K4600 Missing Blue

Fynflood

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I have a K4600 here, missing blue. I reflowed ALL connectors, adding fresh solder, removing and re-soldering to make sure it's all fresh, and hit any other questionable spots - though I didn't see and obvious bad joints. I replaced 401 on the neck board (The original tested ok), no luck there. I also replaced TR204-TR209 on the interface board. Still no luck, but I am able to see the squares from the TPG now.

I've got a little sync and fold over things to address as well, but that feels like an easier fix - I'm just trying to sort the Blue atm.

Is there a way to swap green or red with blue so I can see if the tube is capable of blue? It's my understanding that shuffling the yoke wires around isn't a good idea.

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have you tried turning up the Blue Cutoff to see if it's an issue with the color drive? if you turn it up and the whole screen turns blue, you can rule out the color drive circuitry within the neckboard. if you're losing blue in the signal processing you'll need to test the video signal card parts. in which case you would trade out the blue transistors for red or green, see if it stays or carries.

I've never had problems like this on K4600s but have a fair amount on K4900s.
 
Shorting the guns is easy enough. K on the neck stands for cathode. So you should see RK, GK, BK. You momentarily short out the cathode of the color in question with ground. Screen will fire that color.
 
have you tried turning up the Blue Cutoff to see if it's an issue with the color drive? if you turn it up and the whole screen turns blue, you can rule out the color drive circuitry within the neckboard. if you're losing blue in the signal processing you'll need to test the video signal card parts. in which case you would trade out the blue transistors for red or green, see if it stays or carries.

I've never had problems like this on K4600s but have a fair amount on K4900s.
Shorting the guns is easy enough. K on the neck stands for cathode. So you should see RK, GK, BK. You momentarily short out the cathode of the color in question with ground. Screen will fire that color.
This video from @ArcadeJason might help. He shows how to temporarily short the neck transistors for testing.


Hey that's a pretty good trick! Thank you both for that. So, I'm seeing blue when I short it. That's good - I was worried about the tube.

@mecha Blue cut off and drive basically don't do anything - other than shorting to see if blue even exists, I can't make it show me blue at all. I'm pretty confident the video card transistors are good since I just replaced them all (I have 3904s and 3906s on hand). Maybe time to check for broken traces. And maybe I'll clean and reflow again... seems the 3rd time was a charm on the g05 :D
 
Hey that's a pretty good trick! Thank you both for that. So, I'm seeing blue when I short it. That's good - I was worried about the tube.

@mecha Blue cut off and drive basically don't do anything - other than shorting to see if blue even exists, I can't make it show me blue at all. I'm pretty confident the video card transistors are good since I just replaced them all (I have 3904s and 3906s on hand). Maybe time to check for broken traces. And maybe I'll clean and reflow again... seems the 3rd time was a charm on the g05 :D
if you shorted the blue drive transistor and didn't get blue then you may have a broken trace on the neckboard. look it over closely for cracks. or check the adjustment pots, one may be cracked. you can swap those around with red.

do you know if the blue gun on the tube is good?
 
if you shorted the blue drive transistor and didn't get blue then you may have a broken trace on the neckboard. look it over closely for cracks. or check the adjustment pots, one may be cracked. you can swap those around with red.

do you know if the blue gun on the tube is good?
I got blue when shorting, yes. I'll report back after broken trace hunting!
 
Thank you all! This is super helpful! I have a missing red in my K4600 cocktail pac-man and have been following your conversation with utmost interest.
I tried grounding each transistor on the neck board: saw a nice blue, and green, but unfortunately no red. So the red gun is out.
I don't mean to disrupt this thread, but just wondering if this is bad or hard to fix, or if anyone has any further advice or other thread to point to for a bad gun.
 
Thank you all! This is super helpful! I have a missing red in my K4600 cocktail pac-man and have been following your conversation with utmost interest.
I tried grounding each transistor on the neck board: saw a nice blue, and green, but unfortunately no red. So the red gun is out.
I don't mean to disrupt this thread, but just wondering if this is bad or hard to fix, or if anyone has any further advice or other thread to point to for a bad gun.
There is still a chance that your red gun is fine. How do you know the red transistor is good?
 
I tested with a multimeter in "diode" mode: B to C .650 and B to E .950 on all 3; and open the other way C to B and E to B. So that seems ok. The 0.950 seems high, but all 3 neck board transistors behave the same, so I assume they are ok.
I guess I can try and replace the red one? I just thought shorting that transistor and seeing no red on screen was indicating a bad gun. But I am willing to grasp at your straws, and hope it is a transistor... I'm just getting less hopeful.
 
I tested with a multimeter in "diode" mode: B to C .650 and B to E .950 on all 3; and open the other way C to B and E to B. So that seems ok. The 0.950 seems high, but all 3 neck board transistors behave the same, so I assume they are ok.
I guess I can try and replace the red one? I just thought shorting that transistor and seeing no red on screen was indicating a bad gun. But I am willing to grasp at your straws, and hope it is a transistor... I'm just getting less hopeful.
I assume you don't own a tube tester/rejuvenator? That's probably the next step.
 
There is still a chance that your red gun is fine. How do you know the red transistor is good?
you alligator clip the tab of the color transistor to ground (the monitor frame). if it goes full blast of that color and looks vibrant then the electron gun is healthy. if you're missing a color then it's getting lost in the processing stage at the signal card or there's a break in a connection at the neckboard.

usually the header pins where the signal comes in. these monitors are 45 years old now, if they've never been worked on before then that will be a problem with the factory solder.

people overlook that in the coin-op era these got passed around a lot between games to keep the top earners making money
 
Interesting. Any other test I should try? I don't have a tube tester / rejuvenator. Is it maybe worth trying to find a compatible tv tube and trying how that may work?
 
Interesting. Any other test I should try? I don't have a tube tester / rejuvenator. Is it maybe worth trying to find a compatible tv tube and trying how that may work?
try what I've suggested first

tube swap for something that's probably a simple fix like killing a fly with a sledgehammer and I wish people would stop clinging to tube swaps as the first approach to fixing monitor issues
 
Well, darn, I thought I found a bad potentiometer, changed it and hoped, but still no red. I'm getting desperate and about to change anything in the red path…
 
Well, darn, I thought I found a bad potentiometer, changed it and hoped, but still no red. I'm getting desperate and about to change anything in the red path…
I haven't been able to track down my missing blue issue yet either (Haven't had the time lately, sadly). Traced out the neckboard, nothing busted there. I've got another interface card somewhere I've been meaning to try.
 
Sort-of-solved... After changing a variable resistor, adding solder on a few spots and all header pins, and double-checking everything I could in the red path (comparing some values to blue & green), I still found nothing wrong yet no red color.
I finally swapped tubes and that did it. So in my case, I think @TheYeti is right on, I need to find someone with a tester/rejuvenator and see if anything can be done for that dead red gun.
Thank you all for your time and great advice!
 
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