Hi guys's I have a VDM here that the customer bent the one Flyback Amp on the blue channel and it shorted out. I used an older M8000 VDM for parts and replaced the Amp ( U900) and resistor 921 was open. The PJ fired up but only the top half of the grid on the blue channel was showing while a garbled mess was on the lower half or no scanning at all. I was thinking the blue channel of the quad op amp was taken out U 901.
Ant thought on where to look later when I get home form work?
If it was me this is what I would do. It's very easy and quick to narrow it down. First, as always I check line voltages to the ic's They should be correct because the other colors are correct but just to be sure. So check the + and - 15v to U900, + and - 14.5v to U901 and VSS and VDD to U8.
If all is well I would then locate R905 and R705. I would then lift 1 end or each resistor and using small gauge wire to cross the blue and red channels. U901 output to R705 and U701 output to R905. Use the point where you lifted the resistor to solder the wire to and then to the opposing resistor. Then see if the problem remains on blue or changes. This is fast and easy and will tell you if the problem is before R905 or the circuit past R905. Depending on the outcome would determine where to go to next.
Last Edit: Nov 4, 2013 18:56:17 GMT -5 by mastertech
ok , Voltages are fine going to make jumpers now. I confirmed that the resistor points go to the correct op amp pins, the blue channel on the board is visually easy to follow the red travels a bit under and over parts to get the U701.
I would first test R921 and R922. You have to lift 1 to test both because the are mounted parallel and at such a low value you would not get a correct reading with them both mounted. In further looking at these schematics I see a line that goes back to U4 which also puts R908 and R901 and U4 in play. Then every resistor tied to the circuit. R905,R901,R906,R907,R910,R920,R915,R916,R911,R912,R913,R914 Also that R1900 if you locate it, verify populated R1800,R1700 to confirm install. Then D901,D902,D903, The caps can definitely cause anomalies but probably only need to be tested for value and no short condition. Did you have another U900 to change and verify?
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