Electronics > Beginners
Took the Plunge - GoldStar OS-7020 Oscilloscope
olafwagner:
Hi All,
I have been a longtime 'watcher' of EEVBlog, but am new to the forum. I have been an electronics enthusiast for a long time, but decided to 'level-up' to a scope. I took Dave's advice (from one of the videos) to be willing to cut my teeth on an analog scope, so when I stumbled onto a dirty GoldStar OS-7020 2-channel 20Mhz scope, I went for it (even though it didn't come with any probes, power cords or documentation at all). I attached a picture below.
I have cleaned it up, and the scope looks much better and it is working, but it does need urgent calibration. When I switch it to XY mode I don't get a 'dot' but rather a 'dash' on the screen that I can move around.
At this point I am trying to track down a service manual - because I don't want even think about 'experimenting' by turning trimmers without knowing which does what.
Anyway, happy to have joined this community.
james_s:
That isn't a calibration issue. Either you're picking up noise on the X input or the astigmatism of the beam is way off. I don't see an astigmatism control on the front panel so I'd first look at the noise possibility. Is the V/div set the same on both channels? What happens if you short out the X input?
olafwagner:
Yes, the v/div is set the same on both channels, and have even shorting x-input has absolutely no effect. So I am hoping it is simply astigmatism, but I don't have the service manual yet, so I don't know where to 'fiddle'.
That being said, I just found the service manual for the GoldStar OS-9020, and while they are somewhat different scopes, that manual might show me the 'region' of the astigmatism control, and then I'll try to find the same on the 7020's schematics.
RoGeorge:
To see if it's because of the spot's astigmatism, connect the calibrating signal to one channel (with a simple wire if you don't have probes) and look at the raising and falling edges of the signal. The trace on the raising/falling sections should be even thinner than the horizontal regions of the trace.
The astigmatism can be seen even better with a sinusoidal signal, but the square calibrating signal would do it, too.
olafwagner:
The calibrating signal highlights a number of issues:
- There are no visible vertical lines just horizontal ones
- The lines overlap a bit (I assume this also points to an astig. issue)
- Calibration: This signal is .5v and my scope is set to .1v/div so there should only be 5 divisions yet it shows 6 (similar calibration issue on the time axis, it is slightly out)
- There is a visible 'retrace' at the bottom (is that what it is called when the beam returns to the left?)
- The image is a bit 'bowed'
- There is a slight 'overshoot' on the voltage (I assume this is somewhat normal with analog scopes)
For reference, I also hooked up the Goldstar's test signal to my cheap digital scope (please excuse the dust) where I can see that the calibration signal being generated is pretty clean.
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