| Electronics > Repair |
| BK Precision 2120 trace issues |
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| vexatag:
Yes, if I move it from left to right quickly the trace distorts, I wasn't able to capture it on camera but it almost looks like a sine wave or a poorly ramped square wave. Here's the video. |
| bdunham7:
OK, that isn't a problem. The reason that it distorts is that you don't really have a diagonal line there, you have a rapidly moving dot and persistence of phosphor (and vision) makes it look like a line. When you move it that quickly it can look like that. If you were able to move it quickly and evenly enough, you would even see a coherent waveform. I was concerned with whether there was a stable sort of distortion (bad CRT) or if it moved other than straight back and forth (something really wierd). Since it doesn't do those things, that part is fine and you can proceed with further testing. If it helps, as you are turning the CH2/X volts/div down, you can leave the CH1/Y setting much higher, like 1V/div, if that keeps the line under control. That diagonal line is noise in the input amplifiers somewhere and it is correlated (in sync) between the two channels. First suspect would be bad power supply caps, so after you do the horizontal deflection tests, you can check the power supply ripple by measuring each power supply voltage on DCV and then ACV. ACV should be really low. I would try both of your meters just to see if you get different results. |
| vexatag:
Results of testing Q808 base with different volts per division: 0.1 v/div: 1.57 to 2.16 50 mv/div: 1.56 to 2.75 20 mv/div: 1.56 to 4.52 10 mv/div: 1.58 to 6.80 Results of testing power supply voltages with the DM383: 5V: 4.93 V DC, 0 V AC +12V: +12.23 V DC , 0 V AC -12V: -12.05 V DC, 0 V AC 24V: 24.4 V DC, 0 V AC 140V: 141.8 V DC, 0 V AC 260V: 265 V DC, 0 V AC Power supply voltages testing, Aneng AN860B+: 5V: 4.935 V DC, 0.000 V AC +12V: +12.23 V DC, 0.001 or 0.002 V AC -12V: -12.03 V DC, +0.002 or +0.003 V AC 24V: 24.49 V DC, 0.000 V AC 140V: 142.0 V DC, 0.000 V AC 260V: 265.2 V DC, 0.000 V AC |
| bdunham7:
OK, but we also need to know what happens to D1 and D2 when the base voltage changes on Q808. Just the 20mv/DIV and 10mv/DIV will do. If it changes like it did on the Q809 test the horizontal amp is good. I thought about your issue with the line getting long and as you are doing the above test, try this. Set X and Y to the same (high) setting so you get that small 45-degree angle line. Then when doing the above test center that line so that it is goes through the center point on the screen with about half above and half below. Then when you do the readings, turn both down together and just let the line get long but only pay attention to the point where it goes thru the horizontal center line. That intersection should move left and right just like the dot would if it weren't all stretched out. Does it do that or does it do something else? If it gets faint or blurry, just turn up the intensity, dim the lights and make your best guess. |
| vexatag:
Right, my bad. D1 testing: 20 mv/div: 144 V to 96 V 10 mv/div: 144 V to 65 V D2 testing: 20 mv/div: 155 V to 186 V 10 mv/div: 152 V to 241 V As for the line, when the test load is applied the entire segment moves up and to the right, even at 5 mv/div. When checking CH1 in XY mode with a bnc lead in CH2, it's displaying a horizontal line of about the same length as the diagonal line from CH2. Normal mode is still just a trace, but I thought I should point that out. |
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