 is at the other end of the AC/DC supply circuit and still gets 165v.

 

[cbuschief]

If you have 165 vdc...then thats what I would expect to find at R103. If you unplug the neck board does your B+ go up? If not...I would check the caps and resistors around the T2 transformer area.

 

[TheBasement]

If you have 165 vdc...then thats what I would expect to find at R103. If you unplug the neck board does your B+ go up? If not...I would check the caps and resistors around the T2 transformer area.

I unplugged the neckboard. B+ did not go up, it stayed at 112V. I pull out every resister around T2, hot and IC4 and ohm'd them out. At least 20 of them, large and small. They all measured correctly. I do not have a way to test capacitance, but all electrolytics have been replaced. I did test for shorted C32, C34, C35 and C36 around T2 area, none were shorted.

There is a .47ohm mod (factory i think) going from Hot base to pin 4 of T2 on the underneath. The wire jumper has been clipped. This also ohm'd out to .47ohm. The photo from modessit of common k7000 issues shows a similar looking resistor in the same spot on the underneath of the board.

I'm really all for a do it yourself on repairs. However, this has me stumped. I'm really up for sending this to someone with more expertise to pay and repair if possible. I'd prefer not to have to pull the yoke and purity rings, but just send the chassis and neckboard. It is a med. res. k7915.

If someone else has more things I could try I will certainly keep going. I just don't know what else I can do with a volt and ohm meter.

 

[modessitt]

Is C36 a 6100pf? If not, try that.

I assume you have 120ac input? If it was low, it may result in a low B+.

Sometimes the B+ adjustment pot gets out of whack from the glue.

Have you replaced IC2?

 

[TheBasement]

Is C36 a 6100pf? If not, try that.

I assume you have 120ac input? If it was low, it may result in a low B+.

Sometimes the B+ adjustment pot gets out of whack from the glue.

Have you replaced IC2?

I replaced C36 with 6100pf. I also tested the AC coming in which was spot on. I didn't think this monitor had a B+ adjustment pot, but an HV shutdown pot. I did try twisting that back and forth with no change. I also pulled a leg from D10 up to prevent any shutdown with no change.

I have not replaced IC2. Will IC2 give a low B+? I'm really trying to understand how that's generated. At this point I've received 3 different parts orders from Bob Roberts trying to fix this thing and would hate to just keep throwing money parts at it, but try to understand what's failing here.

I guess I will try one more parts order with Bob Roberts to replace IC1, IC2 and IC3. If that doesn't work I guess its going to Arcade Cup. I know he does quality work, I would just have preferred to fix this myself.

 

[TheBasement]

Received my 4th parts order on this and installed today.

Here is what I put in new this LAST time:
C36 (retrace) again, C38, C51, R104
IC1, IC2 and IC3

Guess what happens when I power it up? Nothing. Nothing but that 112v B+. This is a real shame. I guess I am done with this monitor and it goes to ArcadeCup for repair. I am glad for more experienced people with better tools than myself. I will gladly pay for this to be fixed at this point. I guess its just a shame with all the time, parts and troubleshooting into this that it still has to go out for repair. So that's the end of this monitor. No go.