Tuesday, August 10, 2010

SuperSprint Repair - Pt 9 - Repair complete!

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.

Monday, August 09, 2010

SuperSprint Repair - Pt 8 - Drawing a blank, or not!

Victory! Well, actually just progress. 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), 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 I'm still not certain where the blue 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.

SuperSprint Repair - Pt 7 - Feeling Blue

A fellow collector suggested it may be the tube and reminded me that it is possible that the blue gun is shorted and staying on when the heater has power, creating the blue screen. I'm not sure about that as I thought losing blue when I left the blue gun out of circuit would contraindicate a gun failure?  Also, I was under the impression that a shorted gun wouldn't leave a clean black border between the image and the tube edge. Hmmm.... To be sure I tested H-KG, H-KB, H-KR 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 (red+green=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 (400 series) 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 in the blanking circuit. 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 think I will replace these for good measure.

SuperSprint Repair - Pt 6 - New Voltage Regulator

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.

So far;

- Removed game PCB feed, no change.
- Removed RGB feed from chassis to neckboard, no change.
- Removed Q403 (blue drive transistor), 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?