Author Topic: Took the Plunge - GoldStar OS-7020 Oscilloscope  (Read 7950 times)

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Offline olafwagnerTopic starter

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Took the Plunge - GoldStar OS-7020 Oscilloscope
« on: January 05, 2020, 04:08:27 pm »

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.

 

Offline james_s

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Re: Took the Plunge - GoldStar OS-7020 Oscilloscope
« Reply #1 on: January 05, 2020, 05:24:15 pm »
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?
 

Offline olafwagnerTopic starter

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Re: Took the Plunge - GoldStar OS-7020 Oscilloscope
« Reply #2 on: January 05, 2020, 06:05:54 pm »

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.

 

Offline RoGeorge

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Re: Took the Plunge - GoldStar OS-7020 Oscilloscope
« Reply #3 on: January 05, 2020, 06:18:24 pm »
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.

Offline olafwagnerTopic starter

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Re: Took the Plunge - GoldStar OS-7020 Oscilloscope
« Reply #4 on: January 05, 2020, 06:45:32 pm »

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.

 

Offline RoGeorge

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Re: Took the Plunge - GoldStar OS-7020 Oscilloscope
« Reply #5 on: January 05, 2020, 06:53:24 pm »
That's astigmatism, no doubt, and a very strong one.  That is why the vertical sections are not visible at all, and also the cause upper and lower horizontal sections are overlapping on the horizontal direction.

Online nfmax

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Re: Took the Plunge - GoldStar OS-7020 Oscilloscope
« Reply #6 on: January 05, 2020, 06:59:30 pm »
It looks like you have the intensity set too high - if you turn it down, the retrace should disappear. That may also improve the astigmatism. Is there an astigmatism adjustment on the back panel, or through a hole in the side cover? It was often just a screwdriver operated control.

Edit to add: if the calibration is off in both axes, and the retrace blanking isn't working properly, and there is astigmatism, that all points to an HT/EHT fault somewhere. However, the focus looks OK, and the trace is bright, which makes it unlikely to be EHT.
« Last Edit: January 05, 2020, 07:02:40 pm by nfmax »
 

Offline rstofer

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Re: Took the Plunge - GoldStar OS-7020 Oscilloscope
« Reply #7 on: January 05, 2020, 07:43:34 pm »
Not only is your trace "blooming" with excessive brightness, a 20 MHz scope won't be capable of displaying the rising edge of any decent square wave source.  You might change the time per division such that you only display the area close to the rising edge.  That may help see it but I kind of doubt it.

None of this matters.  Sure, a 100 MHz DSO is a higher end tool (and it is considered 'entry level') but I got along for a very long time with a Heathkit 10 MHz scope.  Given the emphasis on Arduino projects, 20 MHz will be fine.

Enjoy!
 

Offline james_s

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Re: Took the Plunge - GoldStar OS-7020 Oscilloscope
« Reply #8 on: January 06, 2020, 01:56:38 am »
Even a 5MHz scope will display what looks like a reasonable rising edge of a square wave, you just won't be able to accurately measure the rise time. I had a 5MHz Heath scope when I was a kid and it would show a nice clean looking square wave of at least a few tens of kHz. Looking at a square wave on an analog scope of any sort you will mostly see the flats on the top and bottom, the rising and falling edges will be faint but visible when the scope is operating correctly. I think whatever is causing the severely out of round spot in this case is most if not all of the issue.
 
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Offline olafwagnerTopic starter

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Re: Took the Plunge - GoldStar OS-7020 Oscilloscope
« Reply #9 on: January 10, 2020, 11:08:11 pm »

I scoured the boards and found the ASTIG trimmer on the Power Supply Board, and the INTEN ADJ on the Horizontal Board in the 'CRT Drive' section (they have divided the board into logical groups with handy silkscreening) (see pics below)

After fiddling with both of these trimmers (as well as the 'Focus' and 'Inten' controls on the front, I am still facing a few problems:

- I still cannot get the XY down to a 'dot', it remains a 'dash'.
- The 'Inten' control on the front panel isn't doing anything (it doesn't noticeably make the trace brighter or dimmer. Similarly, turning the INTEN ADJ trimmer on the board also doesn't seem to change anything.

I wonder whether these problems are related?

As an aside they also handily silkscreened some of the output voltages for the power supply, so I tested the 5v, 12v, -12v, 50v, 160v and 180v rails and they all seem to be very close. (I haven't measured the HV because I don't have a HV DMM Probe yet)

 

Offline james_s

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Re: Took the Plunge - GoldStar OS-7020 Oscilloscope
« Reply #10 on: January 11, 2020, 01:06:53 am »
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.
 

Offline olafwagnerTopic starter

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Re: Took the Plunge - GoldStar OS-7020 Oscilloscope
« Reply #11 on: January 11, 2020, 03:51:51 am »

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.
 

Offline olafwagnerTopic starter

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Re: Took the Plunge - GoldStar OS-7020 Oscilloscope
« Reply #12 on: January 11, 2020, 04:57:54 pm »

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.
 

Offline james_s

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Re: Took the Plunge - GoldStar OS-7020 Oscilloscope
« Reply #13 on: January 12, 2020, 12:22:57 am »
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.
 

Offline AVGresponding

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Re: Took the Plunge - GoldStar OS-7020 Oscilloscope
« Reply #14 on: January 12, 2020, 11:11:51 am »
Q612?

Its emitter is biased to -12v, so the base is effectively +12v with respect to the emitter...
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Offline Brumby

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Re: Took the Plunge - GoldStar OS-7020 Oscilloscope
« Reply #15 on: January 12, 2020, 12:22:52 pm »
I picked up this exact model late last year - with the service manual - and Olaf has already PM'd me about getting a scan of it.  I've been busy lately, but I should have a chance over the next 2 days to get that done.

It has several schematics.
 
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Offline Brumby

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Re: Took the Plunge - GoldStar OS-7020 Oscilloscope
« Reply #16 on: January 16, 2020, 10:58:28 am »
Hope this will help you out.  (I don't think I missed anything  :D )

https://www.dropbox.com/s/rybj78uwiksgupo/Goldstar_OS-7020_Service_Manual.pdf?dl=0

File size is around 19MB - but I feel this is on the high side.  If anyone can recommend some PDF wrangling software that might deliver a smaller file size, I'd be interested.

I've presented it as it was in the hardcopy, so viewing on screen should be fine, but printing it out as is might not give you what you expect.
« Last Edit: January 16, 2020, 12:06:09 pm by Brumby »
 
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Offline olafwagnerTopic starter

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Re: Took the Plunge - GoldStar OS-7020 Oscilloscope
« Reply #17 on: January 16, 2020, 03:57:53 pm »
Thank you very much, this is absolutely brilliant.

I am going to immerse myself and try to figure things out. This will make for some excellent 'bedtime reading' tonight :-)
 

Offline Brumby

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Re: Took the Plunge - GoldStar OS-7020 Oscilloscope
« Reply #18 on: January 17, 2020, 01:13:41 am »
Glad you like it.  I did try for a decent result - but that's just me.

Enjoy.
 

Offline rsjsouza

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Re: Took the Plunge - GoldStar OS-7020 Oscilloscope
« Reply #19 on: January 17, 2020, 01:32:49 am »
Brumby, can you try to upload to K04bb site? I have downloaded very high resolution scans there (600dpi grayscale for schematics) and the site is a very precious aggregator of manuals.

P.S. I am on my cellphone, otherwise I would do it.
« Last Edit: January 17, 2020, 01:35:06 am by rsjsouza »
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Offline Whales

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Re: Took the Plunge - GoldStar OS-7020 Oscilloscope
« Reply #20 on: January 17, 2020, 01:43:25 am »
Holy crapola thankyou Brumby.  I have the 7040A model (40MHz) and I was coming here to say I've never had any luck finding schems.

olafwagner: DC drifts on mine slowly as it warms up, but apart from that it's in perfect working order.  Shoot me if you want photos or measurements of any parts.

EDIT: previous topic RE my scope
« Last Edit: January 17, 2020, 01:47:38 am by Whales »
 

Offline Brumby

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Re: Took the Plunge - GoldStar OS-7020 Oscilloscope
« Reply #21 on: January 17, 2020, 03:13:15 am »
Brumby, can you try to upload to K04bb site? I have downloaded very high resolution scans there (600dpi grayscale for schematics) and the site is a very precious aggregator of manuals.

P.S. I am on my cellphone, otherwise I would do it.

Done.

I changed the file name to:

Goldstar_OS7020_Oscilloscope_Service_Manual.pdf

... to follow the preferred convention for that site.
 
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Offline olafwagnerTopic starter

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Re: Took the Plunge - GoldStar OS-7020 Oscilloscope
« Reply #22 on: January 21, 2020, 03:17:55 am »

OK, I have started digging into this further now that I have the service manual (many thanks again Brumby, it is perfect).

1. Power Supply
I checked all the low voltage rails, and they are all within spec. Now that I received my HV probe, I checked the HV rail and it seems to be a bit low (or is that high?). According to the service manual, it is supposed to fall between -1,995v to -1,805v but I am measuring -1,650v. I hope I am doing this correctly, is this a DC voltage (service manual doesn't say), and I connected the HV Probe ground terminal to chassis ground. Is this too far out of spec?

2. Working Backwards from Intensity Pot
After looking at the 'HV CRT Drive' schematic, I started at P604 Pin 4 and measured voltages. Turning the pot sweeps from -6.6mV (~0v) through to -11.76V (effectively -12v rail) so I believe the pot works correctly.

While I was scouring the internet for this service manual (prior to getting it from Brumby) I discovered that the Hitachi V-212 is a very similar scope with pretty similar schematic layout. In their service manual, they handily include some interesting scope diagrams for various points on the board and there I saw that they included a diagram for the spot between the 2 nearest transistors to the intensity pot. (I attached it below and highlight the scope diagram in question)

I immediately hooked up my 'cheapo-scope' to my board, and made an interesting discovery. When I connect 'downstream' of transistor Q621 at a point I labeled "P1" in the attached image, then I get a similar signal (is this the blanking signal?)

However, if I connect the scope to "P2" (between the 2 transistors, similar to the Hitachi scope), then I just get a constant flat 1.34v

Does that seem right?
 

Offline olafwagnerTopic starter

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Re: Took the Plunge - GoldStar OS-7020 Oscilloscope
« Reply #23 on: January 21, 2020, 03:19:10 am »
Doesn't seem the last 3 images made it, here they are...
 

Offline AVGresponding

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Re: Took the Plunge - GoldStar OS-7020 Oscilloscope
« Reply #24 on: January 21, 2020, 06:11:34 am »
Looks like q621 might be toast, that constant 1.34VDC is about what I'd expect to see from the bias resistors of q622, there should be a blanking signal at p2 as well as p1
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