Dim oscilloscope screen - Page 1
Forum section: electronics / testgear

author: yugu
author_rank: Contributor
timestamp: November 18, 2014, 09:21:18 pm
content: Hi guys. I bought a $50 analog oscilloscope from ebay (I'm a fan of Dave). It's Instek GOS-635G. It works fine except that the screen is quite dim. When I turn the intensity knob up from the min position, the screen doesn't show anything before 70% position. From 70% to 90% position, the brightness keeps going up. And then it starts to drop if I continue to turn it up. So the maximum brightness happens when the knob is at 90%. But it's still very dim. I can barely see the trace.Does anybody have any clue about what's wrong and how to fix it? I am wondering maybe there's something wrong with the potentiometer (it feels smooth though). But would like to hear what you guys have to say before I open it upThanks!

author: Stray Electron
author_rank: Super Contributor
timestamp: November 18, 2014, 09:32:20 pm
content: Did you try taking the cover off and cleaning the front of the CRT and the back side of the protective glass? You'd be surprised at how dirty they can get, particularly in an old scope. I just bought a well optioned and working Lecroy 9374. I got it for $40 because it looked like the CRT was shot and was barely visible. But after cleaning it it's clearly visible at 64% brightness setting.

author: yugu
author_rank: Contributor
timestamp: November 18, 2014, 09:45:26 pm
content: Thanks for the suggestion! Will clean it once I open it up.

author: tautech
author_rank: Super Contributor
timestamp: November 18, 2014, 10:06:22 pm
content: Quote from: yugu on November 18, 2014, 09:45:26 pmBe careful around the PDA lead on the CRT, it might bite you.Even when off !Best if it is dicharged with a screwdriver and a grounding strap.Find a manual if you can and check HV voltages are to spec.Be warned, they can be as high as 2.5 KV and a HV probe or capable MM are needed for this.There is often high value resistive dividers in the CRT circuitry and the resistors can drift or go OC. Check these.

author: PaulAm
author_rank: Frequent Contributor
timestamp: November 18, 2014, 10:45:20 pm
content: Some CRT scopes have a bias adjustment that can be used to set the brightness adjustment range, some don't. If your scope has one of these, try to find the service manual for instructions on how to set it. You may need a HV probe for your multimeter to set it correctly.Sometimes the resistors in the focus/intensity string drift. If you check these, make sure you have completely discharged the HV circuits before sticking any probes in there (and make sure it's unplugged, as well).Something may have flaked out in the Z axis or blanking. Something could be shot here restricting the intensity range.It's possible the CRT could be bad, but normally you only see that if a scope has been left on 24/7 for extended periods. That would be the last thing I would look at.Do you have a service manual or schematic?

author: yugu
author_rank: Contributor
timestamp: November 19, 2014, 01:07:39 am
content: Thanks guys for the reply. I wasn't able to find the service manual.Here is the internal. it looks quite clean. The potentiometer of the intensity knob is hiding under the crt. I have to remove the CRT to see it.I didn't see the PDA lead of the CRT for discharging. could someone point it out in the following pictures?Any thoughts?

author: tautech
author_rank: Super Contributor
timestamp: November 19, 2014, 01:41:05 am
content: As you have pointed out in the OP, the increase in brightness followed by dimming at max intensity does indeed indicate a problem.Your CRT does not have a visible PDA lead, however use a grounded probe throughout the HV circuitry to discharge stuff around the tube.PaulAm's description is accurate and I suggest you download some service manuals to get a "feel" for how the CRT HV is set up.Also get the Tek pdf in the first post in the Repair board.

author: yugu
author_rank: Contributor
timestamp: November 19, 2014, 04:29:22 am
content: Thanks. I've been playing with it for some time today. So far what I know isThere is nothing wrong with the intensity potentiometer. I've verified it with a multimeter.There is another intensity adjustment potentiometer on the pcb. However, it does not improve the maximum intensity.I will dig into the HV circuitry when I got a chance. Seems the hypothesis is the HV for the CRT is not high enough, right? Without a service manual from Instek, how do I know the expected values?

author: SeanB
author_rank: Super Contributor
timestamp: November 19, 2014, 04:34:50 am
content: More likely resistors in the brightness circuit have gone high in value, and possibly a few capacitors in the power supply are failing and giving low power rails which means HT is low and the control range is low as well.

author: yugu
author_rank: Contributor
timestamp: November 19, 2014, 04:53:38 am
content: Yay found the circuitry for GOS-620FG. Though not the same model, they should be very similar. Really appreciate it if you could take a look to see which elements may go wrong. It will certainly take me a long time to understand the schematics.

author: PaulAm
author_rank: Frequent Contributor
timestamp: November 19, 2014, 05:26:51 am
content: Those aren't very useful. The last page on the user manual has a block diagram; you really need to stuff in the boxes marked CRT Circuit, Z-axis Amp and HV supply. On the bright side, the 635 only uses a 2 KV HV supply.

author: tautech
author_rank: Super Contributor
timestamp: November 19, 2014, 07:58:38 am
content: Quote from: yugu on November 19, 2014, 04:29:22 amIIRC the trimmer is used to set minimum trace brightness while the main intensity is at a low setting.Normally trace is adjusted to a "just visible" value.Many adjustment procedures are done in a specified order to ensure the correct setup.The HV adjustments will not be as critical as the rest of the scope circuitry, but you might strike it lucky and some values are on the PCB overlay as test point values.There would be no harm in contacting Instek for a Service manual or failing that say a PSU and CRT schematic. Those should give the info you need for a repair. (Voltages, test points and component list)

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: November 19, 2014, 12:01:58 pm
content: Well I didn't want to post but that behavior sounds an awful lot like "double peaking". Usually means the tube is at end of life.

author: David Hess
author_rank: Super Contributor
timestamp: November 19, 2014, 12:35:50 pm
content: The schematics do not show any PDA (post deflection acceleration) voltage connection and no PDA lead is visible in the photos so I assume the CRT lacks this which is common in slow oscilloscopes. The documentation says that only the faster oscilloscopes in this series have PDA. High PDA is necessary for a bright and sharp CRT and a good reason to prefer using fast oscilloscopes even slow signals.If the horizontal and vertical deflection sensitivity is correct then the CRT cathode voltage is correct. The manual says the cathode voltage is -2000 volts but from the looks of those schematics, it is lower. Is the horizontal and vertical calibration correct on slow signals?What kind of signal are you using as a test and at what time/div setting? If the repetition rate is low, then fast time/div settings are going to look dim.Make sure the x10 horizontal magnification is turned off and the hold off is set to minimum.When the trigger is set to automatic and the timebase is slow, the trace should be nice and bright.

author: yugu
author_rank: Contributor
timestamp: November 19, 2014, 08:33:28 pm
content: thanks guys!yeah it sounds similar to the symptoms of "double peeking". I hope it's not because of an old tubeTo David: Good point. I test it with a 4v 1kHz sinusoid using 1ms/div and 1v/div. The measurement of the time looked correct to me. I didn't pay attention to the value of the peak-to-peak voltage, though. Let me verify it tonight. Do you mean if the vertical axis is way off, then there is still hope for the tube?Also I will measure the voltage when I have a HV probe.

author: David Hess
author_rank: Super Contributor
timestamp: November 19, 2014, 09:49:07 pm
content: Quote from: yugu on November 19, 2014, 08:33:28 pmIt is sometimes difficult to distinguish double peaking from blooming.Another possibility is that the CRT has become gassy. If this is the case, just operating it for hours to days with the intensity turned down but not off may restore operation.QuoteIf the cathode voltage is low which will cause a dim CRT, then the horizontal and vertical calibration will be off as well. Low voltage will increase the sensitivity; if the cathode voltage is one half of what it should be, then the deflection sensitivity will be doubled.So if the horizontal and vertical deflection is much higher than they are suppose to be, it indicates that the cathode voltage is low (closer to ground) which will also result in a dimmer CRT.A more likely problem is that the repetition rate of the test signal is low. Your 1 kHz test signal should be bright at 50uS/div and slower but progressively dimmer at faster sweep speeds. The x10 horizontal magnification function will also dim the trace to 1/10th of its normal brightness.QuoteIf the horizontal and vertical calibration are correct, then you do not need to bother measuring the high negative cathode voltage because it is correct.

author: yugu
author_rank: Contributor
timestamp: November 19, 2014, 11:17:31 pm
content: The explanation of the voltage makes perfect sense to me now.I will double check the sensitivity when I got a chance. Thanks!

author: yugu
author_rank: Contributor
timestamp: November 20, 2014, 05:49:48 am
content: The sensitivities of both horizontal and vertical axis are correct. I measured the voltage of the tube pins (referenced to ground). They are as followsH=-2.2kvH=-2.2kvK(G1)=-2.2kvG1(k)=-2.2kvP1=-1.65kvp2=77.7vG2=96.5vAttached is the photo of the pins. Do those voltages look normal to you? I will purchase a replacement tube if these data is enough to give it a death sentence.

author: tautech
author_rank: Super Contributor
timestamp: November 20, 2014, 05:57:52 am
content: Those measurements are referenced to ground right?Measure voltage between H & H, that should be the heater filament voltage.

author: yugu
author_rank: Contributor
timestamp: November 20, 2014, 06:08:18 am
content: Quote from: tautech on November 20, 2014, 05:57:52 amyep they are referenced to ground.QuoteThe voltage between H&H is AC 6.7vHere are the parameters for the tube found from internet:Type: KX-S-6 15SJ118Y14Screen Size: 6"Voltage(V): 6.3Current(A): 0.24th Anode Voltage (V): --3th Anode Voltage (V): 2000±1002th Anode Voltage (V): 250~4501th Anode Voltage (V): 2000Cutoff Voltage (V): —35~-85Horizontal: 19~21.6Vertical: 10.8~13.2

author: yugu
author_rank: Contributor
timestamp: November 20, 2014, 07:37:41 am
content: If understand it correctly, the intensity control is adjusting the voltage between G1 and K. I checked the brightness of the screen on different voltages. Here is the result(turning the knob from min to max):V(G1)-V(K) = 68 brightness = 0V(G1)-V(K) = 62 maximum brightness (very dim)V(G1)-V(K) = 44.5 brightness ~= 0.1 * maximum brightnessG1 is control grid and G2 is screen grid, right? what are P1 and P2?

author: tautech
author_rank: Super Contributor
timestamp: November 20, 2014, 07:51:59 am
content: Did you get the TEK PDF for troubleshooting CRO's?Study from P35 on

author: VK5RC
author_rank: Unknown
timestamp: November 20, 2014, 09:37:10 am
content: If it isn't a circuit problem and a spare tube can't be found for reasonable $, there are some guys who have had a little success in getting a bit of extra life out of the tube. KE5FX, not for the faint hearted!http://www.ke5fx.com/crt.html

author: yugu
author_rank: Contributor
timestamp: November 20, 2014, 07:11:57 pm
content: Thanks. I am not experienced in schematics/circuits. So I wanted to directly measure the voltages of the tube, to see if the tube is probably biased. If the voltages are good, and the tube is still too dim, then I will try to replace it (or zap it)Did I miss anything in this approach?Quote from: yugu on November 20, 2014, 07:37:41 amHere V(G1) is higher than V(K). I believe it's because the labels on the PCB is misleading. They probably use K(G1) to indicate the pin could be either K or G1 in different models. And in my case, I should read the labels inside the parentheis...Quote

author: tautech
author_rank: Super Contributor
timestamp: November 20, 2014, 07:28:49 pm
content: Quote from: yugu on November 20, 2014, 07:11:57 pmI have never need to Zap the CRT's of any of the old CRO's I have fixed, but I've always had the schematics.IME its all about faulty componentry, correct voltages and setup.Maybe there is somebody out there that can take some measurements for you of critical test points and PSU voltages/ripple etc.Start a new thread requesting such with the model and make in the title and/or edit this thread and add the same.

author: David Hess
author_rank: Super Contributor
timestamp: November 21, 2014, 03:13:06 am
content: Quote from: yugu on November 20, 2014, 07:11:57 pmThe schematic shows the same thing. I am still puzzling out which voltages were on which CRT elements.

author: yugu
author_rank: Contributor
timestamp: November 21, 2014, 09:51:47 pm
content: Thanks guys. As a beginner, it's much easier for me to isolate the tube out and measure it directly. My theory is that if the voltage bias of the tube is correct, then it should be working properly. Otherwise, there's something wrong with the tube itself. I'm not convinced that I need to measure the low voltage circuit to infer the problem indirectly.If I understand it correctly, the brightness can be controlled by adjusting either V(H1, H2) or V(K, G1). Since the tube is working fine except the brightness, the voltage bias that could go wrong would be these two.However, looking at my measurement, I didn't see anything wrong with these two voltages.+ When I turn the intensity knob from min to max, V(K, G1) dropped from 68v to 42v, and the maximum brightness happens at about 60v. So the the dimness could not be because of this voltage is too high or too low.+ The voltage of the filament is 6.7v. though a bit higher that the 6.3v in the spec, it looks good enough.The other voltages also look good, V(K, P1) ~= 550v, V(K, G2) ~= 2.1kv.I also measured the voltage change of V(K, G1), V(K, P1) and V(K, G2) when I turned the intensity knob. Only V(K, G1) changed and the other two were stable.My conclusion is, the tube is bad. Let me know if I made any stupid error in the above reasoning.I am going to do more experiments on the tube, e.g. increase the filament voltage, zap it, etc.

author: yugu
author_rank: Contributor
timestamp: November 22, 2014, 07:23:57 am
content: An update: I zapped the tube with high voltage for a few minutes and voila, the screen is much brighter now!

author: tautech
author_rank: Super Contributor
timestamp: November 22, 2014, 07:50:16 am
content: Quote from: yugu on November 22, 2014, 07:23:57 amWell done.Did you follow the steps in KE5FX's CRT restore page?Care to accurately describe your procedure?

author: yugu
author_rank: Contributor
timestamp: November 22, 2014, 08:31:02 am
content: Sure. The procedure is very simple (I got it from a friend who fixed lots of televisions):+ Find a high voltage igniter+ Connect K and G1 to the igniter+ Zap itIn my case, I just connect the tub to my gas stove igniter, leave it there with the igniter turned on for two minutes. After that the brightness improved magically. I zapped it for another 10 minutes to see if it would improve more, but found no noticeable improvement.In summary, it's a simple trick, takes little effort to try, and is quite safe (the energy released from the gas stove igniter is tiny). Anyone who has a tube with similar issue may consider give it a shot. Do it at your own risk, though

author: David Hess
author_rank: Super Contributor
timestamp: November 22, 2014, 07:32:42 pm
content: So the CRT is suffering from interface resistance and/or cathode depletion. The former is caused by operating it with the intensity blanked (they made special vacuum tubes designed to operate in cutoff to prevent interface resistance) and the later by long operating hours. I think cathode depletion causes double peaking but interface resistance should just make the trace dim and maybe cause unblanking problems.I find it difficult to believe that someone used an analog oscilloscope this new for long enough to cause these problems.

author: VK5RC
author_rank: Unknown
timestamp: November 23, 2014, 06:17:43 am
content: @yugu Thanks for sharing, hope it goes well! I have some old gear that the tubes are getting a bit soft so it is always nice to have "plan B" confirmed.

author: adamf
author_rank: Newbie
timestamp: October 04, 2015, 01:33:24 pm
content: Hi, and sorry for reopening this old thread; I thought someone could help me, as I have the exact same problem as the OP.I have a Hitachi V-552 oscilloscope, and the trace is very dim -almost completely gone. The intensity knob behaves in the exact way as described by the OP: you can't see the trace until you reach 70%, then the intensity goes up until 90%, and then begins do dim again up to 100%.I read the whole thread, and even though I'm not as good with electronic as the OP, I don't think the problem is with the CRT; at least in my case. The reason is this that at the beginning the problem when on and off; as if a component was failing or loose, rather than a CRT getting old for overuse. I also bought the oscilloscope on Ebay for a similar price, and as I said, the trace was alright when it arrived. After a couple of weeks, I began experiencing the problem, but not continuously; it would be dim some times, specially when turning it on, and then after an hour or so, it would suddenly fix itself; sometimes failing again after a while, and so forth. It behaved like that for a few months, and then it stopped; it stayed dim all the time.I have the schematics, and checked all the components from the 'Intensity' knob and the 'CRT Bias' pot up to the HV circuit; all the voltages and components seem to be fine. I checked the whole area because the PCB looks quite dark in that part of the circuit, as burnt from excessive heat. I also checked the temperatures, and the highest comes from a transistor at 70 degrees centigrade; but the voltages in and out of it seem to be fine. Also took it out and the beta seems also seems to be within specs. I can't check the HV, as I don't have a probe; so now I'm stuck, I don't know where else to look.Anyway, I would appreciate any help or suggestions. Thanks...

author: tautech
author_rank: Super Contributor
timestamp: October 04, 2015, 07:21:57 pm
content: Welcome to the forum.Sounds like you've checked all the usual causes.Have you inspected VERY carefully for any cracked or poor solder joints?As you say there is often quite some heat in the CRT HV, particullary around the HV divider, this heat might have caused joint cracking or even cracked components with thermal cycling. Be sure to give some a gentle nudge to check they are still physically intact.Can you provide a link to the schematic or Service manual?

author: adamf
author_rank: Newbie
timestamp: October 05, 2015, 04:57:58 am
content: Quote from: tautech on October 04, 2015, 07:21:57 pmThanks for the prompt answer.Quote from: tautech on October 04, 2015, 07:21:57 pmI tried this many times, but never worked. For a short time before it completely failed, pushing some buttons of the front panel seemed to have some effect; but didn't last more than a couple of days.Quote from: tautech on October 04, 2015, 07:21:57 pmI checked that part of the circuit, and even found one of the pin connections with cracked joints. But after soldering it, it continued failing; so I guess it had nothing to do with it. I might have missed it, so I'll see if I can take the PCB off again and redo it.Quote from: tautech on October 04, 2015, 07:21:57 pmHere is a link for a PDF copy of the service manual:http://skory.gylcomp.hu/alkatresz/hitachi_v-522-v-523-v-525_service_m.pdfthe HV circuit is on page 60 of the PDF (page 57 of the manual). Only the pages for the V-522 apply, as the manual combines various models.

author: tautech
author_rank: Super Contributor
timestamp: October 05, 2015, 09:02:48 am
content: In cases like these I replace many of the components that are under HV stress. Some are apparent, the hot areas that you have already described. Other components like the HV DC restore diode D1013, find datasheets for the listed Vf voltage and check these with a PSU and a DMM. HV caps do get tired too, in fact most of the CRT circuit is under some voltage and heat stress and just a few dollars will replace most components.I'd check that all ather voltages the HV is referenced to are there in their correct values -8V, +8V, 75V etc.I did have a zener go like D1020 in one CRO I had, so you need to carefully check all the active devices too.You could try to add some hot air of freeze to see if you can localize where the problem lies.Normally I would look hard at all high value and high wattage resistors R1014, R1017, R1024.Patience and perserverence will pay off.

author: Gyro
author_rank: Super Contributor
timestamp: October 05, 2015, 05:36:19 pm
content: QuoteTake it very gently with the freezer spray though - you don't want to get to the point of creating condensation in the HT section!

author: adamf
author_rank: Newbie
timestamp: October 06, 2015, 09:42:49 pm
content: Thanks again for the help.I tested the -8V +8V and 75V, and they seem to be fine.I also took out the PCB again and re-soldered everything in the HV circuit, the zone that looks overheated, connectors, around the 'CRT BIAS' pot, power supply, and everything else that looked suspicious. I think the trace is a tiny bit brighter, but I'm not completely sure; it's hard to tell from memory without measuring it. In any case, nothing compared with the brightness it had when the scope arrived.Next time I'll check the individual components you mentioned; probably need to take them out for testing; as within the circuit everything seems to be fine. Already took out for testing the 3 transistors in the burnt area (the H-AMP circuit); which includes the one which measures 70º C. All betas are well within specs.Never tried to freeze or heat a circuit before; so I prefer to leave that option for last.

author: adamf
author_rank: Newbie
timestamp: October 09, 2015, 10:57:22 pm
content: Quote from: tautech on October 05, 2015, 09:02:48 amHi again,Today I took out the components you suggested for testing, and all of them seem to be fine. The only component I have doubts is the D1013 rectifying diode. I couldn't test it with any of the testing tools I have, but I did as you said and used a PSU. It seems to rectify well from -25V to +25V; which is the maximum voltage of my PSU. It's the first time I test a HV diode, so I'm not too sure I did all the test necessary. In any case, I'm attaching to this post the measurements I took, and these are the only specs I could find:Y10GA Silicon Rectifier(D1013)V(RRM)(V) Rep.Pk.Rev. Voltage=6.0kI(RM) Max.(A) Reverse Current=4.0u@V(R) (V)(Test Condition)=6.0kI(FSM) Max.(A) Pk.Fwd.Sur.Cur.=500mV(FM) Max.(V) Forward Voltage=22@I(FM) (A) (Test Condition)=5.0mI(O) Max.(A) Output Current=5.0On the other hand, I found something that's not right; though I don't think is the reason for the fault. On schematic #6 (page 55) states that R860 should be 1.2K; however, the resistor on the circuit is marked, and measures, 3.3K; also looks like an original component. I noticed it because the resistor next to it (R866) was a bit burnt, but after taking it out it seems to be fine (12.1K).So, I'm stuck again...

author: tautech
author_rank: Super Contributor
timestamp: October 09, 2015, 11:31:27 pm
content: QuoteThat's wrong.V(FM) Max.(V) Forward Voltage=22This means the maximum voltage it will start conducting is 22VHowever that is max Vf and it will likely be on at ~15V.Normally this is well above the voltage a DMM diode test supplies but if it's conducting at low voltages and in either polarity it's most probably shorted.

author: adamf
author_rank: Newbie
timestamp: October 10, 2015, 07:24:12 am
content: Quote from: tautech on October 09, 2015, 11:31:27 pmI got no current when I applied negative voltage. It only conducts when forward voltage is applied, though I could only test it up to -25V, which is a bit short considering the V(RRM) of 6KV.I'll try to get a replacement on my local shop on Monday. Thanks for the help.

author: tautech
author_rank: Super Contributor
timestamp: October 10, 2015, 07:35:50 am
content: Quote from: adamf on October 10, 2015, 07:24:12 amPlace a small load, say 1M ohm across the output of the diode, connect your DMM across the resistor and measure the voltage when it starts to conduct. It should be less than 22V and probably more than 10V.These HV diodes can only handle a few mA so be careful with any tests. You only need to know that they conduct in one direction and what that voltage is, in this case less than 22V.

author: adamf
author_rank: Newbie
timestamp: October 14, 2015, 09:43:41 am
content: Quote from: tautech on October 10, 2015, 07:35:50 amIt starts conducting at +5V. I didn't allow it to pass from 5mA, but anyway I put it back and everything is still the same as before.The problem that I'm having now is finding a replacement for that particular diode. I asked in my local shop and searched on the internet for the Y10GA and can't find that component or an equivalent. Could you recommend me a replacement that would work?

author: adamf
author_rank: Newbie
timestamp: October 14, 2015, 09:48:06 pm
content: I think I found a replacement that could work in a website called HVStuff. It doesn't have a reference, just says: 5mA 6kV 100nS High Voltage Diode HV Rectifier High Frequency.The only difference I can see is the 20V Average Forward Voltage Drop insted of the 22V of the Y10GA. This are the specs on the website:http://hvstuff.com/5ma-6kv-100ns-high-voltage-diode-hv-rectifier-high-frequency-Repetitive Peak Reverse Voltage 6kV-Average Forward Current 5mA-Maximum Surge Current 0.5A-Avg. Forward Voltage Drop 20V-Peak Reverse Current 2uA@25°C5uA@100°C-Reverse Recovery Time 100nS-Dimensions 3mm×8mm(0.12inch×0.32inches)-Terminals Axial LeadCould this one do?

author: tautech
author_rank: Super Contributor
timestamp: October 14, 2015, 10:09:23 pm
content: Quote from: adamf on October 14, 2015, 09:48:06 pm