nashou
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Post by nashou on Nov 4, 2013 13:50:27 GMT -5
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? Added Schematic of the VDM Athanasios Attachments:00-263330-02P.pdf (210.61 KB)
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Post by mastertech on Nov 4, 2013 15:11:14 GMT -5
If you turn up the G2 what does the back raster look like?
Also, is the top grid 1/2 of the whole grid or is it the whole grid compressed into the top area?
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nashou
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Post by nashou on Nov 4, 2013 17:22:48 GMT -5
If you turn up the G2 what does the back raster look like? Also, is the top grid 1/2 of the whole grid or is it the whole grid compressed into the top area? Thanks for the reply. It looks like its a bit folded up onto itself. I made two videos looking into the lens so there is some reflection shadows etc. |
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nashou
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Post by nashou on Nov 4, 2013 18:24:58 GMT -5
Correction on the resistor that was open it was 920 on the + supply side.
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Post by mastertech on Nov 4, 2013 18:53:46 GMT -5
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.
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nashou
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Post by nashou on Nov 4, 2013 19:23:03 GMT -5
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.
Give me a few minutes
Nashou
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nashou
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Post by nashou on Nov 4, 2013 19:30:48 GMT -5
nope still on Blue.
Nashou
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Post by mastertech on Nov 4, 2013 19:34:07 GMT -5
Well now you know your problem is in the surrounding circuit of U900 and it is not U901.
This problem is definitely on this VDM correct. Changing the VDM and no problem, correct?
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nashou
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Post by nashou on Nov 4, 2013 19:42:38 GMT -5
Yep with my VDM which is still on the heat sink there is no issue. He sent me his VDM.
Nashou
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Post by mastertech on Nov 4, 2013 19:49:03 GMT -5
Well start testing the few parts surrounding U900. Also, if you have another STV9379 you may want to try replacing U900 again. You never know, especially with used parts.
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nashou
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Post by nashou on Nov 4, 2013 20:28:29 GMT -5
What is the CA339 Chip for U6 is that just for the Vfail only?
On my board I can not find R1900, it might not have been added. Usually its a piggy backed resistor, its not there so I'll need to see if that resistor value is halved.
Do you think its one of the Caps? the 100uf test ok in value but the others measure much higher but it must be picking up the other caps on the same rail.
D901 is fine as well.
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nashou
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Post by nashou on Nov 4, 2013 20:39:45 GMT -5
One thing I just noticed is the two working amps are much hotter to the touch than the Blue channel.
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Post by mastertech on Nov 4, 2013 20:50:13 GMT -5
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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Post by mastertech on Nov 4, 2013 20:52:26 GMT -5
What is the CA339 Chip for U6 is that just for the Vfail only? Yes, everything past C905 is deflection fail circuit. Of course that's not to say that it's not possible to cause an anomaly but highly unlikely. |
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nashou
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Post by nashou on Nov 4, 2013 20:58:57 GMT -5
I do have another STV9379 to try from the older Marquee 8000 VDM
Should I go that route first?
Nashou
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