| Electronics > Repair |
| BK Precision 2120 trace issues |
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| bdunham7:
OK, those numbers look good. As you can see, the plate voltage difference can be driven quite significantly by Q808, so that proves out the horizontal final amp, at least roughly. There are still some gain issues somewhere, but I think we are done in this area for now, as long as your bodge wires hold up and the resistors don't fry entirely. You'll need new ones eventually. --- Quote ---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. --- End quote --- I'm not sure yet what is going on here, and I'm not sure what you mean by 'normal mode is just a trace'. You have a trace in normal mode??? Or is it noise? What happens when you slow down the timebase to 0.1S/div? So lets look at the input circuits. It is the 'Attenuators and Pre-Amplifier' schematic, followed by 'Vertical Amp and Control Logic'. Something strange appears to be happening here. Try setting the inputs to GRD, XY-mode on, all 4 of the center pushbuttons out, 5mv/div and try to center the dot or whatever is showing. Then reduce the input sensitivity to 10mv/div, and so on. Tell me what you see and make a video if you want--those are helpful--but make sure you get the whole scope face in it so I can see every control. |
| vexatag:
--- Quote from: bdunham7 on July 30, 2020, 12:55:06 am ---I'm not sure yet what is going on here, and I'm not sure what you mean by 'normal mode is just a trace'. You have a trace in normal mode??? Or is it noise? What happens when you slow down the timebase to 0.1S/div? --- End quote --- Sorry, I used the wrong wording. In non-xy mode, channel 1 is still just showing a single dot. Here's the video of me testing the inputs. The thing I had the second bnc leads plugged into was just a breadboard with some component leads and pin headers I've been using as a breakout board for slightly more convenient access to the hooks ever since the ends of the hooks got bent out of shape (hook onto the component lead, press the multimeter lead against the pin header). |
| bdunham7:
OK, a couple of things. First, the video did help, but there are two misunderstandings about scope theory that are apparent. First, I didn't elaborate, but when you switch to the GND, you are disconnecting the inputs internally and shunting the preamplifier to ground. Once you verify that the inputs are unresponsive, you know that that feature is OK and there's no point in supplying any inputs. Second, in XY mode, those 4 pushbuttons don't do anything. They don't change the channel selection, for example, which I saw you pushing. You can and should look at the schematics that I referenced and look at the diagrams of at least the switches to see if you can understand how that works. In any case, we've determined that the problem seems to be on the input side. I also noticed the strange behavior as you were changing the CH2/X attentuator level. It's not uncommon to see a brief jump in the dot as you change ranges, but on CH2/X, it breaks up or jumps at a 45-degree angle--but that channel should only affect horizontal position. So there's some leakage or crossover between channels and it looks roughly equal. We can also try some testing in the normal, non-XY mode. If you leave the inputs grounded, all 4 pushbutton out and turn XY off, what happens? Do you have a 9-volt battery, or preferably, several of them? I'd like to try using a larger input signal through the attenuators to try and track down the area where the noise is generated. |
| vexatag:
Very quick comment, I just wanted to say that when the input is set to DC or AC, the channel selection does switch between the two channels. I don't know if they're supposed to or not after what you've just told me but it's the functionality I've been experiencing so far. Also, I do have several 9v batteries lying around. EDIT: Switching to normal mode with the inputs grounded shifts the dot to the far left of the display, additionally it limits the rightward motion to one square right of center. |
| bdunham7:
--- Quote from: vexatag on July 30, 2020, 08:49:25 pm ---Very quick comment, I just wanted to say that when the input is set to DC or AC, the channel selection does switch between the two channels. I don't know if they're supposed to or not after what you've just told me but it's the functionality I've been experiencing so far. Also, I do have several 9v batteries lying around. EDIT: Switching to normal mode with the inputs grounded shifts the dot to the far left of the display, additionally it limits the rightward motion to one square right of center. --- End quote --- I actually don't know what happens if you push the channel select button in XY-mode, I'd have to look at the schematic. It is possible that it does something like reverse it into YX mode or something weird, but that would be an undocumented feature--or a bug. So, in normal with the dot to the left try switching one channel at a time from GRD to DC, then one at a time to AC, then both to DC and last both to AC. What happens? Then, go back to both set to GRD and connect a BNC-to-clip cable to your EXT TRIGGER connection, trigger level centered and knob pulled out, all six pushbuttons out, COUPLING set to DC/LF, SOURCE set to EXT, timebase to the slowest setting (0.2S/div?) and then briefly connect the 9 volt battery to the cable clips, red to positive (the smaller terminal on the battery). If nothing happens, try turning the trigger knob a few degrees clockwise at a time and repeating. If still nothing, try every position of the trigger knob (5 or 6 places). Next, let's test the vertical amplifiers directly. Turn both channel attenuators counterclockwise to the 5V/div setting. Unground one at a time to the DC setting and give it the same 9 volt input you just did with the trigger. Then do the same in the AC setting. Don't change any other settings the first time, but then push in the second vertical mode button to CH2 and repeat the test on both channels. Lastly, repeat all that except set BOTH channels to DC (you don't need to do AC right now) but run the test one channel at a time, again doing both with the 2nd vertical mode button out, then both with it in. That seems like a big rigmarole, but we're looking for crosstalk between channels as well as vertical amplifier response. The 9-volt battery should give you a little less than 2 divisions of movement where it moves anything, so a lot more or less than that should be noted. |
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