Leader LBO-508 oscilloscope repair - arcing transformer? - Page 1
Forum section: electronics / repair

author: idontseeit
author_rank: Newbie
timestamp: July 28, 2019, 06:07:58 am
content: Hello! Before I get into this, I would just like to preface with saying that I am an electronics newbie. Apologies in advance if I say something that is incorrect (kind of inevitable), or I happen to be misunderstanding a concept. Please please please feel free to correct me on anything!Okay with that out the way... about 2 weeks ago, I acquired an old Leader LBO-508 oscilloscope (here is thelink to the service manual).It was sold in my hometown at a reasonable price and the scope was advertised as completely functional with all calibration and other tests performed.After playing with it a few days with no issues, I happened to walk out of the room for ~20 minutes with the scope still powered on and upon coming back I noticed this sporadic flashing on the CRT display, as well as a periodic "clicking" noise that matched the frequency of the flashing (approximately once every couple seconds). I have some god-awful-quality videos attached:1st vid (shows a bit of the flashing and if you turn up volume you can hear faint clicking)2nd vid (shows more of flashing)So important note to make here: both traces come on and stay on. All functionality (as far as I have checked) works fine. Time divisions, horizontal and vertical adjust, both channels, etc. etc. Yet this flashing is overlaid on the display once the clicking noise occurs and it repeats periodically.Couple more notes:-In addition to the clicking noise, there is a slight audible hum coming from the transformer-The clicking frequency increases the longer the oscilloscope is powered up. Can get as fast as 3-5 times/second.-The clicking seemed to be also coming from the transformer upon removing the cover and listening closely to the noise.After identifying the transformer as a possible culprit, I ran some resistance tests out of circuit (if you look at pdf pg 37 bottom left, I disconnected J103, J104, J105) to see if any windings were perhaps shorted. There didn't seem to be any shorts from any secondary coils to primary coils... however... when I power up the scope and I measure the voltage of the primary 100V, whenever the clicking noise occurs the voltage spikes sometimes 50-70V, and sometimes even up to 300V on the primary line. So maybe the short is only present once line (13) is held at 930V?If I haven't misinterpreted anything, isn't this a dead giveaway that the transformer is shot and perhaps arching to the primary/and or secondary coils in some fashion? Maybe to the CRT filament transformer coils as well? Here is where I am kind of perplexed on the exact science of what is going on inside the transformer. I am assuming that the clicking noise is some sort of arcing but I could be wrong on that.Anyway... I guess I made this post to more document my progression with the repair and just verify what I am interpreting. This is my first experience with any sort of repair so I am definitely open to any feedback ya'll might have or if my logic seems to be greatly flawed.All the best and thank you!-JustinP.S. I would measure the 930 VAC line directly but unfortunately I do not have a multimeter that can go to that high of a probing voltage.

author: jdragoset
author_rank: Frequent Contributor
timestamp: July 28, 2019, 10:16:38 pm
content: The periodic "click" implies a DC arc-over, which could be from the heater winding.Try to uncover (magnetic shields) and observe in a darkened room where the arc-over is occurring.

author: Chris56000
author_rank: Frequent Contributor
timestamp: July 29, 2019, 06:59:26 pm
content: Hi!If the CRT heater winding is the cause of the fault, it's easy to arrange an alternative supply – RS Components do a 6VA 6V transformer which is flash–tested to 4000V, primary to secondary, if you mount one of these on a thick piece of perspex or acrylic, etc., about 20 mm away from chassis metalwork, you can operate the CRT heaters from this replacement transformer, leaving the heater winding of the main power transformer unconnected.Alternatively you can get small encapsulated DC to DC converter modules rated at 3500V isolation, buy one rated at 10W output, with 24V in and 9V out, inserting a (well insulated and protected!) resistor of about 4.7 or 5.6 ohms @ 2W in seres with one of the o/p leads to the CRT heaters!If the ticking/jumping is still present after trying an alternative well insulated 6.3V heater supply, of the paper/plastic film smoothing capacitors in the –2350V e.h.t. supply circuit could be arcing over internallyChris Williams