Analog oscilloscope trace unstable and fading after seconds (Kikusui COS5040TM) - Page 1
Forum section: electronics / repair

author: FoxEcho
author_rank: Newbie
timestamp: December 30, 2020, 02:45:58 am
content: Hi everyone.Today I got an old analog oscilloscope, a Kikusui COS5040TM. It's not working properly, and I'll describe the problem:First off, when turning it on for the first time, the trace was really noisy, even with the input in GND position, for the two channels. Every trace also has two bright spots in the left of the screen.After a couple of minutes, the traces started to face until they totally disappeared.I can get the traces back on if I turn on the scope and let it rest for a while. After that, the traces take a bit of time to appear again, and then again fade away.Oh, and one more thing. This is pretty weird, but If I have the scope upside down, the traces don't appear at all, until I hit it with my finger. Then they appear, and again, go away like the first time.While I could see the traces, I hooked the two channels to the 1 kHz cal signal, and they do show in the screen, always with those two bright spots on the left side.Edit: The weirdest thing happens when I put the scope upside down to check the bottom board. The traces just stop, they become just static points in the screen.I sent an email today to Kikusui to see if I can get the service manual, because they only have the user manual in their web, but I don't have high hopes about that.Has any of you had a similar problem with an analog oscilloscope? Or can point me in a general direction about which part troubleshoot?Since the traces stop when I put the thing upside down, I'm guessing there's a bad solder point or connection so I'll be checking those

author: m k
author_rank: Super Contributor
timestamp: January 01, 2021, 06:45:43 pm
content: Repairing unknown stuff without a manual is always a bit depressing.But you already know quite a bit of what is wrong.Clearly there's a connection problem and since it can be so easily disturbed it can also be something horizontal where its other end is moving.One of those bigger resistors could be it.Components with more legs are usually more steady also, same with resistors that have a leg off from their bodies.Cracked solder can be quite evasive, use flashlight and wooden stick or similar.There a cold spray for electronics can also be your friend, just don't use it too much and keep in mind that cooling takes time, not long though if component is not massive.Mirror is not bad eighter.

author: CaptDon
author_rank: Super Contributor
timestamp: January 03, 2021, 12:41:00 am
content: The entire line of kikusui scopes are notorious for poor solderand bad connections going from board to board at what lookslike molex connectors. I yanked all the connects and did hard wireand it still had weird problems about once a year. Sometimesjust pulling it out of the case to troubleshoot it would cause theproblem to go away. They are also notorious for crappy intensityand focus pots which will totally fail with time. One of the labswhere I worked finally just threw the kikusui stuff away as theygot tired of me using company time to keep those crappy scopesworking. Then they bought some of the shitty Chinese Agilent'knock-off' scopes and I think the first three were dead right outof the box!!! Very frustrating. Rumor has it that those Agilentknock-offs really are Chinese made Agilents that failed qualitycontrol. I just own and use Tektronix stuff although they areinsanely priced if you buy new and their quality has slipped abit over the years. I hate expensive products based on a windowsoperating system!!! Total shit and useless unless you have arestore disk when the unit becomes brain dead!!!

author: bdunham7
author_rank: Super Contributor
timestamp: January 03, 2021, 01:26:44 am
content: https://elektrotanya.com/kikusui_cos5040_sch.pdf/download.htmlI've fixed a number of budget Asian-OEMed scopes like Hitachi, BK, etc without adequate manuals. Even if you do find a schematic, it often is not for the exact version you have in front of you. For your issues, I would strip the case off, turn it on and start poking at it with an insulated stick. Do it in a dimly lit room and the arcing may lead you right to the problem. Solder joints on board interconnects is the first thing to look at.

author: FoxEcho
author_rank: Newbie
timestamp: January 07, 2021, 04:31:47 am
content: I just saw some of your replies, thank you for taking the time of helping me.As you said, one of the problems with the scope was a bad solder connection going to the CRT. Reflowing that solder joint fixed the problem of the screen fading away (or at least it hasn't happened since I fixed the joint).The only problem remaining is the bright spot on the left of the trace.Now, I've noticed some stuff about those bright points now that I can keep the trace for a longer time, so here's my theory about it:What I've noticed is that, changing the time base, sometimes a ghost trace appears, and it seems to be a zoomed version of the waveform in display. This only happens when the time base is at 1 ms for the square wave, but in all the positions for a sine wave I've used to test it.You can see it in the attached pic (btw, the trace is bright, but not as bright as it seems in that pic, the camera seems to be sensitive to that color).After taking a look at the schematic posted and some of the user manuals, I've noticed that the "COS 5041" version of the scope has a delayed time base for waveform magnification, with its own knob, which is not in the 5040. Also, the PCB seems to be the same, and they populate or don't populate some of the components.So what I'm thinking is that maybe some of the components in charge of that waveform magnification are present in the PCB of the 5040, and for some reason the magnification is turned on, but since there's no controls in the front panel, the magnified part is always just the left part of the waveform, which would explain the brightness in that side, and the second waveform visible.It could be any other number of reasons, I've been a bit bussy and tomorrow I'll be able to dissasembly the scope to check the whole circuit, since it envolves taking all the knobs and a lot of work with the connectors etc.

author: CaptDon
author_rank: Super Contributor
timestamp: January 07, 2021, 03:30:29 pm
content: Your problem is that the sub-brightness or beam limiter or what ever they callit is adjusted way too high. You are not seeing 'blanking' so you see the untriggeredresting place of the sweep as well as the retrace. Either your blanking circuit isdefective or the 'brightness range' control is set way to high. Brightness range isNOT the same as the 'brightness' front panel control. the 'sub-brightness' or'brightness range' control is either in the high voltage section or on the C.R.T.socket circuit board. Turn it down until the 'dots' on the left side of the screengo away, usually this adjustment is done with the front panel brightness controlset to mid-range!! Typically during retrace and blanking the CRT grid may be -45volts referenced to the cathode and during active sweep the grid may be around-25 volts referenced to cathode. Don't try to measure the voltage because it isboth a very high voltage (-2000 or more) and high impedance where the meteritself will disrupt the voltages you read.

author: FoxEcho
author_rank: Newbie
timestamp: January 07, 2021, 08:11:24 pm
content: Just a little update. I've checked the circuit that goes to the CRT and all the components seem fine. What I've found out is that if I move the Intensity knob in the front panel, I can move the horizontal position of that ghost trace. I can see that is a magnified version of the squarewave by moving it a bit.In the pics attached, you can kind of see that behind the square wave, there's another trace. In one of the pics you can see first the falling edge and then the rising edge of the next high level. After moving the intensity knob, the other pic shows how the ghost trace moves and then shows first a rising edge and then the falling edge.Another thing I've noticed is that, when the scope is not triggering on any signal, there's still the two bright spot on the left, when I asume it shouldn't show anything on the screen. You can also see that in one of the pics attached.I find extremely weird that the intensity knob let's me move that second trace. I'm still testing the circuit but I can't find any component broken or not working. I'll keep searching and report if something changes.

author: YetAnotherTechie
author_rank: Regular Contributor
timestamp: January 07, 2021, 10:47:17 pm
content: have a look a this threadhttps://www.eevblog.com/forum/repair/gould-os1100-vertical-amp-modulation-issue/

author: CaptDon
author_rank: Super Contributor
timestamp: January 08, 2021, 03:29:10 am
content: You have no blanking!!!!! Can you turn the intensity down to make the traces completely disappear??? You must be able to turn the brightness down with the front panel brightness knob and see the traces completely dim out to nothing!!! If you can't dim them completely out either your brightness range control is up too high AS I SAID located within the high voltage area or somewhere near the wiring to the grid and cathode connections of the CRT, or...... bad news you have a grid to cathode short in your CRT. These crap-kusi scopes are notorious for bad intensity controls and bad divider resistors in the intensity and focus circuits. Your brightness is WAY TO HIGH and you need to find the reason for it. There is a blanking circuit driven by both a signal from the horizontal circuit for retrace blanking and also possibly from the vertical module in the chop mode. You have a BLANKING problem. When the trace is at a normal viewing level you should never see the retrace line (your ghost image) and you should never see the undeflected resting place of the sweep, your two dots on the left side. Once again, you have a Z-AXIS blanking problem. Once you find it and fix it your ghost trace and your left side dots will disappear.

author: bdunham7
author_rank: Super Contributor
timestamp: January 08, 2021, 04:04:21 am
content: Quote from: FoxEcho on January 07, 2021, 08:11:24 pmI'm not sure why it would move either, but as CaptDon is telling you, the problem is that the CRT beam is not shutting off when it should. Can you move those stationary dots off the screen with the horizontal position control? If so, do that to avoid burning holes in your CRT. Then look at your schematic below the CRT on the right side, where it says UNBLNK SIGNAL, that is the area of the schematic you want to pay close attention to. Start with RV653 and Q652.

author: FoxEcho
author_rank: Newbie
timestamp: January 08, 2021, 04:15:02 am
content: Thank you all for the answers.I've disconnected the UNBLK signal completely and the scope is showing no change, so is pretty clear now that is a blanking problem. I'll keep troubleshooting the circuit around that signal.Is not fixed yet but with your sugestions the problem is narrowed down a lot.

author: FoxEcho
author_rank: Newbie
timestamp: January 08, 2021, 04:59:54 am
content: Quote from: bdunham7 on January 08, 2021, 04:04:21 amSo, I moved RV653 a bit and I made the bright points dissapear, and now the brightness/intensity control let's me change the brightness from no trace to bright trace, so I'd say it's working properly.The only issue left is that the traces have like a fuzy or blurry background on the screen even at low birghtness, and the trigger is unstable, but that's not related to the brightness. But I'd say the scope is like 90% fixed.I'll keep testing some other components and I'll probably replace some eletrolitic caps just in case. Thank you all! I'll post any other updates about that blurry background if I fix it, so it can help any other people with the same issues.

author: Mr Scrofula
author_rank: Newbie
timestamp: December 25, 2022, 08:26:41 am
content: Exactly the same thing happened to me the other day when I used my hameg604 in the garage at -4 degrees C ! Ok when I took inside, which was lucky as I couldn’t find the unblinking on the schematic.