Oschilloscope Leader LBO-520A - Page 1
Forum section: electronics / repair

author: xbasedgr
author_rank: Newbie
timestamp: October 01, 2019, 05:37:32 pm
content: Hello everyone!A friend of mine retired and gave me his oschilloscope, a Leader LBO-520A.After 15 minutes of operation, the oscilloscope stopped working.I opened it and i found R437 burned. I replaced it, but it burned again after some seconds.I checked the ESR of all capacitors on the power board, they all seem fine.I checked all outputs of the transformer, all seem fine.I replaced Q436, nothing changed.I tested Q423, it seems working fine.I have about 500V across R437 when i measure dc voltage.Anyone can help so i can repair thios scope?It means so much for me and my friend, he got so sad when he saw it dying..Thanks for your time,Regards, George

author: shakalnokturn
author_rank: Super Contributor
timestamp: October 01, 2019, 09:56:20 pm
content: Hello,You have a good starting point to asking for help by giving the fault history, link to S.M. and extracts of schematic.If you measured 500V across R437, that means it has gone open again. (Either that or it's a very large resistor...)That section is for the high voltage regulation (CRT supply), the main reason for R437 to burn would be excessive current draw on the H.V. winding (15-16 / 560V).Look for problems on either of the branches 2/3 at J402/P402. Mainly components before R428 on the 1.2kV section and the 6 stage voltage multiplier assembly T1326 on the other branch.If you have no experience with high voltage make sure you know what you're doing before going further.If you have no specific H.V. probes there are workarounds.

author: andy3055
author_rank: Super Contributor
timestamp: October 02, 2019, 03:19:40 am
content: Did you check D444?

author: xbasedgr
author_rank: Newbie
timestamp: October 02, 2019, 05:19:41 pm
content: Thanks for the quick responce!Yes I checked D444, it is ok, even with D444 removed, the R437 is getting burned.I have checked everything before R428, caps measure ok capacitance and ESR, diodes measure ok with DMM.I have tried disconnecting the 6 stage voltage multiplier assembly T1326, and still R437 is getting burned. (assembly T1326 is directly connected via a single cable to transormer at point 16)No, i dont have H.V. probes, thats why i cant check the H. V. part.I wonder if a diode still reads ok with the DMM, but at high voltages is getting crazy...?I attach some pics of the H. V. assembly T1326.Caps and diodes measure ok...

author: floobydust
author_rank: Super Contributor
timestamp: October 02, 2019, 06:44:58 pm
content: R437 burning up may mean there is an overload on the main HV power supply.The Marcon oil-filled capacitors are known to short circuit. Check C424-C429 and C431-434 for shorts, R428.Be careful with these HV capacitors, they can bite. Use a ~10k 1W resistor to discharge them before testing, stray HV can damage your multimeter or give you a shock.

author: shakalnokturn
author_rank: Super Contributor
timestamp: October 02, 2019, 08:52:25 pm
content: If the scope has been tested without T1326 multiplier I think we can rule that one out.D444 has been tested for shorts and removed but not tested for open circuit. Even going open I'm not sure it should burn R437, could destroy Q423 though.D417 to D420 have been checked?Try testing transformer isolation between windings 4-5/GND, 6-7/GND, 15-16/GND.

author: floobydust
author_rank: Super Contributor
timestamp: October 02, 2019, 09:26:49 pm
content: Quote from: shakalnokturn on October 02, 2019, 08:52:25 pmNope, disconnecting the CRT HV multiplier does not isolate the -1.2kV voltage doubler, which if shorted would cook R437. R428 would cook if the overload was downstream of the doubler.Anyway, this is where I would start troubleshooting. Using a variac might be best to take some voltage readings or limit smoke.AnotherLeader 302 scope repair threadfound many bad capacitors and HV selenium diodes.

author: shakalnokturn
author_rank: Super Contributor
timestamp: October 02, 2019, 10:07:22 pm
content: Quote from: floobydust on October 02, 2019, 09:26:49 pmAgreed, I still meant what I wrote: If it was powered without T1326 (6kV H.V. mult. assembly) and still fries R437 the 6kV mult. can be ruled out. (Why not? Is my phrasing faulty? Or T1326 gone paranormal?)Also agree on the before/after R428, that's why I pointed to testing components before that resistor.

author: floobydust
author_rank: Super Contributor
timestamp: October 03, 2019, 12:47:53 am
content: Sorry just a misunderstanding about what was being ruled outFor the Marcon capacitors, modern polypropylene film caps work for replacements. C424+C425 as 1.1uF 900V, C426-C429 total 1.2uF 1.8kV same as C431-C434.I would make sure the scope has the correct line voltage switch and fuse (for protection).Use a series light bulb on mains to current-limit so the power transformer does not get cooked, as the HV winding is being overloaded.

author: andy3055
author_rank: Super Contributor
timestamp: October 03, 2019, 05:14:12 am
content: Quote from: xbasedgr on October 02, 2019, 05:19:41 pm

author: xbasedgr
author_rank: Newbie
timestamp: October 03, 2019, 06:29:28 pm
content: Newest update:The scope has the correct line voltage switch and fuse, checked both.Removed Q423 and the R437 is NOT getting burned.Removed Q436 with Q423 in place, the R437 is NOT getting burned. (I think the 4 transistor cirquit at the right of the optocoupler is fine then)Going to replace D412 up to D420, C424 up to C434.I will try to find some replacement caps for H.V. part too...

author: floobydust
author_rank: Super Contributor
timestamp: October 03, 2019, 06:45:49 pm
content: Q423 and opto is not the problem, I'd guess it is the voltage doubler for the -1,230V supply has bad Marcon oil-filled capacitors.The only other way to troubleshoot is to give the scope 1/10 of mains voltage (say 12VAC input instead of 115VAC), bypass voltage regulator (short) Q423 C-E, and see what you get at C431. It should be over -123VDC with no ripple if the doubler is working somewhat. The big capacitor voltages should be equally divided up i.e. not all across say C423 or C434.Make sure you are doing a good test on the Marcons - take them out, check capacitance and leakage. Cheap DMM's will wrongly read high capacitance on leaky capacitors.

author: xbasedgr
author_rank: Newbie
timestamp: November 02, 2019, 09:43:40 am
content: Hello again,I have started buying all the capacitors of the power supply board in order to replace them.I have come to a dilemma at the 6 caps at the HV board.At the schematic they seem to be non pollarised.But what you think looking at them? Do they seem like normal axial pollarised capacitors?

author: andy3055
author_rank: Super Contributor
timestamp: November 02, 2019, 04:27:55 pm
content: Those should be non-polarized. But make sure the voltage rating is the same or above. It is given in the circuit diagram.

author: xbasedgr
author_rank: Newbie
timestamp: January 05, 2020, 09:21:32 am
content: Ooooooook after months of waiting on the bench, the time came to replace all the capacitors at the -1.2kV voltage doubler.And the result...We have a trace! All seem working well!I want to say thanks to all of you that spent your time to advice me where to look and what could cause the problem.

author: andy3055
author_rank: Super Contributor
timestamp: January 05, 2020, 05:28:52 pm
content: Great job! It is looking nice and clean. Thanks for the feedback.

author: shakalnokturn
author_rank: Super Contributor
timestamp: January 06, 2020, 06:00:22 pm
content: How's it doing for risetime?

author: xbasedgr
author_rank: Newbie
timestamp: January 06, 2020, 11:03:59 pm
content: I played around with the scope and it seems working fairly well.Another thing i would ask is this:When i use single shot for triggering, i press reset, screen blank and ready light lits. Good so far.I trigger something on the probe, it momentarilly appears on screen, ready light goes off and screen goes blank again.Same if i use ch1, ch 2 or ext trigger.I dont know if this scope is capable of showing this single shot, i read somewhere about turning intensity up and use a camera with slow shutter to record it...(shakalnokturn: What you mean "How's it doing for risetime?" My english are not the best)

author: shakalnokturn
author_rank: Super Contributor
timestamp: January 06, 2020, 11:14:00 pm
content: I was questioning the frequency response your square wave has quite visible edges to it but that could be the probe compensation output itself.The scope has no form of integrated memory so single shot would be useful to capture a single sweep with a camera.The problem having multiple sweeps is getting exposure right (hard enough with a single sweep) and if signal isn't repetitive you'll have a mess on the photo.

author: xbasedgr
author_rank: Newbie
timestamp: January 16, 2020, 08:07:30 pm
content: shakalnokturn:You were right, one of the probes was the problem.I replaced it with a new one and this is the result.

author: floobydust
author_rank: Super Contributor
timestamp: January 16, 2020, 08:16:58 pm
content: A scope probe has a trimmer capacitor to adjust the compensation, you know that right?

author: xbasedgr
author_rank: Newbie
timestamp: January 16, 2020, 08:51:21 pm
content: Yes i tried it but it did not seem to change something...I will try it again tommorow tho, when i have more time.

author: argonian
author_rank: Newbie
timestamp: April 01, 2021, 01:32:14 am
content: Greetings friends!Recently i bought an Leader LBO with the exact same problem, R437 is getting burnt and also D444 its poped.I tried to replace all high voltage capacitors with 22nF polyethylene caps, now R437 is not getting burned but still no trace is shown on ScreenHave you run into the same problem? I dont have any HV probes so im not able to check the voltage entring the HV module (which supposly should be IN=1kV OUT=6kV)

author: xbasedgr
author_rank: Newbie
timestamp: April 01, 2021, 07:54:20 pm
content: Hello. I solved my problem by changing all the electrolytic capacitors on the PSU board. I did not changed any of the HV ones that are located in the metallic box on the rear of the scope.This is the board where i replaced the capacitors, i cannot find the foto of the other side atm, i will keeep looking...