CRT Newbie - Where did all the RED go?

[asantora]

First off, thanks in advance for looking at this. It's been a while since I've been on this board, but after 10+ years of being away from the hobby, my kids and the kid in me has brought me back.

Picked up a Rival Schools cab (really mk 2) with a 25 inch monitor. CRTs were never an area that I explored before, but after taking the machine home as-is, I was crushed to find that the colors were off. For the most part all screens have a strong blue tint to them. So, I tried a little troubleshooting and now I'm hoping past karma from this board will repay. Things that I tried:

1. First thing I did was adjust the pots on both the chassis as well as the neckboard, no luck.

2. Disconnected the jamma harness and reconnected, nothing

3. Removed the neckboard and reattached, nothing (note, the light isn't very bright in the neck)

4. Broke out the multimeter and pretended I knew what I was doing. Moved to continuity check and touched pins in jamma and connection on chassis and heard a beep for all colors.

5. Broke down, took some pics and looking for help. The test screen shows that red is drastically missing.. note, there's a little something there that doesn't show in the pics.

Based on my research so far, I have a sharp image chassis? Which uses the k7000 cap kit. Not sure what the tube is, but pics are attached. Thanks in advance for any help, looking forward to learning new things.


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[asantora]

Forgot to mention, also plugged in another jamma board that I had laying around. Same issue.


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[pbj]

Bad tube

[asantora]

Can it be rejuvenated?


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[baritonomarchetto]

Do you see those three identical transistors on the neck board? It's very likely that the one controlling red went bad (or something around there). You should check all the solders on the neck board, first. Then check those transistors; then all components on the red line.
Bad tube? Very unlikely.

[asantora]

Baritonomarchetto, thank you very much for the feedback and direction. Will look into this today.


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[asantora]

Okay, checked the neckboard for cold solder joints or anything unusual. All looked clean. Then located transistor q301 which is the red, all looked with solder. Took multimeter to q301 and got reading approximately 1530, switched probe and got 1030. Then went to other two transistors and got 1530 and 1130. Wait, shouldn't I get the same reading? Not really sure what this unit of measurement is, but could it mean that the transistor is bad for red q301? Please educate me.


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[pbj]

You can ground out the transistors one by one and it should flood the tube with that color.  Or move one of the other ones to the red transistor spot.

From looking at your color test picture, I can faintly see red on it which indicates to me that gun is shot.

[baritonomarchetto]

Transistors must be check out of circuit (or with one of the methods pbj suggested, even if i would avoid tests with the monitor switched on if you are not confident with it). If the red one is ok, check all resistors on the red line.
I am still prone to exclude a bad gun (I do not see any red in the pic, but it could be me...)

[Mike A]

There is a bit of red in there for sure. If the pictures are to be trusted.

[big10p]

Listen to what baritonomarchetto is saying here. Unlikely to be a bad tube. I'd replace the red transistor just to rule it out, if nothing else. I'd also test/replace the red cutoff and gain pots on the neck board.

[asantora]

You guys are awesome.  A few comments:

1. There is a tiny hint of red in the test screen.

2. Transistors - Okay, now I understand about testing in circuit. Thanks for teaching me that.

As for grounding out the transistor to test - how does one do that? Is that with the monitor on? If so, I'll go the discharge the anode route and swap spots with another transistor.


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[lilshawn]

also, your blue and green drive is unbalanced. the gradients should be matched. I would turn the green down a bit.



swap the green and red transistors. if you have red now, it's a bad transistor.

if still no red, (swap back transistors) and swap the red and green video input wires. if "green" lights up in the red area, you are getting red signal but no red output from the tube. tube gun is bad. you can try rejuvenation.

[pbj]

You touch chassis ground to the metal heat sink tab of the transistor.  Doing that to each individual transistor should flood the tube with that color.

These 'unlikely to be a bad tube' people obviously haven't dealt with many 25" monitors. 

[asantora]

Alright, so I connected a new wire to the chassis ground and touched to blue - flood, green - flood, red - very dim flood. Not sure where to go from here. Thanks again for everyone's help.


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[asantora]

Shawn, thanks for the note on balance. As for input wires, video input into the chassis? Neckboard from chassis?


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[lilshawn]

since you have a labeled and color coded wires on the neck board (R-in G-in etc.) I'd just do it there.

[lilshawn]

as a side note. i'm not 100% sure which chassis that is. you should have a label on the big main chassis someplace that has a model number (like Sharp image or SI-###) if you could post that it would be helpful.

also consider there may be a video amplifier IC (LM1203 or the like) on the main chassis (or a set of discrete transistors) that has a bad red channel on it.

[pbj]

Bad tube.  Shorting the transistors eliminates virtually all involved chassis components.

You MIGHT get lucky having someone rejuvenate that gun, but it's never going to be as strong as your blue and green.  You'd have to turn down their levels until they got close to the red, and then crank the brightness and contrast up.

[asantora]

Thanks for all of your help.

Given that I have SOME red tint. Any hopes of a rejuv? Anyone in NJ/ PA watching this have one? Is it worth me buying the rejuvenator?


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[lilshawn]

(sorry, i missed the shorted transistor test results post.)

rejuvenating is really hard on the tube. you basically drive many times more volts into the heater and get them nice and hot, then blast them with high voltage to (hopefully) blow/burn off anything on the gun causing the output to be low.


sometimes you blow out the heater.

sometimes you blow out the gun.

sometimes it works....and if it works, it's a crap shoot how long. could be a minute...a week.. 30 years.

if this monitor is all you got, and all you can get...a rejuvenation might be the only answer. I would call around and see if any place has one you can borrow or will do it for you. it's probably not worth your money to buy one for a one off.

[asantora]

Thank you everyone for all of your help. Bummed, but learned a lot from all of you.  Will be seeking for a new friend with a rejuvenator and seeking a new tube as plan b.


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[baritonomarchetto]

Nice to see you are on the right track

[kardenm]

Good call pbj.  I would have guessed that the red transistor on the neck board was bad.
How did you know right from the start?
Is that a really common occurrence for these monitors?

[lilshawn]

i think it was because there is a tiny trace of red on the screen. when transistors fail, they usually go open (black) or shorted (full red screen)

this was low output.

[pbj]

Yep, it was the fact I could see faint red in the picture.  (Only visible on my iPad, btw.  Couldn't see it on a PC monitor).  Shorting the transistor is one of those tricks nobody wants to tell you about because you're encroaching on their ability to make money in the 80s as a TV repairman as soon as they perfect that time machine.



Rejuvenator will improve things a little.

 

[grantspain]

short red gun to ground to prove tube is ok,very quick short to ground should produce incredibly red screen with flyback lines

[asantora]

So, haven't found a 25 inch monitor yet, so I decided to hook up an lcd monitor to the cabinet using a gonbes converter. Oddly enough, I found that some colors are missing... not red, but some colors. Any correlation? Could there be something else wrong with my cab? Pics included.



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[pbj]

Looks like a settings issue with your converter.....

[asantora]

Alright, thanks, figured I'd give it a try


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[lilshawn]

depending on the converter used, there is sometimes an adjustment for the color clamp cutoff. maybe it's maladjusted.

I know the CGA to VGA convertor boards (GBS-8220) have an adjustment that needs to be tweaked to have the pic display properly on some machines. (Clamp and ClampSP)

your convertor may have a similar adjustment.

[asantora]

Thanks Shawn. Adjusted the clamps to 3 and 4 as well as 55 and 56 + some other combos... no dice. Frustrating hobby


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[asantora]

Messed around with the gonbes video converter a bit and just no luck. So I bought a new power supply, jamma harness, rgb to ntsc converter and made a pseudo supergun for testing and things are looking great.

Now... the next challenge. I would like to decase the tube tv that I acquired and install it in the arcade cabinet. In order to do so, my understanding is that I will cut the cord on the tv and wire it to the cords coming out of the isolation transformer. This will prevent the mounting ears or other components from being HOT. (By the way, I have yet to figure out where to mount the tv's pcb / chassis, any ideas welcomed). Did I get this right?

Assuming so, I have 3 wires coming out of the isolation transformer that were originally hooked up to the arcade monitor. 2 purple 1 green. I suppose that each purple goes to either (doesn't matter) of the two power cord spliced wires from the tv. But as for the green, which I assume is ground, it was never connected to the arcade monitor. Do I leave it as-is, not connected? Or should this be attached somewhere, like a metal frame for a chassis? A pic of the wires from the isolation transformer attached. Thanks for all of the help.


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[lilshawn]

yes, the green ground should be attached to the metal frame.

whether or not you use the isolation transformer on your TV is up to you. the only reason some monitors needed it is because of the way the older style monitors used a voltage regulator to produce the B+. it needed to not be referenced to neutral becasue neutral is tied to ground.

modern monitors and TV's now use a switch mode power supply that don't need to be isolated.

[asantora]

Thanks Shawn, I appreciate all of your help. Hopefully I will be able to return the favor soon. 

One more question as I'm seeing different approaches on various websites. Should I discharge my current arcade monitor and/or the tv before mounting? I'm thinking it's a safety precaution and may not be necessary.


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[pbj]

There's no real need to discharge them.

[asantora]

Thanks all, you guys are awesome


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[behrmr]

Thanks Shawn, I appreciate all of your help. Hopefully I will be able to return the favor soon. 

One more question as I'm seeing different approaches on various websites. Should I discharge my current arcade monitor and/or the tv before mounting? I'm thinking it's a safety precaution and may not be necessary.


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If you are going to disassemble the monitor or separate the tube from its chassis you can discharge the tube as a precaution.

[asantora]

Time to make a discharging tool


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[jtb1987]

Messed around with the gonbes video converter a bit and just no luck. So I bought a new power supply, jamma harness, rgb to ntsc converter and made a pseudo supergun for testing and things are looking great.

Now... the next challenge. I would like to decase the tube tv that I acquired and install it in the arcade cabinet. In order to do so, my understanding is that I will cut the cord on the tv and wire it to the cords coming out of the isolation transformer. This will prevent the mounting ears or other components from being HOT. (By the way, I have yet to figure out where to mount the tv's pcb / chassis, any ideas welcomed). Did I get this right?

Assuming so, I have 3 wires coming out of the isolation transformer that were originally hooked up to the arcade monitor. 2 purple 1 green. I suppose that each purple goes to either (doesn't matter) of the two power cord spliced wires from the tv. But as for the green, which I assume is ground, it was never connected to the arcade monitor. Do I leave it as-is, not connected? Or should this be attached somewhere, like a metal frame for a chassis? A pic of the wires from the isolation transformer attached. Thanks for all of the help.


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asantora, can you clarify specifically what was able to solve your problem? I think I have the same problem as you discuss in your original post but have no idea where to start. See attached