I have a WG K7600 Series Monitor that has developed a keystone raster. This occurred after C204 on P456 (Yoke circuit board) shorted and blew the fuse within two to three seconds. After Ireplacing C204, eveyrthing returned to normal, but I was left with a keystone raster. I've probed all over the board and exchanged some components, but nothing has had any affect on the raster. Can anyone offer some suggestions as to the cause or what I might have overlooked?Components Replaced
- All major electrolytics
- C36 (retrace cap)
- C38 (width)
- L2 (Linearity)
- R69 - 3.9K (value was off considerably)
- D15, D18
- R98, R99 (values out of spec)
Voltage and Oscllator Measurements
- 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 width.
- The sides of the raster are straight (not wavy).
- With the exception of the shape, all other aspects of the image are normal.
- Horizontal and Vertical oscillator frequencies as measured on DMM seem normal
- B+ is steady at 123 VDC
- All voltages on pins of vertical and horizontal ICs are within tolerance
- Anode voltage measures 19 KVDC
- Collector voltage on HOT measures about 126 VDC (this doesn't seem right)
- All of the "Do Not Measure" voltages do not exceed 130 VDC (doesn't seem right either)
- Vertical Resistance is 13.6 Ohms. Each leg measures 7.8 ohms.
- Horizontal resistance is 4.6 ohms.
- I've reversed the leads to the horizonal coil, but the keystone raster did not invert (although the image reversed as it should).
- Adjusting the width coil will widen the image, but it retains its keystone shape.
- The deflection coils have not been moved or jarred.
- Solder connections have been reflowed and cleaned with flux solvent.
- The flyback has no cracks. It does not arc, smell, etc.
- The circuitry around IC2 is slightly different than that shown on the Wells-Gardner schematic for the 7600 series. Pin 5 is grounded on the schematic, but Pin 5 has a bypass capacitor to ground on my PCB.
Since the keystone did not invert when the horoizonal leads to the deflection coil were reversed, that would seem to indicate the coil is bad. However, I am doubtful that is the case. I have also read that their are circuit problems that can still cause this keystoning.
I do not have an oscilloscope or ESR meter so I'm unable to determine if there are any malformed waveforms or leaky capacitors.
Unless I start spending a lot of money, I've run out of ideas. I hoping someone with a lot more experience at this than me, might be able to lead me out of my haze..