Leader LBO 520A Oscilloscope

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This is a 30 year old dual channel 35 Mhz scope I was given.
It is simply a challenging retirement project.
I am a hobbyist, not a technician.
 From old TV days, I am aware of the hazards of HV.

The unit is very clean inside and outside but R437 [4.7 K] in the early  
stages of the power supply  fried immediately when I powered on.
Replacement did the same.
No other burnt components are observed.
Here's the schematic: https://imgur.com/E6WV4yD

I have an ESR and all caps seem OK.
All the diodes I tested seem OK as well.
I removed and tested the Q423, the diode/limiter D 444 and optocoupler  
which are connected to R437.  All appear good.

The 560V winding of the transformer is not grounded but I assume  
measurements stated on the schematic are to ground.

R 437 reads -270V [to ground] but the schematic says should be 1.8V  
[hard to read.].  [I would have thought such a large variance easy to  
spot.]  The 15, 40 and 150 [schematic says 130 but board says 150] volt  
rails seem accurate and steady.

Before I abandon the project, does anyone have experience with A Leader  
scope this old?  Does the schematic provide any clues that I am unable  
to see?

Thanks for any comments.

John Keiser





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Re: Leader LBO 520A Oscilloscope
On Monday, 2 April 2018 00:10:42 UTC+1, John Keiser  wrote:
Quoted text here. Click to load it

circuit diagram unintelligible.


NT

Re: Leader LBO 520A Oscilloscope
On Monday, 2 April 2018 00:37:30 UTC+1, tabby  wrote:
Quoted text here. Click to load it


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

Re: Leader LBO 520A Oscilloscope
snipped-for-privacy@juno.com says...
Quoted text here. Click to load it

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!


Re: Leader LBO 520A Oscilloscope
On 4/1/2018 1:10 PM, John Keiser wrote:
Quoted text here. Click to load it
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.
https://imgur.com/a/QURla


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Re: Leader LBO 520A Oscilloscope
A decent print for the 520 but not A is at  

https://elektrotanya.com/leader_lbo-520_2x5mv,30mhz_delay_line_5inch_oscill
oscope_1977_sm.pdf/download.html

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.

Re: Leader LBO 520A Oscilloscope
Reading fast I missed the part where you disconnected the CRT, so forget that part.

Re: Leader LBO 520A Oscilloscope
snipped-for-privacy@gmail.com says...
Quoted text here. Click to load it
suspects.  
Quoted text here. Click to load it

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.


Re: Leader LBO 520A Oscilloscope
Perhaps you missed this part :

Quoted text here. Click to load it
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.

Quoted text here. Click to load it
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.

Re: Leader LBO 520A Oscilloscope
snipped-for-privacy@gmail.com wrote:
Quoted text here. Click to load it

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.

Re: Leader LBO 520A Oscilloscope
I wonder what happened to our OP here.

Re: Leader LBO 520A Oscilloscope
On 4/4/18 8:35 AM, snipped-for-privacy@gmail.com wrote:
Quoted text here. Click to load it

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

--  
"I am a river to my people."
Jeff-1.0
We've slightly trimmed the long signature. Click to see the full one.
Re: Leader LBO 520A Oscilloscope
On 4/4/2018 3:57 AM, Fox's Mercantile wrote:
Quoted text here. Click to load it
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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Re: Leader LBO 520A Oscilloscope
Quoted text here. Click to load it
the entire world is out to get you. "

I didn't do that.

Re: Leader LBO 520A Oscilloscope
On 02/04/2018 00:10, John Keiser wrote:
Quoted text here. Click to load it

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.

Re: Leader LBO 520A Oscilloscope
responding to
http://www.electrondepot.com/repair/leader-lbo-520a-oscilloscope-176993-.htm ,
Did u had any luck repairing? I have the same oscilloscope with the exact SAME
problem. :(

Re: Leader LBO 520A Oscilloscope
responding to
http://www.electrondepot.com/repair/leader-lbo-520a-oscilloscope-176993-.htm ,
Btw if you still need to repair it, contact me via message, i managed to get my
own fixed.

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