Electronics > Beginners
Took the Plunge - GoldStar OS-7020 Oscilloscope
james_s:
The delta is exactly it, voltage is the potential difference between two points. Think of a bird sitting on a power line, it could be said that the bird is "rated" for a few tens of volts and yet it is able to sit on a 7200V line without consequence. That example is not perfect since it represents an open circuit, but if you had a wire with say 7180V on it nearby, you could safely touch both of those and have just 20V across you as long as you weren't near earth.
olafwagner:
My capacitors arrived today (after another lengthy wait), so I immediately replaced C821 as well as a bodge wire to replace the damaged pad. ('10a' & '10b' below) This went very well, and all the voltages are now in spec, and no more snaps, crackles and pops. Also, I noticed (for the first time today) that when I switch the scope off, the 2 Neon Lamps (that I mentioned in an earlier post) certainly flash a few times after the power is released, so I assume that is working precisely as designed.
However, the scope still basically functions precisely as it did when I first got it, but after all this fiddling and measuring, I think that there are basically only two problems (which still may be related). I think apart from these 2 problems, everything else can be addressed with a good calibration. The problems are:
#1 Trace Intensity
I am still unable to get the trace to get the trace to disappear using either the INTEN pot on the front of the scope, or the INTEN ADJ trimmer inside. I have noticed that while the INTEN pot on the front doesn't make a significant impact on the brightness of the trace (it remains bright), it DOES have an impact on the retrace - interestingly when the INTEN pot is turned to the left (minimum), then the retrace is clearly visible dashing off to the left, however, when I turn the INTEN pot to the right (max), then the retrace disappears entirely. The HV rail measures at -1,831V so that is nicely within spec.
As per my earlier post I need to figure out a way to test the diodes, but the fact that one can get the retrace to dissappear makes it seem like the unblanking (or is that blanking?) circuit is mostly working.
#2 Trace is a 'Dash' and not a 'Dot' (in the X-Axis)
The trace on the display is a dash - almost half as wide as a graticule mark, which you can see in '10g' below. This can be seen clearly when I slow down the trace, but even more so when I 'stop' the trace entirely in X-Y mode. (I intentionally used the flash for that pic because without it the trace look far more 'bloomy' than what it looks like in person)
This is the issue that I more concerned about, and I fear that this is the reason why the previous owner of this scope gave up on it. My fear being that the problem is the CRT itself. While comparing the service manual for a similar Hitachi scope to mine, I see that they included scope diagrams for the X+ and X- signals (which as per my layman's understanding of CRTS are the ONLY thing that deflects the beam on the x-axis). In the case of the Hitachi, the X+ signal is a nice sawtooth, and the X- is a nice corresponding 'upside-down' sawtooth. I hooked up my cheapo-scope to the same 2 points on the horizontal PCB, and I got the exact same clean sawtooth ramps that I saw in the spec (I overlaid my scope images in attachment '10d' below).
I then wondered about X-Y mode, and my fundamental assumption is that this bypasses the sweep gen and chopper circuits, and simply send the x-input signal to the CRT (after it has been attenuated and amplified). When I switched the scope to X-Y mode, I got a flat trace on my cheapo-scope even after I zoomed in a lot (to account for the noisiness of this digital scope's signal) and saw that the signal is flat. (refer to '10g' and '10h' below)
Now the X+ and X- pins on the horizontal board connect directly to the CRT, there are zero components 'upstream'. This means that everything 'downstream' can be excluded from my problem (CH1 input, sweep gen, horizontal amp).
I also hooked up my cheapo-scope to the 18v rail that feeds the HV transformer, so see if there is any ripple, but it also comes through totally clean.
Conclusion (and next steps?)
It is accurate to say that if a CRT gets the suitable HV input, and gets no signal on its X+,X-,Y+,Y- deflection plates, then it should show a 'dot' in the center of the screen right? Also, is the problem here the CRT itself, or can some of the HV circuits (the blanking circuit or HV amp) impact the trace?
After a cursory browse on Ebay I see that there are used replacement CRT's available, but they are far more expensive than what I paid for this scope, and without being sure it is too risky.
olafwagner:
Immediately after posting this I have one more hunch regarding 'Problem #2 - Dash vs Dot' in my prior post.
When I turn the Horizontal position pot, the trace (or dash) move left and right very smoothly, which means that this is affecting the horizontal amp correctly, however, what if there is an 'imbalance' within the horizontal amp itself? What I mean is, assume there is a problem within the horizontal amp, would it not be theoretically possible that the X+ and X- values are not being amplified the same. If this were off, surely this would cause a 'broader' output rather than a dot?
To test this theory I did the following test - now it is a bit unscientific, because all the values are relative (even more, they are relative to the impact of the horizontal adjustment pot) but I wanted to use this to illustrate:
I flipped the scope into XY mode, and moved the trace to the center. Based on my prior post I know that there is a flat (constant) signal being sent to X+ and X-, so I decided to measure this with my DMM to see EXACTLY what was being pushed out of X+ and X-
1. Center Trace
With the trace straddling the center division of the graticule, I measured these voltages (refer to 11a):
X+: 93.4V
X-: 83.8V
2. Right Trace
I then moved the trace so that it straddles the 4 divisions to the right of center, and got (11b, note that the green dash in the picture is merely a 'MS-Paint' illustration of where the 'dash' was):
X+: 125.0V
X-: 51.0V
3. Left Trace
I then moved the trace to the left so that it straddles 4 divisions to the left of center and got (11c, same note on MSPaint as above):
X+: 61.8V
X-: 114.6V
While it makes sense that X+ increases (and X- decreases) as the trace moves further to the right, and vice-versa to the left, my question (and theory) is - maybe the voltages are too far out relative to one another (eg: when the trace is at the center, maybe the 2 voltages should be more similar in order to have less 'deflection'.
Does this seem logical?
If yes, I need to delve into the horizontal amp (schematic attached as '11d' below) and somehow compare what is happening in one 'half' versus the other to see whether any components that should be behaving the same are behaving differently.
(Frankly it feels that I am grasping at straws here, but I don't know where else to turn).
AVGresponding:
It does seem odd that the voltages aren't symmetrical when the trace is centred. What about the Y deflection voltages? Are they symmetrical?
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