EZ OS-5020 Oscilloscope Not working right. - Page 1
Forum section: electronics / repair

author: coolduder1001
author_rank: Contributor
timestamp: August 08, 2021, 09:21:25 pm
content: This is my first oscilloscope and I've already run into an issue. When I tried to use it for the first time I don't get a line across the screen, and when I wired CH1 to to the test point next to the power switch I could just faintly see something moving in the line. It's almost like everything in the vertical space was squished into the middle. Also the intensity knob and X pos. knob do nothing. I took the scope apart to see if anything was obviously blown and I can't find any issues, other than one resistor (r428) has a little discoloration at it's joints. Other than that I don't see any obvious issues and I don't really know where to go from here.I have a folder here with pictures of the MB and a video of me trying to operate the front:https://drive.google.com/drive/folders/1JYNL4ASaM53rDvzh0avbxfCi7vpnpRsL?usp=sharingThank you for any help.

author: wn1fju
author_rank: Frequent Contributor
timestamp: August 10, 2021, 12:28:01 am
content: 1. Try to find a service manual online with schematics.2. A couple of your pictures seem to show a connector that isn't plugged in (did you disconnect it intentionally for the photo?).3. The first step in the repair of pretty much any piece of test equipment is to check the power supply voltages. Look for sagging DC voltages and/or excessive ripple on each supply rail. There is a good chance that one or more electrolytic capacitors have failed.4. Be careful where you stick your hands. The CRT uses enough high voltage to zap you pretty good. If you don't know what you are doing, STOP.5. If all the supply voltages check out OK, you have to start debugging the various vertical and/or horizontal circuits. Unfortunately, the best way to do this is with a separate (working) oscilloscope.

author: coolduder1001
author_rank: Contributor
timestamp: August 10, 2021, 04:40:30 am
content: 1. I tried finding schematics for this scope with no luck. I found one for the os-9020 which is completely different since the horizontal and vertical have dedicated boards on that model. All I could find was the manual for the scopehttps://www.testequipmentdepot.com/ezdigital/pdf/os5020-5020c_manu.pdf2. Yes, I forget to put that back. I was checking the power supply voltages while it was disconnected, I forgot to write down the readings.3. I did check the power supply voltages while all the wires were connected and I have it starting at R801A going down. From what I could find these are not the correct voltages but I can't find out what is. Someone has a similar issue but with different voltageshttps://www.electronicspoint.com/forums/threads/repair-help-on-lg-os-5020c-oscilloscope.276494/. I did check the capacitors on the power supply board with my component tester and they were all within spec with no bulging or leaking. From what I can remember running the board disconnected from the mainboard the voltages looked more normal, but I still have no ide if they were.1 - (260v)2 - (5v) -This one should be correct3 - (20v)4 - (GND) -This one should be correct5 - (-20v)6 - (0v)7 - (210v)8 - (0v)4. I know CRTs are scary so I'm avoiding it as much as possible. I don't think it's an issue with the tube itself anyway.5. Well the power isn't ok sooooo...PS: I am a bit concerned about that resistor (R428) next to the connector I disconnected. It looks like it got really hot and discolored the board a little. I want to look for replacement just in case, but I can't really tell what kind it is. I also can't tell the color to good so I can't read the color code.

author: wn1fju
author_rank: Frequent Contributor
timestamp: August 10, 2021, 11:47:15 am
content: It is probably very similar to the OS-9020, so I would use those voltages as the "normal."All of your voltages look quite a bit high (except the ones that are at zero!). For instance, you should be getting +12 and -12, not +20 and -20 volts.I would disconnect all the P/S connectors to the boards and then check the voltages being produced. Check them for DC and for AC. Excessive AC points to an electrolytic and/or diode problem. Next, I would check the external (I think there are 3 of them) pass transistors for shorts. For instance if you have 20V being input to the 12V regulator circuit via the pass transistor, and the transistor is shorted, you could get 20V out. Note: I had a similar EZ-scope and was getting 15 and -17.5V instead of 12 and -12V. All I got was a small circular trace on the left edge of the screen. Once I corrected those voltages (pass transistor short), the scope worked fine.If all the open circuit voltages are OK, then go the corresponding connectors on the various boards and measure the resistance to ground for each pin. You want to know if there is something on a board that is dragging down the P/S.It may be that the discolored resistor is a low ohm current sense resistor. If the voltages are too high, or the current draw is too much, and the regulators aren't working, you could stress that part. Most overheated resistors I've seen actually measure fine, so measure it with an ohmmeter and see what you get.I wouldn't run the scope for too long with those measured voltages. They may inadvertently stress some other parts.

author: Chris56000
author_rank: Frequent Contributor
timestamp: August 10, 2021, 07:02:12 pm
content: Hi!I posted the schematics for this Oscilloscope about two or three weeks ago!https://www.eevblog.com/forum/repair/htc-os-5030-oscilloscope/Chris Williams

author: coolduder1001
author_rank: Contributor
timestamp: August 11, 2021, 03:14:25 am
content: Ok I checked the voltages again with the power supply disconnected.1 - (230v)2 - (5v)3 - (22v)4 - (GND)5 - (-22v)6 - (0v) -----7 - (150v) |--- (2.5v AC)8 - (0v) -----The 7805 (15v in 5v out), 7812 (20v in 12v out), 7912 (-20v in -12v out). The last chip on the heatsink is a 2SA1668 transistor and I don't know much about how it's wired, I don't see any voltages on it when it's on

author: perieanuo
author_rank: Frequent Contributor
timestamp: August 11, 2021, 05:17:39 am
content: hi, from op seems your vertical deflection is dead, trace the power supply for vertical boardso focus first on this vertical board i see in sm for OS-9020G page 60 the voltages are +-12VDC, so maybe this module or his input are the culprit (i didn't watched the video but the photo says V deflection is dead)higher voltages means somebody doesn't operate and of course V goes higheri'm not sure the schematics are correct for your model, but may be used as guidance for your case, usually they just 'tune' a little bit between models of same product series.please refer to that manual if you can, it's simpler for us to trace this than to trace R801A or R428 in some old pdf, we can't search text in that pdf, at least not after 1 hour of ocr work on that pdf.or draw on paper what you reverse engineernow i'm sure the schematic 134639836-LG-OS-5020G-Oscilloscope-Service-Manual.pdf is not the same, you have some picture with P210 connector and R428 comming from pin 6/P210 and in the schematic page 59 we have R428 2K tied to +12V rail. if any resistor is fried there should be R429, but in that V defl preamp i doubt resistors are hot.i'll dig a little more see if i got something valuable

author: coolduder1001
author_rank: Contributor
timestamp: August 11, 2021, 05:22:30 am
content: I just compared it to the os-9020 diagrams and they're the same. I can tell cause the power board for both the one you sent (465F8-8.PDF) and the lg os-9020 are a perfect match. Plus on my board there is only one 7805 on the PS itself and no transistors. Both of these are also missing part (NL801) I have no idea what this component is. It looks kind of like a Christmas light with two metal rods in it, it also flashes when you turn the scope on and I don't know if it's supposed to do that.

author: coolduder1001
author_rank: Contributor
timestamp: August 11, 2021, 05:25:53 am
content: I don't think this os-9020 schematic is helpful at all. Because my PS does output +-20v, but the voltage regulators are moved to the main board except for the 7805. The 7812, 7912, and 2s1668, are all on the mainboard.I just added some close-ups of the PS board to the folder.

author: perieanuo
author_rank: Frequent Contributor
timestamp: August 11, 2021, 05:50:33 am
content: if you think schematic is ok, do you have the right voltages see page 67 Power Supply (14)?vertical deflection preamp/amp works with those +-12Vdc,+140Vdc, +5Vdc see connector W705and btw, +-12V are regulated voltages, founding +-20V instead is not ok at all

author: Labrat101
author_rank: Unknown
timestamp: August 11, 2021, 05:54:39 am
content: It looks kind of like a Christmas light .It's a spark gap . Or neon stabilizer . Used to stop high voltage surges. Momentary flash at power on is not uncommon.

author: coolduder1001
author_rank: Contributor
timestamp: August 11, 2021, 06:19:46 am
content: I said it's not helpful. Look at the pictures of the PS board and tell me if that matches the schematic.

author: Labrat101
author_rank: Unknown
timestamp: August 11, 2021, 06:46:58 am
content: As mentioned above .You have a bad ceramic cap in the vertical . . The circuit diagrams. Is not complete .I will look tonight when I have a large screen to look closer.Also what voltage are you . 115v or 240v ?

author: perieanuo
author_rank: Frequent Contributor
timestamp: August 11, 2021, 07:07:37 am
content: Quote from: coolduder1001 on August 11, 2021, 06:19:46 amit is helpful, it doesn't need to be identical, is totally ok for guidancei always pointed youQuote, your board and 9020 schematic are a little different, but that doesn't mean you can't use those schematics to find the problemsomeone with 5020 found different voltageshttps://www.edaboard.com/threads/repair-help-on-lg-os-5020c-oscilloscope.347582/, CHECK PS like i said on my last post and reportdon't search for 'bad ceramic cap' or any other gizmo before validating 100% the supply voltages, and so far you haven't found the correct values expected. each thing at the time, learn the correct approach to repair electronics

author: coolduder1001
author_rank: Contributor
timestamp: August 11, 2021, 07:51:12 am
content: I'm using 115v.

author: coolduder1001
author_rank: Contributor
timestamp: August 11, 2021, 08:01:52 am
content: CHECK PS like i said on my last post and reportThat's what this is.1 - (230v)2 - (5v)3 - (22v)4 - (GND)5 - (-22v)6 - (0v) -----7 - (150v) |--- (2.5v AC)8 - (0v) -----These are the voltages I got when I disconnected the Power Supply board from everything else in the case. I can't see how the components on this board will give me a +-12v. Unless the transformer after it's rectified gives out a perfect +-12v.

author: perieanuo
author_rank: Frequent Contributor
timestamp: August 11, 2021, 08:12:37 am
content: it doesn't matter 220 or 110 or 50/60hzi repeat, page 67, you have to check those voltages or at least try to reverse that part, for example diode D924 will tell you the correct regulated voltage, i doubt they switched from 12 to 20V, but anything is possible

author: perieanuo
author_rank: Frequent Contributor
timestamp: August 11, 2021, 08:15:03 am
content: btw, i don't see that +55Vdc they mention in page 67, just near the +140Vdc

author: ambrosia heart
author_rank: Regular Contributor
timestamp: August 11, 2021, 08:37:15 am
content: LG = EZ OS 5020.Comparing voltages and see photos.

author: Labrat101
author_rank: Unknown
timestamp: August 11, 2021, 11:48:06 am
content: This is the service Manual for your Scope . This scope was build by LG and was rebrandedAlso known:LG PRECISION GOLDSTAR OS-5020 OS5020 OS 5020https://elektrotanya.com/lg_precision_goldstar_os-5020_oscilloscope_sm.pdf/download.html#dlPlease enjoy . All your Power supply voltages are out . and the 56 volt could well be your problem .PS. I am not sure what country you are from . But please check your Grammar before posting as you have given quite a few negative answers . We are all he to help . We can't actual see what's on your bench and what test equipment you have at hand . I am also assuming you are a novice in electronic by some of your questions . Yes it is frustrating when something goes wrong .As already said the power supply is the first priority .There are also differences in Models as in version numbers. So some components maybe changed due versions update or batchhttps://elektrotanya.com/lg_precision_goldstar_os-5020_oscilloscope_sm.pdf/download.html#dl

author: ambrosia heart
author_rank: Regular Contributor
timestamp: August 11, 2021, 12:12:08 pm
content: https://elektrotanya.com/lg_precision_goldstar_os-5020_oscilloscope_sm.pdf/download.html#dlWrong schematics. NO OPAMP in power supply.

author: Labrat101
author_rank: Unknown
timestamp: August 11, 2021, 12:21:32 pm
content: Quote from: ambrosia heart on August 11, 2021, 12:12:08 pmLook Again there is . even in his photo ..

author: coolduder1001
author_rank: Contributor
timestamp: August 11, 2021, 05:49:37 pm
content: WOW it looks like you have the actual manual, I can't find that thing anywhere. I just get people sending me the os-9020 manual thinking it's the os-5020.

author: Labrat101
author_rank: Unknown
timestamp: August 11, 2021, 06:59:18 pm
content: Quote from: coolduder1001 on August 11, 2021, 05:49:37 pm9020 read page 3 your scope is upgradable the PCB is similar ie as OS-9000 ,3000srs seriesIf you read page 3 it has the versions and corrections list . and models coveredPlease Note the schematic is correct but you will find component numbers may differ . due to version updates .Also the AC board is not shown as being separated in the schematic . The main power control is on the main board .Sorry you will have to do a bit of trace work .There will also be components layout differences ie your Hot resistor 428 on some models its 2 resistors in parallel .I hope this helps . also read the section explaining how sections work .Have fun & good luckPS update I would check the 2SA1668 as this supplies the Vertical voltages . This transistor has probably gone leaky or failed causing the resistorR428 to run Hot

author: coolduder1001
author_rank: Contributor
timestamp: August 11, 2021, 10:12:17 pm
content: That's kinda what I guessed that's why I mentioned it here.Quote from: coolduder1001 on August 11, 2021, 03:14:25 am

author: Labrat101
author_rank: Unknown
timestamp: August 11, 2021, 10:37:52 pm
content: Quote from: coolduder1001 on August 11, 2021, 10:12:17 pmAlso your Christmas tree light as you called it is a Neon 90 volt regulator and its connected to the collector of that transistor . which has an isolated base they fail . The neon should flicker when the transistor is working there maybe another resistor in series with it . There is also a small chance it damaged the opamp driver . which has a diode protection . check the diode as well .and there are some 22uf 50v caps . I can't remember the exact values but check the small caps in that area as well . one may have a pore ESR.