Electronics > Beginners
Took the Plunge - GoldStar OS-7020 Oscilloscope
james_s:
I bet they are. The fact that the intensity pot doesn't do anything clearly indicates that something is wrong. Can you find a schematic for the scope? Even a the schematic for a vaguely similar scope might be enough to give you an idea. At the very least you should be able to find a pinout for the CRT and you can look at the intensity and blanking circuits. Look around for open resistors or a shorted transistor. At the very least figure out the intensity problem before you bother troubleshooting anything else.
olafwagner:
I am still working on tracking down a schematic for this EXACT model. The closest I have is the GoldStar OS-7020A, which seems to be very very similar, but all the component numbers are slightly different than what I see on my PCB's.
I attached a schematic (below) for the H.V. CRT Drive, where I have tracked down the 'INT ADJ' trimmer in the model, and the connector to the INTEN pot towards the left.
Unfortunately I am not skilled enough to know where the Intensity and Blanking circuits are (or what they even do), therefore the only way I would go about this would be to work my way backwards from the pot and trimmer and check each resistor along the way.
olafwagner:
I have a few questions for those who are able to read schematics like 'sheet music' :-)
(With reference to the attached schematic with highlights)
- In order to handle this PCB safely, I assume I only need to ensure that I discharge C691, C692 and C697 (They are all circled in red, and they are high voltage caps and looks like 1000F each)
- I assume all the 0.01u caps (circled in yellow) are of no concern due to low capacitance values
- I understand the +12V and -12V rails on the schematic, but in your experience, what does +12DEC and -12DEC mean? I see there is a small schematic (lower left, circled in pink) which looks like +12DEC is simply +12v passed through a 10 ohm resistor and a 22u cap to ground. Does 'DEC' mean 'decreased' or maybe 'decoupled'? What does that mean and what is typically the purpose?
- How does Q612 (circled in orange) work in this configuration? The base is permanently grounded so I guess this transistor never switches on and off? Is it always on?
- I am trying to decipher what the upper portion of the schematic (circled in light-blue dotted) is supposed to do, but frankly I have no idea. I assume this has something to do with blanking, because there is a connection to the 'Ext Blanking Input' (connector on the rear of the scope) to the left, and eventually connects to P615 Pin 5, which is labeled as the 'Blanking' pin to the CRT
This is going to be a fun journey. I wish I knew more, but as long as I learn something every day, eventually I will get my head around this.
james_s:
I haven't seen that 'DEC' nomenclature before but looking at the circuit I think your guess of "decoupled" is absolutely right. The resistor and capacitor form a low pass filter and prevent transients on one side from affecting the other, it's a bit like the spring and shock absorber on your car. As long as you have the specified voltages present I wouldn't worry about that right now. I would start at the two intensity pots, first check the voltage on each of the two outer pins relative to ground and verify that it's reasonably close to what's shown in the schematic, if it's not then we can start looking to see if the circuit you have is different or if there's actually a fault there. Then check the voltage at the center pin of each of those pots, it should vary between the levels on the side pins. Not sure what that transistor with the base grounded is doing, I'll have to think about that, maybe one of the analog experts here will know.
AVGresponding:
Q612?
Its emitter is biased to -12v, so the base is effectively +12v with respect to the emitter...
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