Leader LBO-522 Beam not reatching the screen - Page 1
Forum section: electronics / repair

author: Pikasoo
author_rank: Newbie
timestamp: June 06, 2025, 01:28:31 am
content: My LBO-522 has been a great help to troubleshoot other stuff but today it suddenly stopped displaying the line.After a bit of test i can see the green light moving on the X axis when placed in slow dividerIt's hard to show since it doesn't really appear on picture. Its a kind of reflection of the light moving from left to right. Playing with all knobs and the Y center doesn't do much, going fully up or down make the glow effect dim.It's as if the beam cant reach the screen and get dissipated on the tube.When trying to film it, the beam randomly popped up on the left, so i guess something in the horizontal circuit is wrong. and the display is way on the left but the knob cant bring it back properly.on visual inspection nothing seam to be blown up.video of the issue:https://youtu.be/OdV7HGt3Ywg

author: andrewtaylor
author_rank: Contributor
timestamp: June 06, 2025, 04:51:45 am
content: Hi, try to geht the schematic, and start with check the DC voltages in the Low voltages power supply section.

author: Zenith
author_rank: Frequent Contributor
timestamp: June 06, 2025, 11:30:29 am
content: An instruction manual with calibration instructions and schematics is available online from the usual places. I couldn't find a service manual but I didn't look very hard. The schematics look clear and have expected wave forms in places.After eyeballing it to look for anything burned, damaged, or disconnected, as andrewtaylor says, the place to start is with the power supplies, making sure they produce the correct voltages and with acceptable ripple. If the power supplies aren't right, other fault finding is a waste of time. When fixing a scope, you can quickly find you need another scope, which need not be a very good one.Be careful with the EHT. It can give you a very nasty nip.

author: Pikasoo
author_rank: Newbie
timestamp: June 07, 2025, 01:22:44 am
content: got the schematics, and yep, i wish i had an oscilloscope to repair it...For now i can at least test the low voltages.

author: Zenith
author_rank: Frequent Contributor
timestamp: June 07, 2025, 07:59:29 am
content: With multimeter and some thought, you can do more than check the supply lines. But as you said, an oscilloscope is useful for troubleshooting other things, and now your oscilloscope is itself one of the other things. You could for instance see whether the sweep is being generated properly, and whether what comes out of the X amp is what it should be.

author: Pikasoo
author_rank: Newbie
timestamp: June 07, 2025, 01:10:32 pm
content: Well, the issue is fixed but i dont know for sure what it was really..By measuring the voltages i didnt find anything useful and got to the 4 transistor controlling the X axis. So i decided to de-solder them and swap them around. After then the line displayed properly, witch let me believe that one of them is failing and is now at a less demanding place or i had a crack on the solder i didnt see and it's now fixed.At least i have it back for now!Thx for the helps!

author: Zenith
author_rank: Frequent Contributor
timestamp: June 07, 2025, 03:46:56 pm
content: Bad soldered joints can cause a lot of problems. It's also possible one of the plug in connectors was faulty because of tarnishing over the years, and you either unplugged it and re-plugged it, or just moved it slightly, so making it give a good contact. It's also possible you have a crack in a printed circuit track. Faults like that can be hard to pin down and they can be heat sensitive or vibration sensitive. It would be worth looking with a magnifying glass.I don't buy the theory that one of the transistors was weakening, and moving it around gave it a load it was better able to handle.Anyway, congratulations on a good result.