Electronics > Repair
BK Precision 2120 trace issues
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vexatag:
Oh and R825 was 8.6k
vexatag:
Measurements!
Checking to ground:

* Left pin: 825 mV
* Right pin: 38.2 V
Neg on left, pos on right:

* X-pos neutral: 146 V
* X-pos clockwise: 73 V
* X-pos counterclockwise: 220 V
* X10 mode neutral: 115 V
* X10 mode clockwise: 1.3 V
* X10 mode counterclockwise: 260 V
Slowing the sweep:

* X-pos neutral: 141 V
* X-pos clockwise: 145 V
* X-pos counterclockwise: range between 136 V and 138 V
* X10 mode neutral: 141 V
* X10 mode clockwise: 1.3 V
* X10 mode counterclockwise: range between 136 V and 138 V
Measuring the left leg of R834 (when viewed from the front of the scope) at 0.5mS/div in XY mode gave about 1.6 V through the entire x range and X10 mode, but at 0.2S/div in normal mode gave -1.42 V at neutral and -2.32 V when the xpos knob was rotated fully clockwise.
bdunham7:

--- Quote from: vexatag on July 24, 2020, 11:59:48 pm ---Measuring the left leg of R834 (when viewed from the front of the scope) at 0.5mS/div in XY mode gave about 1.6 V through the entire x range and X10 mode, but at 0.2S/div in normal mode gave -1.42 V at neutral and -2.32 V when the xpos knob was rotated fully clockwise.

--- End quote ---

I meant the left side on the schematic, IOW the side that is not connected to the base of Q808.  It would help if you had a signal generator that could feed in a very slow ramp or triangle wave of about 1 volt and 0.1 Hz, but the sweep generator in the 0.2S/div mode will maybe do.  What I'm looking for is a smooth change from low to high that repeats itself.  This may be difficult to spot depending on how fast your meter updates.
vexatag:
Oops, my bad. Doing the proper test (checked the leg labeled B for continuity with either side of R834, then hooked the positive lead to the leg without continuity and the negative lead to the chassis), it started at about -15 mV but has been falling over about three or four minutes. I wish I could have a visual of this data, I understand now why they say you need a scope to fix a scope.
bdunham7:

--- Quote from: vexatag on July 25, 2020, 05:20:55 am ---Oops, my bad. Doing the proper test (checked the leg labeled B for continuity with either side of R834, then hooked the positive lead to the leg without continuity and the negative lead to the chassis), it started at about -15 mV but has been falling over about three or four minutes. I wish I could have a visual of this data, I understand now why they say you need a scope to fix a scope.

--- End quote ---

Yes, another scope is the best way!  What did you measure R834 at?  It is only a 68R resistor so I would not expect a continuity meter to show open. 

So what we are trying to do is trace signals.  If you look up at the Trig & Sweep section, you'll see that S605 is your XY MODE switch.  In normal mode, the sweep generator sends a ramp signal to the horizonal final amp and that is amplified and sent to the horizontal deflection plates causing the beam to sweep from left to right.  In XY mode, the CH2/X signal is fed instead, causing the beam to move according to the signal.  Since neither of these functions work and they are at least partly independent of each other, I was going on the assumption that your problem lies somewhere in the horizontal final amp.  Now we should take a step back and verify as many of those assumptions as we can, and tracing a signal into R834 is on very important point.  The other would be verifying as best we can that the CRT is responding to the horizontal deflection voltage.  So, let's try a few things.

First, unplug PG106 and power up the scope.  Where is the dot?

Second, tell me what equipment you do have, other than your DMM.  Power supplies, electronic parts such as potentiometers, batteries, wire, solder, breadboards, etc etc.  We need to rig up a signal we can trace through the scope.  Even a battery and a 10K potentiometer might work.
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