Electronics > Repair
BK Precision 2120 trace issues
<< < (20/36) > >>
bdunham7:
Yes, use the pos lead of the Aneng to the red clip on your BNC.  In your video, I think you are just picking up noise, you have it set at 10mV/div.  So now that we know that 5 volts is enough of  change at the base of Q809 to deflect from one side to the other, lets see what we get when we use a proper input.  You have about 0.5 volts, so if we set the CH2/X input to 100mV/div (I think that is 0.1V/div on your scope) then your meter's output should deflect the dot 5 divisions (if it were working). 

So do that.  Set it to 0.1V/div on CH2/X, use XY mode as before and test at Q808 base, emitter, D2, D1 and the side of R835 that is not connected to Q808's emitter.  Then we can compare the base voltage change and the D1-D2 change with what Q809 produces.
vexatag:
I figured they're probably just poorly shielded cables, I'll pick some decent ones up eventually.

As for the measurements (set as you outlined, connecting pos-pos on the test load):
Q808 emitter: 0.96 V to 1.55 V
Q808 base: 1.53 V to 2.14 V
D1: 143 V to 135 V
D2: 154 V to 161 V
R835 (Q809 side): 0.96 V to 1.55 V
bdunham7:
OK, does the dot move when you do this?  How far? 

If it does move, the go down to 50mV/div and do the same test again, including recording all the same voltages, then go down to 20mv/div and do it again, then 10, etc until you get either get a difference between load and no load of at least 5 volts on the base of Q808 or you get the dot to move at least halfway, preferably all the way, across the screen (you can recenter as needed to give it room to move).

vexatag:
In XY mode, the dot is not a dot but a 45 degree angle line as I mentioned here


--- Quote from: vexatag on July 28, 2020, 07:15:36 pm ---Testing without a test load in XY mode on channel 2 at 0.5 V/div (so I know where the center is, it's a 45 degree line at lower levels)

--- End quote ---

When I apply the voltage, the line shifts position up and to the right about 1 CM, in addition to getting larger and fainter. I don't know if this changes anything in what I should be looking for.
bdunham7:
The 45 degree line is what happens when you feed the same signal to both channels in XY mode.  There are noise and signal issues here that we haven't gotten to yet--I saw your videos and something is....wierd.  But for now I want to see whether the horizontal final amp is fixed before moving on.  So just to confirm, if you move the 'dot' or whatever it is with the XPOS knob, you apparently can get it all the way to the left or right.  Does it change shape or anything when you are moving it with the XPOS knob?
Navigation
Message Index
Next page
Previous page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod