Newbie needs advice on a 25K7000 dead chassis

Teknotoyz

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Well, got a good deal on a cabinet with a dead monitor.
Monitor is a 25" K7000 with a blown fuse.
So I pulled a newbie screw up and put a 3 amp fuse in (2 amp normal) to see if there was any life... Magic smoke was released from the area of the rectifier diodes. :(
After removing the neck board, the plastic key is missing from the tube, but pins are straight and all there so I assume the board was installed correctly, stole one off another tube for now.

OK, so NOW I looked online and found the monitor flow chart, Bob Roberts info, and Aaron's sticky on the K7000 and learned a bit of knowledge before making more issues for myself.

As it turns out, I also have a 19" K7000 I will need to rebuild, also dead.
Time to become schooled.


I have ordered replacement parts from Bob Roberts:
-1N4007 diodes for the rectifier section.
-2.7ohm 7 watt resistor
-Voltage regulator
-Flyback (only ordered 1)
-Cap kit
-HOT (1398) IC

Details (both are K7000, I'll call them by the tube size):
No fuse on the 19 and blown fuse on the 25
The HOT measures bad on both chassis's (low resistance from chassis to center pin).
The 19 has the flyback with the white knobs Aaron seems to love.
Both the 2.7ohm 7 watt resistors are measuring around 3.4ohms

So what do I do first?
I know I have to replace the HOT on both the 19 & 25.
If the flybacks appear physically OK, do I try them?
Aaron recomends replacing the white knob flyback regardless.
How do you know the voltage regulator is good or bad?
Is there any way to test the rectifier diodes IN the circuit?
I measured continuity, but get a 211kohm reading on both ways across all the diodes on 19&25.

There is no visible magic smoke source, so I'm pretty worried about the diodes.

I'd like to start with the known bad items, like the cap kits and HOT's. But I don't want to have another bad component kill the new ones.

Can anyone give me some advice on what to do?
I've never had to replace a flyback before, sounds like HOT's get taken out often with them.
 
Hmmm, I wrote up a short reply yesterday. Where did it go?



Anyway, if you suspect the bridge then you need to check the diodes. Using the diode test on you multimeter test each doide both directions. One direction should read somewhere between 400 and 600 while the other direction should read around 1200.

In my opinion all white knob flybacks should be replaced. They are prone to failure and replacing them while the chassis is out is cheap insurance.


Also, I rarely see the ICs fail without something driving them to fail. If you have a shorted HOT or VR then you need to identify why. The most common things I have been seeing lately are shorted C36 or C38 caps. These are not included in cap kits. Test them for continuity and if you have a reading close to zero then they are shorted and need to be replaced.

Other than those things make sure to check for cold solder joints as detailed in the K7000 sticky. Go ahead and desolder the areas shown, clean them if necessary, and then resolder.
 
Also, I rarely see the ICs fail without something driving them to fail. If you have a shorted HOT or VR then you need to identify why. The most common things I have been seeing lately are shorted C36 or C38 caps. These are not included in cap kits. Test them for continuity and if you have a reading close to zero then they are shorted and need to be replaced.

I finally got around to working on the chassis.
I did the shotgun approach, not happy about the rectifier diodes as they all turned out good and are 5A vs the 1N4007's 1amp rating.
Would anyone reinstall the old, or go ahead with the new lower current replacements? This is a 25 inch K7000 (2A main fuse)

I replaced the flyback, VR, HOT, and installed a cap kit.
Trying to get everything while the chassis is out.

Not sure what in the heck I was thinking, newb move as you gave me the answer.
Plugged it in, fuse popped and the new HOT is gone.
NOW, I read closer and follow your advice on C38, shorted.... dammit, answer was right in front of me!

I wanted to thank you for the help, have to get a new C38 cap before I can proceed again.
(sigh)
 
Well, I guess it's not dead anymore.... Surf planet seems to be a real dog of a game...
Played a very red game, left-right-button, not too impressionable.

Well, back to the issue.
Monitor now fires up to a red tinted screen, no pot adjustments on the neckboard help.
So at least the fuse blowing has ceased, HOT was bad and cap C38 was shorted.
Made it to the last step in the flowchart.

Anyone know what the "Substitute 1C1" means?
Any pointers on the next step will help, do I replace all 6 transistors?
(or just red, or just blue/green?)

Thanks again!!!

Screenshot2011-07-31at101228PM.png


IMG_2979.jpg
 
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Substitute IC1 means replace it, preferrably with a known good IC from another chassis.

Have you tested the tube yet? One color overwhelming the others is often a tube problem, and can usually be fixed with a rejuvenator.

Other things I have seen cause a color to appear shorted are the wrong parts (transistors specificly) installed previously and a shorted component in the color circuitry. Luckily, when troubleshooting color problems there are two almost identical circuits to compare your results. Swapping parts from one circuit to another is a good way to see if the problem follows, just make sure and swap one component at a time...
 
not happy about the rectifier diodes as they all turned out good and are 5A vs the 1N4007's 1amp rating.

Keep the new 1N4005's in it. Maybe someone with more knowledge can chime in but I always consider these to be kind of like fuses in that you don't want to have one with a larger value where a lower valued one is needed.
 
Don't suppose you've got another 25" K7xxx chassis you can borrow a neckboard from? That will rule out the neckboard as part of the problem.

To rule out the chassis as the problem, swap the neckboard from your 25" chassis to your 19" chassis and plug it into your monitor. Sure your image will be tiny on your 25" tube, but if the red problem goes away, then you know the problem isn't the neckboard, it's the 25" chassis.

To narrow even further, place your 25" chassis on the 25" tube with another 25" neckboard. If the issue goes away, then it's your 25" neckboard.

If the issue persists through both experiments, then it's likely a short in your tube's red gun. Get ahold of a rejuvenator and see if you can recalibrate the CRT guns.
 
If you have another working chassis of this type, swap the neckboards to verify the issue is on the neckboard. Then swap the red drive transistor with the blue one to see if the image changes to all blue. If so, replace it. If not, then report back.

BTW - if you unplug the 5-pin blue/white wire connector near the flyback that connects to the neckboard, does the red go away?
 
Well, I guess it's not dead anymore.... Surf planet seems to be a real dog of a game...
Played a very red game, left-right-button, not too impressionable.

Well, back to the issue.
Monitor now fires up to a red tinted screen, no pot adjustments on the neckboard help.
So at least the fuse blowing has ceased, HOT was bad and cap C38 was shorted.
Made it to the last step in the flowchart.

Anyone know what the "Substitute 1C1" means?
Any pointers on the next step will help, do I replace all 6 transistors?
(or just red, or just blue/green?)

Thanks again!!!

Screenshot2011-07-31at101228PM.png


IMG_2979.jpg

No retrace lines on that pic that I can see....I don't think it's tube related. My best guess is the neckboard.
 
If you have another working chassis of this type, swap the neckboards to verify the issue is on the neckboard. Then swap the red drive transistor with the blue one to see if the image changes to all blue. If so, replace it. If not, then report back.

Many thanks to Modessitt for his help with my monitor, but I took his advice above, combined it with laziness, a desire to learn, and came up with a different approach. I desoldered one leg of all the color drive resistors at the neckboard. I then put up a colorbar pattern and took a jumper with clips and swapped out the signals red for blue, red for green, and so on. By making a chart of which signal/gun pairs worked properly, and which one's did not, I could tell exactly where the problem was.

Once I was done, I knew my red gun was weak and my green drive transistor was failing. I replaced all three Q's anyway (they were cheap) making the exercise pointless, but I learned a lot in the process.

Have you tested the tube yet? One color overwhelming the others is often a tube problem, and can usually be fixed with a rejuvenator.

I read somewhere that color that extended outside the raster field is typically an H-K short, and the screen shot certainly seems to be edge-edge. Pull the neckboard and check for an H-K short.
 
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I read somewhere that color that extended outside the raster field is typically an H-K short, and the screen shot certainly seems to be edge-edge. Pull the neckboard and check for an H-K short.

Actually, a chassis short can cause that too, as it overdrives the gun...
 
If you have another working chassis of this type, swap the neckboards to verify the issue is on the neckboard. Then swap the red drive transistor with the blue one to see if the image changes to all blue. If so, replace it. If not, then report back.

BTW - if you unplug the 5-pin blue/white wire connector near the flyback that connects to the neckboard, does the red go away?

Lots of good advice from everyone!
The red does go away with the blue/white cable unplugged. It was a black screen with occasional flashes (still had red tint in the flashes).

I swapped 201 and 202 on the neckboard with no change.

I do have another 25" K7000 in my Neo Geo, due for a cap kit anyway, so I'll pull it and try to rule out the tube.
 
Well, the flashes may indicate a need for rejuvenation, but if the problem was the tube, the red would have stayed when the blue/white connector was unplugged.

Your next step is to check Q1 on the chassis (2N3904's are cheap, so you may just want to replace them). You can always swap it with Q2 if you want to verify it's good or bad (screen changes to all green). After that, replace IC1...
 
You mean you can still BUY semiconductors at Rat Shack?



They still have a small selection of generics. I was talking to our local manager about how little remained, and and he said corporate was bringing component parts back to the main product line. During our chat I lamented the loss of learning opportunities for kids that we had in the build it yourself electronic project kits, and he went into the back and pulled out a whole case of assorted kits. He said they were coming back also and he hadn't even had time to make space for these kits.

I bought a 3W mono audio amp kit for $19.95. Too expensive for what it was, but I wanted to show support for a return to their roots.
 
I bought a 3W mono audio amp kit for $19.95. Too expensive for what it was, but I wanted to show support for a return to their roots.

Might work as an audio amp for bench testing boards that don't have on-board amps - i.e. Centipede, Crystal Castles, Mr. Do...
 
Tube is now confirmed good, I pulled the other monitor and swapped chassis's.
The picture came up normal with it.
OT, my other chassis (dated 1990) has the white knobs on the Flyback.... (see pic)
Never had an issue with it, plan on installing a cap kit while I have it out due to slight curl at the top.
Would anyone change the flyback as I hear the white knob versions are a bad deal.
Only part is, I really don't have a spare fly right now.

Back to the original chassis, I replaced Q1 with no change, but need to replace Q2 & Q3 as I have a sneaky idea that red is actually the only color that IS working (only had one 3904 on hand). I disconnected both green and blue inputs and no change to the picture. I ended up ordering (12) 3904's from Bob Roberts at 25 cents each vs RS's $1.19 each, maybe should have supported them, but I needed some additional cap kits too (extra 3904's are for a WG4600 chassis).

If it comes down to replacing IC1, does anyone socket the PCB?
 

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