Leader LBO 520A Oscilloscope

circuit diagram unintelligible.

NT

I can't read your schematic, very low res...

but anyways, I would say it could be one or two things since you obvioulsy have checked the caps and diodes..

The Tranformer may shorted to a HV wouding...

The heater in the CRT may have shorted to the cathod, unplug the CRT and see what happens with the voltages.

If the heater did short, install a small transformer just for the heater and let it float..

if you can provide a better schematic to look at maybe you would get better help!

Until we get a readable schematic, as a general principle one can always disconnect as many loads as poss to find out which one is sucking the thing to death.

NT

I have disconnected all loads, including the CRT. Still radically wrong voltage appearing at R437. Here's a better [but much bigger file] version of the schematic. Thank you for reviewing and comments.

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A decent print for the 520 but not A is at

Scroll down and get to where it says "processing". Watch that until it says "Get manual" and click there to download. I guess that rigamaroll is to ke ep the bots out.

Anyway, R 437 feeds Q 423 which appears to be a simple pass regulator. It i s in the return for a 560 volt winding that is doubled for negative voltage , which would only be used for the cathode circuit of the CRT. This regulat es that.

Disconnect C 424 or 425 and see if the resistor is still stressed. If not l ook for a short around D 412 through 415. If there is no short try it conne cted but without the CRT socket on. Look for arcing in the socket. If none, disconnect the filament winding from the power transformer at pins 5 and 4 and see if the resistor is comfortable then. If so, suspect the power tran sformer for internal leakage to ground via that winding. That is actually a mong the most likely suspects.

If it proves to be the transformer it is no doubt unobtainium, but there is a way. You need an isolated transformer to feed the filament, a 6 volt AC wall wart would work. Or a regular hipot transformer will work. If you use a wall wart make sure it puts out AC, not DC.

If it is not the transformer, it is not likely to be the CRT, so that leave s he circuitry associated with that doubler for the negative source. Any sh orted cap for example. When they are in series like that, one shorting will impose excessive voltage on the rest.

If all that is good then move on to the circuitry associated with Q 420.

The problem has to be around there somewhere. If the picture you supplied i s your only print, get the manual for the 520 as it seems to conform to wha t you said. It should be close enough. I checked the 522, 510, whatever and none of them are close, there is no R 437 anywhere nearby in any of them.

The voltage at the junction of D 417 and 418 would be telling once you star t disconnecting. Right now with the short it will be high. Once the short i s gone by disconnecting things it will be low because due to the lack of ou tput voltage the feedback will force Q 423 into full conduction. That is wh en you found it.

Reading fast I missed the part where you disconnected the CRT, so forget that part.

As it will be a linear ps presumably, before frying anything else, run on a variac at 50% or a step-down site transformer and ratio the voltages.

suspects.

why can't I go to those websites any more without down loading their down loader first?

I don't want toolbars installed, TSR's running, adds popping up on my PC all over the place...

What a shame the NET has turned into a field of garbage.

Perhaps you missed this part :

ys "Get manual" and click there to download. I guess that rigamaroll is to keep the bots out. "

Don't click on anything else. It does take a bit of time, but the text DOES change from "processing" to "Get manual". Those letters are the link to ac tual manual.

That bit of test is underneath the picture on the left. Small print of cour se. At least they don't use that %$&*# captha shit.

all over the place... "

Me too. In fact my older browser that I LIKE it vulnerable to a button inst alled by eBay. It is not all that easy to disable. Pricks. So when someone puts up a link to an eBay page I right click and save location and go to Ch rome, which I can't strand, and do a "Paste And Go". I need Chrome for cert ain sites. Then I got IE for downloading videos and music because FF gets i t in some weird format, Chrome doesn't get them at all but IE gets them in native MP4 so I don't have to convert them. And they're smaller.

I prefer FF 18, some people like even older versions. I got 15 on my old XP box but that is currently not on the net, but it will be. However I will n ot use it for casual surfing because of the obvious reasons. I need it main ly to be on the house network because it has extensive media and other file s. The net down there would be limited to youtube, places to get datasheets and manuals. Qnd not even that much youtube. The IE got damaged by a virus years ago, and the highest you can get is 8 for XP so I just ain't too wor ried about it.

At any rate, the link it there but it is just a bit hidden. It doesn't even look like a link but you can tell by the cursor.

I hung in there with Firefox 28 until it was just non functional on many sites, FF even managed to snuck in some updates via Thunderbird which screwed it up. The latest rendition of FF is Chromewannabe and microsoft Edge is such a useless piece of crap. try Seamonkey, its FF28 on steroids, most FF add-ons will work and so far I really like it.

I wonder what happened to our OP here.

Probably gave up after you hijacked the thread to complain how the entire world is out to get you.

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Jeff-1.0 
WA6FWi 
http:foxsmercantile.com

No, I didn't give up and appreciated your encouragement. The power supply board components all seem to test good with an ESR meter and DMM. I unsoldered several suspects to be sure. Being discrete components, this is something I hope to puzzle out. I am fairly convinced the issue is not on the power supply board itself. I have the 520A service manual but the schematic varies from the actual production board even though the board is marked for both 520 and 520A. This slows me down. I will update if I solve the problem. Thank you.

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the entire world is out to get you. "

I didn't do that.

responding to

, xBased wrote: Did u had any luck repairing? I have the same oscilloscope with the exact SAME problem. :(

responding to

, George wrote: Btw if you still need to repair it, contact me via message, i managed to get my own fixed.