Electronics > Repair
BK Precision 2120 trace issues
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vexatag:
Ok, measurements have been made. All measurements were made with the negative lead on either D1 or D2 and the positive lead on the component being tested.

From D1:
L802: 71 V in either state
R839: 71 V in either state
Q809 base: -193 V in either state
Q809 emitter: -256 V in either state
R836: -193 V in either state

From D2:
L801: 162 V in either state
R838: 162 V in either state
Q808 base: -190 V in either state
Q808 emitter: -104 V in either state
R835: -104 V in either state.

The only component I wasn't able to test properly was Q807, it's small and sits too close to the pcb for me to clip a test lead to it. I suppose the next step is to test at the resistors directly adjacent to the base and emitter?
bdunham7:
Sorry, I think I wasn't clear.  That helps, but it is confusing for me to try and calculate what I need.  Can you do this again with the negative DMM lead to chassis ground and the positive at the test point?  Are you in XY mode and have verified that you can 'move the dot' in the Y channel again before switching to the X channel at the same V/DIV?  Something isn't working that was working before.  Don't worry about Q807 at this point if it is too hard to get a safe measurement there.

EDIT:  Check at the base of Q808 first and verify that it changes with and without an input.  If it does not, stop right there.

EDIT2:  Check both sides of R835 and R836.  One side of each is connected to the other but I'm not sure it is obvious which just by looking at the board.
vexatag:
Having bnc leads connected to both channels is tripping the system up, but that's an issue to troubleshoot later.

I wasn't testing in XY mode, there's a lot of new concepts for me and things are escaping my notice. Y and X channels both have full motion from the position knobs. Additionally, the X position knob is now going off the screen both left and right.

Testing in XY mode with the BNC lead plugged into CH1/Y, DMM1 negative lead on chassis ground:
Q808 base goes from 1.527 V without a load and 1.532 V with a load. (it's hard to be specific, the exact reading can change just by moving the bnc cable around (I guess this is why you don't get the cheap cables?))
Q808 emitter: .961 V to .955 V
L801: 265 V either way
L802: 265 V either way
R835 (side not connected to R836): .960 V to .969 V
R835 (side connected to R836): -2.30 V to -2.29 V
R836 (side connected to R835): -2.30 V to -2.30 V
R836 (side not connected to R835): .488 to .488
Q809 emitter: .514 V to .514 V
Q809 base: 1.08 to 1.08
bdunham7:
So go back to normal mode and see if you get a sweep with the trigger level knob centered and pushed in.  If not, try switching the COUPLING to 'LINE' and you may have to switch source to EXT as well--but try both ways.  Make sure all 4 buttons below there are out.  Set the timebase to one of the slowest settings.
bdunham7:

--- Quote from: vexatag on July 28, 2020, 05:08:34 am ---
I wasn't testing in XY mode, there's a lot of new concepts for me and things are escaping my notice. Y and X channels both have full motion from the position knobs. Additionally, the X position knob is now going off the screen both left and right.

--- End quote ---

That's very good news.


--- Quote ---Testing in XY mode with the BNC lead plugged into CH1/Y, DMM1 negative lead on chassis ground:

--- End quote ---

Oops!  You were supposed to check that you could deflect the dot vertically with CH1/Y but do the measurements on CH2/X.  But you don't need to redo everything.  The scope may be working at this point.  If not, just redo this test and look at the base and emitter of Q808.  Everything else looks good and the fact that you can get full control with the XPOS knob proves out most of the rest of the horizontal final amp.

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