Electronics > Repair
BK Precision 2120 trace issues
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bdunham7:
Yes, just connect the battery to the EXT trigger BNC with your handy cables, switch the mode to EXT and then follow the trail as far as the slope switch with your DMM in the usual way.  If the board is out, you'll have to find a ground point, but the BNC or battery negative should do fine.  And you can change the switch back and forth from EXT to ALT to make sure your battery signal comes and goes. 

As far as motivation goes, you've been working on that thing for a while.  I'll still be here later if you want to give it a break for now!
bdunham7:

--- Quote from: vexatag on July 31, 2020, 11:39:30 pm ---I can't find R610 on either the Trig and Sweep Generator schematic or anywhere on the circuit board.

--- End quote ---

R610 on the schematic is on the left side right next to the "FROM BD503 LINE 60 HZ".  It is all run together as R61027K.  I don't see in on your board either, nor on the board diagram in the manual, but it should be connected to R611 and might be off-board or connected by a wire.  It should be near a wire coming from the power supply---if you look at that schematic you'll see it coming off of the top of bridge rectifier BD603--'to R610'.

I've noticed the first version discrepancy--the trigger op amp U601 in the manual is U615 on your board.  There might be more, so if you see anything clearly different let me know.

EDIT:  R610 is on the low voltage power supply board.
vexatag:
Re: I've been working on this a while, it's the sort of thing where if I set it down I won't take care of it for many many months, this thing is big enough that I want it to be working


--- Quote from: bdunham7 on August 01, 2020, 12:40:27 am ---I've noticed the first version discrepancy--the trigger op amp U601 in the manual is U615 on your board.  There might be more, so if you see anything clearly different let me know.

--- End quote ---

Weirdly, U615 is also called U601 on the opposite side of the PCB.

Voltages for each part I could find in the circuit, listed in the order I tested them. Some are probably only used for other triggering modes but I tested them anyway. I tested with the coupling switch set to DC/LF and the source set to ext, I was able to have the board out with everything except the CRT inputs plugged in. All numbers are in volts, none of that mv rubbish.
Battery voltage: 9.39
Voltage at the switch: 9.16
R601: 9.16 to 9.37
C601: 9.16 to 9.37
R602: 0.00 and 0.00
R605: 9.16 to 9.08
R603: 9.16 to 0.00
R606: 9.09 to 4.745
C603: 9.16 to 0.482
R604: 0.199 to 0.00
R607:  0.200 to 0.200
C630:  0.415 to 0.417
R609: 4.024 to 4.804
C604: 9.08 to 4.787
C606: 4.097 and 0.00

I went ahead and checked the resistance of the resistors that had an input voltage of ~9V and an output voltage of 0V, all were within spec.

The other testing will happen tomorrow.
vexatag:
R610 measured at 7.3 V and 1.67 V AC, I also found two points on S603 that measured 1.67 V AC only when the coupling was set to LINE.
bdunham7:
OK, remember those two points, that is the input side of the switch.

So I think the next thing to do is make a test tool.  You'll need a 9 volt battery, a clip or something to reliably wire to the battery (I snip them off of dead or obsolete devices) and the 10K potentiometer you said you had earlier.  Wire up the 9 volt battery to the outside (fixed) terminals of the potentiometer,   then attach one wire to the center (variable) tap and one to the side where the negative battery lead went.  You should now have a short-circuit proof variable voltage source, which you can confirm with your voltmeter.  It might be worthwhile to take the time to make the whole thing neat and durable.

Next, connect it to the EXT trigger in with DC/LF/TV-V and try to find a signal right where you found the AC signal on S603.  If everything works, you should be able to turn  your pot up and down and see the voltage change in sync at the center input terminal of S603.

Last for now, move your voltmeter lead to the switch terminal exactly opposite of wherever  you find your signal in the last step.  By 'opposite', I mean in the other row--there's two rows of 4 or 5 terminals, same postion/opposite row.  Then, with the scope turned on and the trigger level knob not it AUTO (pulled out), you should be able to turn it up and down and see a similar change in voltage at that point.  These are the inputs to U601 (or U615??) which is the trigger comparator--it turns on when the voltage on the input side exceeds the voltage on the trigger level side--or vice versa if SLOPE (-) is selected. 

The rest of the trigger logic circuit is more complicated and I'm still trying to figure out a way to have you test it with what you have on hand.
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