Need assist with K4915 chassis

[SuperSprint]

Both. If you look at the schematic, you'll see that TP91 and pin 9 of the flyback are on the same trace.

I see that now. I'm on two xeroxed pages and missed a trace switching from pg1 -> pg2. Thanks, that makes things much clearer. B+ is the output from the AVR section and supplies power to the fly's primary winding. From there is looks like the primary's field is controlled by the HOT shorting C363 (4300pf), subsequently causing the primary field to load up or collapse.

But it's usually recommended to measure it at R503 on the side closest to the fuse, as it's usually easier to get to when the monitor is installed in a cab...

Makes sense. TP91 is tricky even when the monitor is on a bench.

I really do appreciate all the time you and others have spent with me on this. I know I could have just shotgunned it, but I also wanted to understand how it works. Thanks to you guys, I feel more comfortable that I can do some basic troubleshooting on a troubled chassis.

 

[SuperSprint]

Repair update: With a new flyback installed I decided to give her a test run. At first, she displays a playfield that I was happy to see! It was a little out of focus, small, and washed out but otherwise correct. A few degrees on the focus pot, and she looks more in focus. A few degrees on the "Screen" pot, no change. Suddenly, the screen goes bright yellow-green with brighter horizontal lines every 1" or so. I shut her down and check all connections again but find nothing amiss.

My second try is a repeat of the first. Starts out normal, goes green. I let her run awhile, and she gets to a weird red/green/yellow watercolor mix that looks like modern art. Best I can describe it is a rear projection screen that focussed about 1/2 way between the neck and the tube face. I let her continue to run as it has been 10yrs, and all the caps are new. I plan to allow about 10-15 min to normalize. After about 5 min, the screen goes dark for about 60 seconds and comes back with a picture (attached), but it has a definite blue tint.

Checking B+ reveals it's only 100VDC and Fromm's points to the voltage regulator. This is a new regulator, but it's a slightly different part number (STR381 vs STR380) and was installed prior to replacing the fly. Not having another, I decide to try the original STR380. Bad move. She blew the main cabinet fuse immediately on power up. After replacing the main fuse and STR381, she no longer has a picture or heater glow. To make matters worse, while testing various voltages I see a ribbon of smoke and smell R503 overheating. I shut her down, and find the HOT is shorted too. Man I hope it's just the HOT and VR IC. Tomorrow I'll call Zannen again and order another VR and HOT... I'll keep troubleshooting, but if you have any insights please share them

Note: Circular "waves" or "denim pattern" are a camera artifact and not visible on screen. And while it may not be apparent, this appears to be heavy blue, normal red, and little-> no green.

 

[SuperSprint]

While doing some parts cross ref today, I found the following info;

STR380 - ECG1548 - Hybrid TV Voltage Regulator, 123 V, 1 A.

STR381 - ECG1546 - Hybrid TV Voltage Regulator, 130 V, 1 A. TO3 3 pin.

Zannen sub'd a STR381 for a STR380. I assumed (perhaps unwisely) that it was a proper substitution or they wouldn't have sent a 381. Would a B+ of 130VDC (spec for 19" is 123V) been significant enough to cause damage or give the distorted picture? I'm assuming no to the latter as it's my understanding that shrunken picture is caused by low B+.

 

[SuperSprint]

Shameless bump....

 

[modessitt]

Did the STR381 actually give you a B+ of 130v? I know the schematic indicates 123 on that line, but the flowchart shows 130...

EDIT - BTW - was your old STR380 bad? Still have it? might want to throw it in to see what happens...

 

[SuperSprint]

I From what I understand 130V is for a 25" tube, and 123V is correct for a 19". My copy of the manual is specific to a 19" and it calls for a 380 and 123V. I have a Fromm schematic for a 25", and it calls for 130V. Not sure about the part, but I believe it specs a 381.

B+ was 100V on the 381. When I got a shrunken picture and low V on a 381 that had been used with the bad fly, I decided to tried the old STR380,. It blows the cabinet main fuse, killed the new HOT, and turned R503 into a space heater. Now I have no picture or heater glow. I'm guessing the 380 is toast and praying I didn't kill the fly.

When my new VRs come in (ordered 5), I'm going try it with a new VR and HOT together. If that fails, I'm going to cut the jumper that separates the power supply from the rest of the board and put in a 1A fuse like the use in a G07 (f901) to protect the VR from a blown HOT. It will also allow me to separate the B+ and get it working properly before putting it back in service.

Lesson learned. Rebuilds get a new VR, fly, and HOT, needed or not before power up.

 

[SuperSprint]

Would someone mind looking and seeing if these two jumpers exist on their 4900/4915 chassis. It's at the rear edge near the vertical position control.

 

[modessitt]

Just went a looked. I have a K4900 chassis here that has the same jumpers....

 

[SuperSprint]

Thanks. They definitely looked like an afterthought and I thought it might be a hack. You can probably tell I brushed 'em with a wire brush so I could get a better look at 'em.

 

[LyonsArcade]

The blue is going to be something in the blue circuit, of course... it'll probably be a bad transistor (the big ones on the neckboard). If you have a diode check, check the blue one vs. the red one which seems to be working alright. You may also have a bad green one, or a bad green pot. OR you may have a bad blue pot. I don't think this is your tube because there's no retrace lines, and the image stops at the edge of the image the board is sending it. If it was the tube's blue gun shorting on, the entire tube all the way to the edges would be blue.

 

[SuperSprint]

Thanks. The green screen I mentioned was pure green, with brighter (~1-2pixel) lines every 1" or so that extended about 1" past the green square into black space. The lines were perfectly horizontal with no "flyback" diagonal line connecting them. I've attached an drawing if that helps. It appeared on the first 2-3 power ups (before I fried the HOT) after about 30 seconds of normal but shrunken image. That stayed about a minute, then it went totally dark for another minute or so. It filled the whole presented area, but that was shrunken in about 2" from all edges. As I mentioned, I believe the shrunken image was due to 100V @ B+

 

[modessitt]

remove the green drive transistor on the neckboard. if the picture stays green, you have a tube problem. if it goes away, swap the green drive transistor with the blue drive transistor and see if the picture changes to all blue. If so, replace that drive transistor. if not, unplug your video connector to see if the green goes away. If so, then you most likely have a game PCB issue. If not, then trace the green input voltage through the chassis to the IC to the neckboard to the neck socket, checking for a short somewhere...

 

[SuperSprint]

Thanks again to everyone who's been helping me. As I mentioned to Modessitt, this is more to me that just getting one monitor running. If that were the deal, I'd just pay someone and be done. Definitely faster and probably cheaper. I'm asking all these questions (and grateful for your answers) because I want to learn how to diagnose and repair monitors.

@Modessitt - Thanks. I'll check the drive circuits as soon as the VR arrives and I get the heater lit again. I don't know if it makes any difference in the diagnostic, but the screen I pictured only above stays for ~30sec. After that, it's the blue tinted screen from earlier in the thread. Here's the sequence of events;

00:00-00:15 Black screen
00:15-00:60 Normal color, faded, shrunken game screen.
00:60-01:30 Green Screen pictured above.
01:30-02:30 Black screen
02:30-xx:xx Blue tinted game screen seen above.

Also, this was my machine for a good while before it died so it's not a "mystery monitor". When it died, the picture was perfect and it just started playing blind suddenly. Testing revealed a dead HOT and flyback. That's where this thread starts.

 

[SuperSprint]

Update: Replacing the VR & HOT brought my B+ back up to 122.5V got me raster. Not white raster, but bright blue. Still not out of the woods yet, as all I can get is a blue screen with regularly spaced horizontal and diagonal lines (see green pic earlier in this, substitute blue for green). Lines are present in the screen shown, but not visible in the photograph

I removed the Q403 (blue drive - neckboard) and the main body of the screen is light blue with bright blue diagonal lines. Top and bottom edges are well defined with a black border. L&R are scalloped in, forming a sort of letter "I" shape. Removing the RBG connection between the main board and the neck board cleaned up the edges nice and straight once, but that may be a fluke as I haven't been able to reproduce it again.

 

[SuperSprint]

So far;

- Removed game PCB feed, no change.
- Removed RGB feed from chassis to neckboard, no change.
- Q403, colors lightened, images remained the same.
- Lifted R415 where it meets the neckboard. Color changed to yellow, but the image is the same.

I'm stumped. Fromm diagram suggests Q201, Q202, but with the neck feed disconnected, they are out of circuit. Am I seeing a failure in the oscillator and hence no change when it's disconnected from the neckboard?

 

[Oryk Zinyo]

Have you tested the tube? It is possible that the blue gun is shorted and staying on when the heater has power, creating the blue screen.

 

[SuperSprint]

I haven't. But wouldn't losing blue when I left the blue gun out of circuit contraindicate that?

Also, I was under the impression that a shorted gun wouldn't leave a clean black border between the image and the tube edge.

 

[LyonsArcade]

Your tube isn't shorted. After searching, I found a picture of a shorted tube, so everybody will know what it looks like. Once you see it, youll be able to tell pretty quickly if a tube is shorted or not. If a transistor shorts on, you won't see the retrace lines. You can get the effect of the transistor shorting on by turning your pots all the way up, like turn your reds all the way up, and it will NOT look like this picture. Notice too how the color goes all the way over the tube, all the way up to the corners and everything... it doesn't always do this, sometimes it'll stop wherever the image stops.

This is from last year, a guy posted up that his monitor was doing this and was trying to figure out what was up. Red gun is shorted, this is a K7000 but they all look similar when it does it!

If your tube has a WEAK gun, you may get one color that won't go bright enough without smearing or looking horrible.

 

[SuperSprint]

Foreward: I hope I'm not boring everyone with my updates. I know that I would enjoy reading it, so I thought others who are also new to monitor repair (now, or future) might like to share in the diagnostic process.

Thanks Lyons. That's what I suspected from the descriptions, but a picture sure does help! As I expected, I tested H-KG, H-KB, H-KG and found no shorts between the heater and any of the colors.

Next, I unseated two of the resistors before the neck socket, removing input to those guns. With only green remaining, I got the image above, only in green. When I jumpered in Red off the green drive next circuit (pre-resistor), same image in yellow. When I jumpered in blue, I got a light cyan. From this, it appears that all my tube colors are functioning properly as yellow and cyan are the secondary colors of the respective pairs.

Not knowing where to turn, I tested all the resistors on the neck board (in spec), checked the pots against one another (same), swapped Q401 (green) and Q403 (blue), and re-flowed ALL of the neckboard connections. When tested, I get the same image pictured above, only in white (YEA!!). Not there yet, but it appears I may have solved the dominant color issue.

Before calling it quits, I pulled and tested Q201, Q202. I don't have a control to compare them to, but they tested similar to one another with ~0.694 across one junction and open across all others. I may replace these for good measure.

 

[SuperSprint]

Victory! Well, actually just progress. For those who might be following the repair.

I replaced Q201 & Q202 in the blanking circuit. That gave me a picture (FINALLY!), but it was still heavily blue biased (see pic 1). For good measure, I also replaced Q205 & Q208 in the main board blue drive circuits. For a few glorious minutes, I had correct color. Then it returned to blue biased again???

Next, I swapped the Red and Blue neck board transistors (Q401, Q403) with no change. After that, I pulled all the pins from the RGB feed to the neckboard (except T) and began playing mix-n-match with jumpers. No matter what happened, sending a signal to the blue input resulted in a blue that was too strong. Diagnosis; Problem is not in the mainboard.

To diagnose the problem further, I lifted the final current limiting resistors (3) feeding the RGB pins on the tube. No picture (black) as expected. But to my surprise when I cranked up the screen control near max, the image began appearing faintly on screen. (cross talk?). Anyway, I jumpered from the each drive circuit to KR, KG, KB. Consistently, whenever any signal was applied to KR or KG, it looked normal (pic 2). But when that same signal was applied to KB, I got an overly bright blue screen (pic 3). Diagnosis; Problem is not in the neckboard.

At this point, I believe there is a problem with the blue gun. But, testing it against H, and every other pin on the neck I get no short and >1Mohm resistance. And the blue ends at the well defined edge of the raster. However, if I turn up the screen, I get raster lines that are blue, and a blue "background" where there should be white raster. By playing with the drive pots and the screen control, I manage to get a proper picture (pic 4 & 5), but the blue pot is all the way down and the other pots are all the way up. Also, the screen seems set a little high at about 11o'clock. At 9 o'clock there is practically no image at all. It's usable, but I wonder about running these pots hard at one end or the other.

Conclusion: It's playable but m still not entirely sure where the problem lies. I had speculated that the problem was a "hot" blue gun and a weak red and green because of how I set the pots. However, with a good picture on screen, final K voltages are R-87.2, G-88.7, B-101.0 vs 88.3VDC spec for all. What confuses me is that *dropping* the blue gun to 88.3 dramatically increases blue bias and creates raster lines on screen. Perhaps's an inverse coupling of K voltage and screen brightness is normal and I just don't understanding completely how the CRT works.

Any thoughts at this point would be graciously welcomed.

P.S. I played my game last night for the first time in 10 years. I am VERY grateful to all who helped me on this project.