Trapezoid Image on WG K7000 Monitor after Recap

fiddler

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Does anyone have an idea as to the cause of a trapezoid image on my WG Monitor? See attachment.

Here are the game specs:
Game: William Defender
Monitor Model: Wells-Gardner 19K7689 Mfg Date: Aug 1991

Problem Description
I had a full screen image on my monitor, but it had some retrace issues so I decided to recap all of the electrolytics on my board. After the recap, my retrace issues disappeared, but I was left with a trapezoid image. The top of the image covers about 3/4 of the screen width. The bottom of the image covers about 1/2 of the screen.

After the recap, I performed the following tasks before powering the board:
1. Rechecked that every cap was mounted in the correct orientation,
2. Reflowed all high power connections
3. Examined the board under 10x magnifier for any shorts or lifted pads.
4. Cleaned board with flux cleaner to remove all solder and dirt residue.

Observations
B+ and other high voltages are normal (within 1v).

Voltages measured at diodes and transistors in the horizontal section are within 350mV of stated specifications except for Collector voltage on Q10. It is reading 10V higher which I attribute to R89 showing a value of 3.3k instead of 3.9k I think the heat has altered its value, but it was this way before the recap, so I don't think it is the cause now.

There are no cracks or leakage on my flyback.

The resistance of the deflection yoke windings measure as follows:
Vertical: 13.4 ohms
Horizontal: 4.8 ohms
I don't know whether this is correct as I am unable to find any published specs for these.

I lack the tools to measure the inductance of the horizontal linearity coil (L2), but the resistance measurement is nearly zero. I'm unable to find a published spec for this device, but the low measurement might indicate it is shorted. Removing it from the circuit and shorting R99 does not affect the image.

The horizontal size coil (L1) appears to be working properly.

I lack access to a scope to observe waveforms, but the oscillators all seem to be running at the correct frequency as measured by my DMM.

Full Disclosure
In the interest of full disclosure, my recap exercise could be the cause of whatever component has now failed. After the recap, no image was displayed and the 1.5A fuse on the board would blow within a few seconds. I traced the problem to a cap in position C204 on the neck board that was installed backwards. The cap was oriented correctly with the markings on the PCB, but the markings on the board are in error. So the shorted cap was causing some higher currents than normal to flow through part of the flyback, the deflection yoke, and both horizontal coils. Once I removed the cap and replaced it with another known good one, the image was restored to the monitor, albeit a trapezoid one. Before the fuse would blow, that well known electronic burning smell was slightly evident, but I was unable to isolate it to a device.

Conclusion
I had a full picture before the recap, so I would have expected the same after the recap. I have not altered any convergence settings, since I don't see how they could have been affected by the recap. I could possibly see a need for a slight tweak, but the magnitude of the picture change suggests something more fundamental is wrong. The only thing that looks suspicious to me is the linearity coil (L2), but I can't be sure. Would a failure of L2 device cause this problem? What have I overlooked? Any suggestions would be greatly appreciated.

Thank you
 

Attachments

  • Defender Capture.JPG
    Defender Capture.JPG
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