Broken Scope - Leader LBO-508 - Page 1
Forum section: electronics / testgear

author: rj88
author_rank: Contributor
timestamp: December 02, 2015, 12:59:08 am
content: Hi folks,I have acquired a broken scope - a Leader LBO-508.Symptoms:1. Power light on, nothing on screen, was working OK, but seemed to be very badly calibrated - on both channels the voltages were measured at about half of what they actually were.2. When I measured the calibration signal, I saw a square wave of the wrong voltage (again too small), and the 'on' part of the cycle was 'tapered' rather than flat (hope that makes sense)3. While hooked up to the calibration point, the signal slowly faded away, and now I don't get any signal at all.4. I had been playing with it for a while with strong signal, until I hooked it up to the calibration point - it's as if that killed it (or it might just be coincidence)Should I chuck it or is there hope?I don't have enough background in scopes to know where to start, however I have attached the service manual for this model if anyone is inclined to take a lookAny advice much appreciated. Thanks

author: gearhead
author_rank: Contributor
timestamp: December 02, 2015, 01:08:19 am
content: Do you have a signal generator or anything so you can check how the scope is operating without using the CAL on the scope?

author: rj88
author_rank: Contributor
timestamp: December 02, 2015, 01:10:37 am
content: Quote from: gearhead on December 02, 2015, 01:08:19 amUnfortunately, I am now gettingnothingon the screen at all(have edited original post to make that a bit clearer)

author: gearhead
author_rank: Contributor
timestamp: December 02, 2015, 01:17:30 am
content: You can download a service manual for that scope here:http://elektrotanya.com/leader_lbo-507_508_sm.pdf/download.htmlYou'll need to be able read schematics, and have the proper tools to go through this unit. Start with the power supply. Remember there is HIGH VOLTAGE driving the CRT. IT CAN KILL YOU! If you have to test CRT voltages you need a high voltage probe for your DMM or maybe a Triplett 630-NA that can measure up to 6000vdc directly. And clip your leads on before powering up the scope so you don't risk being shocked.But start with the power supply section first. It won't work properly without proper voltages being supplied.

author: rj88
author_rank: Contributor
timestamp: December 02, 2015, 01:21:11 am
content: Quote from: gearhead on December 02, 2015, 01:17:30 amMy DMM is a Fluke 15B, can only go up to 1000V. I might be out of my depth here

author: gearhead
author_rank: Contributor
timestamp: December 02, 2015, 01:22:38 am
content: Here's another link for the service manual if the other doesn't work:http://bee.mif.pg.gda.pl/ciasteczkowypotwor/Leader/LBO-507%20&%20508.pdfYou can go through the low voltage supply with you're meter. Like I said, if the voltages aren't there, the scope won't work.The SM has a troubleshooting flow chart you can go through that lists specific sections to check. The very first piece of electronics I bought and fixed was a Tektronix 2337 scope. Gotta learn somewhere.Biggest problem I had is that I had to buy another scope to be able to go through the circuits and look at the waveforms to track the problem down. Your unit sounds like a power supply issue, mine was an IC chip on the sweep board.

author: rj88
author_rank: Contributor
timestamp: December 02, 2015, 01:38:52 am
content: Quote from: gearhead on December 02, 2015, 01:22:38 amThanks for that. I will go through the first 4 test points (I can't do the 5th because I can't measure 2000V)What I found strange was that the signal just kind of faded away, has anyone encountered this before?

author: gearhead
author_rank: Contributor
timestamp: December 02, 2015, 01:43:47 am
content: I haven't, but it sounds like a power issue. Check the low voltage points 1-4 and post your findings for each.Also rj88, I have seen where there's been a problem with cracked solder joints on the large pins on the PCB connectors.Something to look for while you're in there. If a joint looks like it has a little circle around it, then that joint needs to be re-flowed.Some cracks are VERY hard to see and require a magnifier to locate.Many guitar amps I've repaired had bad solder joints that once re-flowed worked great.

author: dan3460
author_rank: Frequent Contributor
timestamp: December 02, 2015, 03:06:37 am
content: You have to start with the basics. The manual probably has a table of voltages that are expected around the scope. Leave the high voltage at aside for right now. See how the voltages on your scope compares with the numbers on the manual.It is very common that the power supply will function for a while, after many years of not use, and then give up.Check voltages and report back.

author: rj88
author_rank: Contributor
timestamp: December 02, 2015, 03:35:38 am
content: Quote from: gearhead on December 02, 2015, 01:43:47 amQuote from: dan3460 on December 02, 2015, 03:06:37 amThanks to you both - will report back ASAP.

author: rj88
author_rank: Contributor
timestamp: December 02, 2015, 10:28:18 am
content: OK here's what is going on:Tp1, should be -15, actual -20.5VTp2, should be +15, actual +15VTp3, should be +210, actual +209.7VTp4, should be +320, actual +333VSo TP1 and TP2 should be within 5%, TP3 and TP4 should be within 10%. There appears to be a problem with TP1...Any further advice would be very much appreciated!

author: dan3460
author_rank: Frequent Contributor
timestamp: December 02, 2015, 03:59:02 pm
content: There are 3 transistors that control the -15V, there are also some lytics around the circuit. I will look around Q101,102 and 103. I would probably change, or least test, C102 and C105. After changing/test, if voltage still off I will make sure that the resistors around those components still on specs. The last resource will be to change the transistors. The diagram shows a bunch of voltages around the transistors, check those voltages.

author: rj88
author_rank: Contributor
timestamp: December 04, 2015, 05:00:23 am
content: Quote from: dan3460 on December 02, 2015, 03:59:02 pmThanks so much - I need to get myself a 22uF 50V, as soon as that arrives I will swap them both out. They don't look to me like they are bulging or anything obvious like that, but it's worth a try!

author: rj88
author_rank: Contributor
timestamp: December 08, 2015, 12:41:21 am
content: Quote from: rj88 on December 04, 2015, 05:00:23 amSorry, took longer than expected.So C102 was fine and C105 was bad. Swapped over C105 but didn't make a difference.Measured some voltages too -please see attached pic, have also included the full section that the pic came from. Issue appears to be at the base of Q102 / collector of Q103.Thanks very much for any advice!

author: dan3460
author_rank: Frequent Contributor
timestamp: December 08, 2015, 02:41:56 pm
content: With most of the voltages around the transistors ok, my next suspect will be the Q103.My thinking is as follows, and people with more knowledge can correct me, the emitter of Q103 is held at the correct voltage by the zener diode. I think the voltage at the base is close enough but the transistor (Q103) is not on and it is not pulling current from the base of Q102, which in turn should be pulling current from the collector from Q101.

author: gearhead
author_rank: Contributor
timestamp: December 08, 2015, 03:09:39 pm
content: The voltages at those transistors etc are pretty close. I would go through and checkthe voltages in the Intensity section (top LH corner of schematic, J122 area)and see what you have. You need to measure the -1930v below Q111 (I think that is what it's labelled.)And the other voltages around that area. Otherwise you may be chasing yourself in the dark.Without the HV to the CRT it won't display anything.Too bad you don't have another scope to use. And a High Voltage probe for your DMM.You really need to look at the waveforms at the test points. And measure the HV supply.You can build a HV probe. You're only measuring in the neighborhood of -2000 volts.AND you need to clip the leads onto where you're testing and keep your hands off of it. Then turn the scope on and see what the voltage is.Here's a couple links to DIY HV probe for DMM:http://www.n2rit.com/homebrew/HVProbe/!HvProbe.htmlhttp://webcache.googleusercontent.com/search?q=cache:qGkehCo5oSEJ:www.repairfaq.org/sam/hvprobe.htm+&cd=4&hl=en&ct=clnk&gl=us

author: dan3460
author_rank: Frequent Contributor
timestamp: December 08, 2015, 03:35:21 pm
content: I respectfully disagree: One thing that I have been told several times before: "you need to make sure that voltages are correct before you start poking around other parts of the circuit" Having the -15V off by 5V will be sufficient to have transistors off or on when there are not supposed to be.Once all the voltages from the power supply are ok, then proceed with gearhead's advise. In my experience, 100% of the scopes that I have rebuild (20-25) the high voltage has been fine, even when on several of them I could not see anything on the screen.Please be careful around the HV areas, the voltage can kill you.

author: MarkL
author_rank: Unknown
timestamp: December 08, 2015, 06:02:01 pm
content: Dan, I think you're right about Q103. The collector looks open.rj88: If you haven't done so already, take the voltage reading directly on Q103's collector lead to make sure there's not a bad joint or a crack in a trace.

author: rj88
author_rank: Contributor
timestamp: December 08, 2015, 09:54:45 pm
content: Quote from: dan3460 on December 08, 2015, 02:41:56 pmMakes sense to me - have got some BD140 transistors coming - PNP VCB=-80V, VCE=-80V, Ic=-1.5A, should be equivalent I think.Thanks again, will update on the voltages asap!

author: rj88
author_rank: Contributor
timestamp: December 08, 2015, 09:57:53 pm
content: Quote from: gearhead on December 08, 2015, 03:09:39 pmThanks for your help - I can potentially get some external help on measuring the high voltages, in the meantime I might as well change the transistor, will only take a few minutes

author: rj88
author_rank: Contributor
timestamp: December 08, 2015, 09:59:04 pm
content: Quote from: MarkL on December 08, 2015, 06:02:01 pmWill try that before I swap it out - thanks again to everyone, this is such a helpful forum.

author: rj88
author_rank: Contributor
timestamp: December 08, 2015, 10:32:20 pm
content: Quote from: gearhead on December 08, 2015, 03:09:39 pmI like the look of this:http://www.n2rit.com/homebrew/HVProbe/!HvProbe.htmlWould these be suitable?http://au.rs-online.com/web/p/through-hole-fixed-resistors/2142342/

author: rj88
author_rank: Contributor
timestamp: December 09, 2015, 10:33:26 am
content: So I just swapped out Q103 (collector voltage -21.2v) with a BD140 and I am now getting almost exactly -15V at test point 1. Which is great, however there are still no traces unfortunately. Is the consensus that I need to now measure those high voltages?Very much appreciate all the help...

author: MarkL
author_rank: Unknown
timestamp: December 09, 2015, 06:38:34 pm
content: Great that the -15V is working now.I would first go back and confirm all the other voltages that you can test are still ok.Make sure you have all the right settings so you would see a trace. Set max intensity, trigger level AUTO (push in), channel positions set to mid, sweep set to time (1ms per div is fine).Do you see anything at all on the screen, even when powering up or powering off? Any blip showing temporary life?Measuring the high voltage would be a valid next step. However, it's not required just yet. There's still a few more things you could do depending on your DMM. (I need to point out I am NOT a fan of home-brew HV probes.)What kind of DMM do you have? What is its max rating for AC and DC voltage?Does it have a diode test feature?Also, the CRT has a filament in the neck near the socket in the back. You should be able to see it glow dull red. Look at it from different angles, including from the rear, and with the lights off if necessary.

author: rj88
author_rank: Contributor
timestamp: December 12, 2015, 03:12:20 am
content: Quote from: MarkL on December 09, 2015, 06:38:34 pmThanks very much Mark, sorry for the delay* Trigger settings all standard* Nothing on screen when powering on or off* The CRT filament does indeed glow red* My DMM is a Fluke 15B with a 1000V limit and it can test diodes.* Will test other voltages, and will get access to a high voltage test soonVery much appreciate the help. What else were you thinking?

author: MarkL
author_rank: Unknown
timestamp: December 12, 2015, 04:31:04 am
content: Yep, it sure sounds like there's no HV.I was thinking you could check some of the HV components with the diode test function on your meter. You can check all the transistors and diodes pretty quickly.On the DMM diode test, diodes will show 0.7V in one direction and not the other, and transistors can be checked as two diodes, one from base to emitter and and the other from base to collector.HOWEVER... Looking at the circuit in more detail, you've got 5 x 0.5uF capacitors charged up to 1000V each and the problem is I don't see bleeder (discharge) resistors in some parts of the HV circuitry. So, some of the capacitors can remain at dangerous potentials for an indeterminate amount of time. They would have to be manually discharged and then verified with a measurement to ensure safety.I was also thinking, depending on what meter you had, you could measure the high voltage secondary which is 1020VRMS, but that's beyond the rating of the 15B. I can't say it would be wise even if it is only 20V over.So, if you already have an HV probe coming, I think that would be the better option at this point. Besides checking test points, you can use the probe to safely discharge the capacitors before proceeding with DMM component checking.

author: rj88
author_rank: Contributor
timestamp: December 12, 2015, 05:18:52 am
content: OK, sounds like a good plan, will err on the side of caution and wait for the HV probe (I am quite attached to the Fluke!)

author: gearhead
author_rank: Contributor
timestamp: December 15, 2015, 06:06:42 am
content: rj88, remember, clip your leads (GROUND) in place when you check that HV. Then power the scope on. Don't take a chance of any leakage or arcing, cause you may not live to tell about it. AND ONLY ONE HAND ON STUFF AT A TIME. DO NOT TOUCH ANYTHING GROUNDED WHILE PROBING! NOT THE TABLE, CHASSIS, OR ANY OTHER EQUIPMENT.

author: rj88
author_rank: Contributor
timestamp: December 15, 2015, 09:43:00 am
content: No problem - I have worked around HV before, thanks for the advice

author: dan3460
author_rank: Frequent Contributor
timestamp: December 15, 2015, 08:53:25 pm
content: One more thing before you go poking around HV, what happens when you press "Find Beam" button? Maybe some one asked this before and I miss it.

author: rj88
author_rank: Contributor
timestamp: January 04, 2016, 05:05:46 am
content: Sorry for the delay, back now, high voltage probe arriving any day now, will update ASAP and will try the "Find Beam" button!gearhead:thanks very much for your PM, I replied but it isn't coming up in my Sent Messages for some reason - hope you received it.

author: rj88
author_rank: Contributor
timestamp: January 23, 2016, 12:47:37 am
content: Hello there, sorry for the huge delay, if anyone is still willing to give me a hand, I have tested some HV points:TP5 - around 2.5VTP6 - 110-118VTp14 - around 3VThe 1930V point that gearhead referred to is oscillating around +/-4VSo clearly a problem on that end! Any suggestions much appreciated.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: January 23, 2016, 04:36:13 am
content: So you have no HV. The Troubleshooting chart says:Check CRT Bias circuit.Check Q112, D112-D119, C118-C121.Myself, I would start with the 930 volt winding output of the main transformer, first unplugging/unsoldering the connection at J103-P103 to isolate the transformer's output from the rest of the circuit.Note that the CRT filament is lifted to HV by connection to the emitter of Q112. I had a similar fault in an old Tek RM503 -- the HV was leaking from the filament winding in the main transformer to other windings and I had low/no HV, no trace and it was also blowing the line fuse. I solved the problem by disconnecting the filament winding altogether and installing a separate transformer for the filament supply, with good isolation between its primary (line voltage) and secondary (half of a 12V ct secondary). Works perfectly now, the hardest part was finding room inside the case for the new transformer.ETA: I'm referring to this version of the SM:http://bee.mif.pg.gda.pl/ciasteczkowypotwor/Leader/LBO-507%20&%20508.pdf

author: alsetalokin4017
author_rank: Super Contributor
timestamp: January 23, 2016, 09:40:08 pm
content: By the way... this thread should probably be in the "repair" section rather than the "test equipment" section.

author: rj88
author_rank: Contributor
timestamp: January 23, 2016, 10:37:51 pm
content: Quote from: alsetalokin4017 on January 23, 2016, 04:36:13 amThanks very much for the suggestions, I will try to test as many of the voltages as I can

author: alsetalokin4017
author_rank: Super Contributor
timestamp: January 23, 2016, 11:32:39 pm
content: You should probably first check the Q112 transistor, and the diodes and capacitors in the voltage multiplier circuit _unpowered_ and out of the circuit if possible.