Electronics > Repair
BK Precision 2120 trace issues
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vexatag:

--- Quote from: bdunham7 on July 30, 2020, 10:07:45 pm ---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?

--- End quote ---

There is no visual difference between any of these settings in normal mode.


--- Quote from: bdunham7 on July 30, 2020, 10:07:45 pm ---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). 

--- End quote ---

Nothing at any of the positions on the trigger knob.


--- Quote from: bdunham7 on July 30, 2020, 10:07:45 pm ---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.

--- End quote ---

I could only get the dot to jump upwards when directly connected to the channel in question (only on channel 1 when the bnc was plugged into CH1, CH1 was set to DC, and the CH1/CH2 switch was not pressed in).

Also, I checked the manual and the four vertical mode switches are in fact supposed to be disengaged when XY mode is engaged. So yes, something is wrong on my end :)
bdunham7:
OK, now we are getting into a tricky area.  Give me some time and when I have a chance, I'll look at the schematics and try to figure out how much you can test with what you have.  You can have a look at the Trigger and Sweep Generator schematic, and then look at the board for those same parts and just visually check everything and see if there are any more burned parts or physical damage.  The circuit in question is very near the previous area that you repaired, so maybe you'll get lucky and spot something. 

Basically what I'm looking at is the Trigger and Sweep Generator, along with the Timebase Generator, are pretty much self contained.  The LINE and EXT trigger options don't go through the regular channels and they feed the sweep ramp directly to your old friend R834, and we know things are good from there.  The operation of the vertical amps in the previous test make me optimistic that those are functioning well enough for now, even though the XY-mode seems to be screwed up.

So, have a look.  See if you can find R602, upper left hand corner of the Trig/Sweep schematic, near the trigger source switch.  See if you can verify that there is -12V on both sides of it.

vexatag:
Found R602, it's either near or in the repair job someone else had done. There's no obvious charring or anything to make me think it was damaged. The side connected to ground measured -12V, but the other side measured -9.36V. Checking the resistance, it's 4.68k ohms, so the drop is probably coming from somewhere else in the circuit?
bdunham7:
That seems OK, although if you switch the SOURCE to ALT, the drop should go away and both sides would be -12V.

So we're headed into a tricky section here, especially since there could be some variance between the schematic and your circuit board due to the different versions I alluded to much earlier.  It hasn't happened yet, but it is one more thing to worry about.  I'm still pondering how to test this without another scope, but there are a few things you should check first just so we don't miss something simple.

The upper left portion (on the schematic, not the board) of the Trig & Sweep Generator schematic is the basic input switching part.  You can follow a 9V input into the EXT input through the switches all the way to the SLOPE switch S603, and you can do that with the power off.  See if you can figure out how to do that just looking at the schematic.  On the BK2120 I had, the COUPLING and SOURCE switches were damaged and bent from some unknown abuse, but straightening them out seemed to fix them, so that is one of the first things to check.

Next, those bodge wires near this area.  It occurred to me that they might be some sort of unsuccessful repair or modification or patch, instead of an actual repair that worked.  Can you look carefully at those wires and tell me exactly where they are attached in the schematic?  Is there any other damage to the board, or missing parts or ???

Last, let's have a quick look at the LINE trigger option.  If you see the input on the left edge marked FROM BD503, LINE 60HZ, find that point on the board and R610.  This is power on--you should have an AC voltage on both sides of R610 and then the same voltage at the upper center terminal (on the schematic--you have to find where this is on the switch) of S603.  The voltage will only be at S603 when the COUPLING switch is in LINE, so you can differentiate it from noise by turning the switch from LINE to some other setting and back (slowly--AC readings take time to settle) and so on.  The LINE signal will be attenuated at each step through the system, so don't worry if it is lower as you go along--just see if it is there and comes and goes in response to the switch.
vexatag:

--- Quote from: bdunham7 on July 31, 2020, 02:54:55 pm ---The upper left portion (on the schematic, not the board) of the Trig & Sweep Generator schematic is the basic input switching part.  You can follow a 9V input into the EXT input through the switches all the way to the SLOPE switch S603, and you can do that with the power off.  See if you can figure out how to do that just looking at the schematic.  On the BK2120 I had, the COUPLING and SOURCE switches were damaged and bent from some unknown abuse, but straightening them out seemed to fix them, so that is one of the first things to check.

--- End quote ---

Somewhat confused as to what you mean by this, hook the battery up to the ext port and check for voltages at each component in the circuit? It's been a hot day and my motivation has been low, but I'll remove the circuit board and try to figure out the bodge job. I can't find R610 on either the Trig and Sweep Generator schematic or anywhere on the circuit board.
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