Electronics > Beginners
Took the Plunge - GoldStar OS-7020 Oscilloscope
olafwagner:
I have been prodding around the board some more measuring voltages, but I must admit I am at a bit of a loss.
Here is a list of Summarized List of Signs and Symptoms:
- After switching on, it takes 30-35 seconds for the trace to appear (I would expect 10-15 to be more typical, this seems long)
- HV Rail measures -1,650v. (Should be in the range of -1,995v to -1,805v.)
- Intensity Pot (and internal trimmer) cannot get the trace to disappear. (It seems to alter the brightness ever so slightly, but not even close to disappearing.)
- Blanking Signal not getting to the Blanking Amp
- Trace is a 'dash' and not a 'dot' (very obvious in low-speed sweeps or XY mode)
I tried to make some progress on any of these issues by doing the following:
1. Blanking Signal
- I measured a lot of voltages around the blanking amp trying to figure out why the blanking signal never gets 'through' Q621. (all measured voltages appear in red in the attachment below)
- I also pulled Q622 off the board and popped it into my component tester, and it is also recognized correctly.
- I noticed in the Hitachi spec that they also have an oscilloscope diagram for what I assume is the output of the blanking amp (before it hits high-voltage), which I am calling "P3". When I attached my scope there I only get a flat signal which I have measured to be 13.5V
- I also tried to measure diode CR621 (1SS83) in circuit with my DMM in continuity mode, but I do NOT get a 'beep' in either direction. Should I get a beep? Or must these be removed from the board and plugged into a component tester to test?
- Lastly I ordered a couple of replacement 2SC1907's from Ebay, so I could simply replace Q621 and Q622 and see whether that does anything.
2. Intensity Controls
I previously measured all the voltages around the INTEN pot (and included the measured voltages on the schematic)
- Today I measured all the voltages around the internal INTEN ADJ. trimmer. I assume they appear correct. I stopped short of diode CR516, because that is inside the high voltage area of the board that has a nice protective shield over it.
3. HV Rail
Here I am seriously out my depth (and do not want to electrocute myself) so I can only do very little testing.
- I measured a few voltages on the input side that goes to the HV transformer (just to make sure the 18v rail was getting here properly)
The 'simplest' issue that I need to progress on is the blanking signal, so my only hope there at the moment is simply replacing Q621 and Q622 and see what happens.
james_s:
If those are HV diodes then the forward drop will be too high to measure with the diode check on a DMM.
Something else that would be worthwhile to test is the CRT itself, check to make sure there are no shorts between elements that should not be connected.
olafwagner:
OK, things just got weird.
I wigged the CRT socket connector a bit (to make sure it is properly connected, and reinstalled transistor Q622, reinstalled the horizontal board into the case and turned it on.
Now all of a sudden I have a VERTICAL problem. The trace is making a vertical line that is the thickest at the center graticule, and if I fiddle with vertical position, only the length of the thin line at the top gets longer and shorter. The fatter part remains in the center of the screen.
Also (and more alarmingly), I re-measured my HV rail, and now I get -2,302v!
What the heck is going on.
james_s:
That's odd, sounds like the CRT socket and associated wiring needs a careful inspection.
olafwagner:
I carefully unplugged and removed the CRT, and checked all of the pin combinations. The only combination that I got a 'continuity beep' on were pins 1 & 14, which I believe is the heater, so I assume that is correct. Also, with nothing visibly wrong with the CRT and nothing rattling around inside, I carefully installed it again.
I checked the CRT socket board and all the pins and harness, but I cannot find anything wrong here. (All continuity beeps out, nothing seems shorted)
I then turned my attention to the HV circuit (because something there must be causing this spike to -2,302v. I opened up the cover to the HV section, and checked all the resistors (they all check out fine within spec), but noticed resistor R697 was missing. I then removed the PCB and looked underneath.
Firstly I discovered that I am certainly not the first here. There is some liquid residue on the board, which I assume is flux that someone before me didn't clean off. It also looks like a lot of the solder points may have been remelted (perhaps whoever had this scope before me assumed there were dry solder joints.
Secondly I found resistor R697 installed underneath (perhaps by the same person that left the other evidence). However, even though the schematic calls for a 3M resistor, there is a 1M resistor installed. This resistor is part of the FOCUS circuit, and the resistor on the other side is a 16M, so assume the 1 vs 3M won't make a huge difference, and is probably not related to my sudden high voltage issue.
Should I remove and test all the 2KV and 3KV ceramic capacitors and test them? (I assume Ceramic caps are very reliable?) Should I remove and test the diodes? (I assume these are far too high voltage to be able to 'beep' in circuit)
Also, what role do the 2 Neon Lamps (NL601 and NL602, type NE-68 which is either an old or unobtainium model) play? I see they are wired across the Blanking and Cathode rails (in series with one another), is this some kind of voltage regulation? How would I go about checking them? Should I see them 'glow'? (because I don't)
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