Electronics > Repair
BK Precision 2120 trace issues
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vexatag:
Q808 emitter: 0.94 V to -5.59 V
Q808 base: 5.61 V to 4.25 V
bdunham7:
If you go back a few pages to your reply #61, you were able to get about a 5 volt swing on Q808.


--- Quote ---At the base, voltage when the test load was connected was -4.37 V, voltage when the test load disconnected was 0.92 V. At the emitter, voltage when the test load was connected was -5.01 V, voltage when the test load disconnected was 0.28 V.
--- End quote ---

Those numbers looked OK for Q808, but the latest ones don't.  So either there's an issue with your input method or something else is going on.  So check everything as best you can--make sure you can move the Y channel about half a screen like before, then move the BNC to the X channel.  I'm not sure what you meant by two BNCs 'tripping the system', but that doesn't sound good!

Next, the fact that you have full X-position control is very good because it shows the CRT and a lot of other stuff is working.  So we are down to Q808 and its input signal.  I'd like you to see if you can get a proper signal change on Q808 like we had before.  Try on both sides of R834 to make sure you aren't losing it across that resistor.  Also, perhaps first, give me the voltages on D1 and D2 (neg-to-ground, pos to test point) three different ways--with the XPOS set so the dot is on the left edge of the screen, centered, and on the right edge.

This scope has obviously been worked on before and one possibility that we have to consider is that a previous unsuccessful repair attempt may have included some adjustments to the trim pots that are now going to need to be corrected.
vexatag:
Here's a Youtube video to what I mean by "tripping the system up". I needed something to test with and these were the cheapest leads that shipped from the USA (since international shipping is a bit of a mess right now). I'm assuming there's very little shielding, if any, and the actual probe clips have either bent out of shape or snapped off (it's hard to tell which), so I'll definitely be replacing them once the scope is able to generate a solid line across the screen when unplugged.

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)
D1, dot left: 198 V
D1, dot center: 142 V
D1, dot right: 89 V

D2, dot left: 98 V
D2, dot center: 155 V
D2, dot right: 210 V

Testing on channel 2 at 10 mV/div, using my second multimeter in ohms mode as a test load
R834 (side not connected to Q808): 1.4 V to -4.6 V
R834 (side connected to Q808 base): 1.6 V to -4.3 V
Q808 B: 1.58 V to -4.16 V
Q808 E: 0.98 V to -4.40 V
bdunham7:
Which leads did you order?  Link? They don't look like they are necessarily junk, but maybe that is an issue for later.  It looks like you have an actual XY pattern going on there, but something is off.  What are those cables connected to on the other end????

The good news is you will know a lot more about oscilloscopes by the time we're done here.  The bad news is I think your scope is a beaten, cobbled wreck that is going to fight us all the way to the end.  No matter, I've fixed worse.  Sometimes I think the most important quality in a repair tech is sheer stubbornness. 

So, if you start with the dot on the left, nothing connected, put it in the non-XY normal mode (button out, right?) and set the timebase to 0.1S/Div, all 4 pushbuttons out, CH1 to 10mv/DIV with DC coupling, and the trigger centered and pushed in, what does it do?  And if you give it an input with your DMM in ohms just like the test before, does it trigger? (Does the dot move horizontally at all?)  If not, how about if you select trigger coupling "LINE"?

If not, then there's two separate things needed to track this down.

First, the inputs.  Something is not right. It looks like the polarity is inverted from what I was expecting, but that alone shouldn't be a serious problem. Still, take the one you're using as the source (the Aneng I presume) and connect it directly to the other DMM, pos-to-pos, neg-to-neg and test the output voltage of the ohms function, including whether or not it is a negative reading.  Then continuity-check your BNC cable to make sure you know which clip is connected to the center pin of the BNC connector.  I want  you to make sure in future tests that the positive source voltage is connected to the center pin--and if your voltage was negative in the first step, that means the negative lead of the DMM.  Then, can you verify that when connected like this to the scope, the DMM reads approximately 1Megohm when connected to either channel?

Second, it looks like we are getting some signal through to Q808, but I've no idea what the actual signal level should be.  So go back to XY mode, set the dot in the center to start and do the same measurements as the last step of last time, only on Q809 and at the base, emitter and the other side of R836.  Don't worry about R832 unless it is super easy to get to.  Then we'll compare the voltage swing at the base that it takes for Q809 to deflect the dot all the way from one side to the other to see if the 5-volt or so swing we had at Q808 should be enough to see(in which case Q808 is bad) or if the signal needs to be a lot more (in which case the horizontal final amp is fixed and we move on...).





vexatag:

--- Quote from: bdunham7 on July 28, 2020, 07:57:17 pm ---Which leads did you order?  Link? They don't look like they are necessarily junk, but maybe that is an issue for later.  It looks like you have an actual XY pattern going on there, but something is off.  What are those cables connected to on the other end????

--- End quote ---

These bad boys are hooked up to... nothing at all. All the changes in the display are from me moving the cables around.



--- Quote from: bdunham7 on July 28, 2020, 07:57:17 pm ---So, if you start with the dot on the left, nothing connected, put it in the non-XY normal mode (button out, right?) and set the timebase to 0.1S/Div, all 4 pushbuttons out, CH1 to 10mv/DIV with DC coupling, and the trigger centered and pushed in, what does it do?  And if you give it an input with your DMM in ohms just like the test before, does it trigger? (Does the dot move horizontally at all?)  If not, how about if you select trigger coupling "LINE"?

--- End quote ---

With settings dialed as specified and nothing connected (no lead hooked up) it's just a dot. With a lead hooked up to CH1/Y, it's a ~20mm vertical line. With one lead of the multimeter hooked up to the test lead, the line goes from top to bottom, growing thinner and dimmer. With both leads of the multimeter hooked up, it is impossible to notice. Trigger coupling "LINE" mode has the same results.


--- Quote from: bdunham7 on July 28, 2020, 07:57:17 pm ---If not, then there's two separate things needed to track this down.

First, the inputs.  Something is not right. It looks like the polarity is inverted from what I was expecting, but that alone shouldn't be a serious problem. Still, take the one you're using as the source (the Aneng I presume) and connect it directly to the other DMM, pos-to-pos, neg-to-neg and test the output voltage of the ohms function, including whether or not it is a negative reading.  Then continuity-check your BNC cable to make sure you know which clip is connected to the center pin of the BNC connector.  I want  you to make sure in future tests that the positive source voltage is connected to the center pin--and if your voltage was negative in the first step, that means the negative lead of the DMM.  Then, can you verify that when connected like this to the scope, the DMM reads approximately 1Megohm when connected to either channel?

--- End quote ---

My source is the Aneng, hooking it to the other meter pos-pos and neg-neg gives 0.499 V. Checking the bnc, the red jack is the center and the black jack is the outset shield. I've been connecting neg-pos on my measurements with the bnc so far, should I connect pos-pos in the future? And yes, both channels are around 1 Megaohm.


--- Quote from: bdunham7 on July 28, 2020, 07:57:17 pm ---Second, it looks like we are getting some signal through to Q808, but I've no idea what the actual signal level should be.  So go back to XY mode, set the dot in the center to start and do the same measurements as the last step of last time, only on Q809 and at the base, emitter and the other side of R836.  Don't worry about R832 unless it is super easy to get to.  Then we'll compare the voltage swing at the base that it takes for Q809 to deflect the dot all the way from one side to the other to see if the 5-volt or so swing we had at Q808 should be enough to see(in which case Q808 is bad) or if the signal needs to be a lot more (in which case the horizontal final amp is fixed and we move on...).

--- End quote ---

Test of changes when adding a load:
Q809 base: 1.06 V to 1.08 V
Q809 emitter: 0.508 V to 3.34 V
R836 (Q808 side): -2.27 V to 3.46 V

Deflection test at base of Q809:
Left: 3.85 V
Center: 1.09 V
Right: -1.80 V
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