Leader LBO 520A Oscilloscope

J

John Keiser

Guest
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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In article <8sydnUgLQ6B0-1zHnZ2dnUU7-YHNnZ2d@powerusenet.com>,
johnkeiser@juno.com says...
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





---
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus
Click to expand...

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!
 
On Monday, 2 April 2018 00:10:42 UTC+1, John Keiser wrote:
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
Click to expand...

circuit diagram unintelligible.


NT
 
On Monday, 2 April 2018 00:37:30 UTC+1, tabby wrote:
On Monday, 2 April 2018 00:10:42 UTC+1, John Keiser wrote:

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

circuit diagram unintelligible.
Click to expand...

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
 
On 4/1/2018 1:10 PM, John Keiser wrote:
>
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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A decent print for the 520 but not A is at

https://elektrotanya.com/leader_lbo-520_2x5mv,30mhz_delay_line_5inch_oscilloscope_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 keep the bots out.

Anyway, R 437 feeds Q 423 which appears to be a simple pass regulator. It is 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 regulates that.

Disconnect C 424 or 425 and see if the resistor is still stressed. If not look for a short around D 412 through 415. If there is no short try it connected 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 transformer for internal leakage to ground via that winding. That is actually among 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 leaves he circuitry associated with that doubler for the negative source. Any shorted 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 is your only print, get the manual for the 520 as it seems to conform to what 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 start disconnecting. Right now with the short it will be high. Once the short is gone by disconnecting things it will be low because due to the lack of output voltage the feedback will force Q 423 into full conduction. That is when you found it.
 
Reading fast I missed the part where you disconnected the CRT, so forget that part.
 
On 02/04/2018 00:10, John Keiser wrote:
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





---
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus
Click to expand...

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.
 
In article <639090a1-3412-4d07-96c0-1cfd964b91d8@googlegroups.com>,
jurb6006@gmail.com says...
A decent print for the 520 but not A is at

https://elektrotanya.com/leader_lbo-520_2x5mv,30mhz_delay_line_5inch_oscilloscope_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 keep the bots out.

Anyway, R 437 feeds Q 423 which appears to be a simple pass regulator. It is 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 regulates that.

Disconnect C 424 or 425 and see if the resistor is still stressed. If not look for a short around D 412 through 415. If there is no short try it connected 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 transformer for internal leakage to ground via that winding. That is actually among 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 leaves he circuitry associated with that doubler for the negative source. Any shorted 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 is your only print, get the manual for the 520 as it seems to conform to what 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 start disconnecting. Right now with the short it will be high. Once the short is gone by disconnecting things it will be low because due to the lack of output voltage the feedback will force Q 423 into full conduction. That is when you found it.
Click to expand...

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 :

>"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 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 actual manual.

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

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

Me too. In fact my older browser that I LIKE it vulnerable to a button installed 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 Chrome, which I can't strand, and do a "Paste And Go". I need Chrome for certain sites. Then I got IE for downloading videos and music because FF gets it 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 not use it for casual surfing because of the obvious reasons. I need it mainly to be on the house network because it has extensive media and other files. 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 worried 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.
 
jurb6006@gmail.com wrote:
Perhaps you missed this part :

"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 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 actual manual.

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

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

Me too. In fact my older browser that I LIKE it vulnerable to a button installed 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 Chrome, which I can't strand, and do a "Paste And Go". I need Chrome for certain sites. Then I got IE for downloading videos and music because FF gets it 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 not use it for casual surfing because of the obvious reasons. I need it mainly to be on the house network because it has extensive media and other files. 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 worried 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.
Click to expand...

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.
 
On 4/4/18 8:35 AM, jurb6006@gmail.com wrote:
I wonder what happened to our OP here.
Click to expand...

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
WA6FWi
http:foxsmercantile.com
 
On 4/4/2018 3:57 AM, Fox's Mercantile wrote:
On 4/4/18 8:35 AM, jurb6006@gmail.com wrote:
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.
Click to expand...
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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This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus
 
responding to
http://www.electrondepot.com/repair/leader-lbo-520a-oscilloscope-176993-.htm ,
xBased wrote:
Did u had any luck repairing? I have the same oscilloscope with the exact SAME
problem. :(
 
responding to
http://www.electrondepot.com/repair/leader-lbo-520a-oscilloscope-176993-.htm ,
George wrote:
Btw if you still need to repair it, contact me via message, i managed to get
my own fixed.
 
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