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Leader LBO-505 Scope problem

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AA9GG

New Member
Hello all....
I just picked up a Leader LBO-505 scope (with manual!) at a hamfest for $5. There is no trace display. I'm assuming it's a HV problem. I get all the voltages at the testpoints, but I did not measure the HV testpoint(+1900vdc!). That's my problem....my meter(s) don't go that high. Any tricks/tips for measuring this? I guess what I'm looking for is basically a go/no go test. Ideas?
 

mvs sarma

Well-Known Member
Hello all....
I just picked up a Leader LBO-505 scope (with manual!) at a hamfest for $5. There is no trace display. I'm assuming it's a HV problem. I get all the voltages at the testpoints, but I did not measure the HV testpoint(+1900vdc!). That's my problem....my meter(s) don't go that high. Any tricks/tips for measuring this? I guess what I'm looking for is basically a go/no go test. Ideas?
Nice price. perhaps the seller knows its history and can now reveal it. so that any pre tested and replaced components etc can be cross checked.

make a chain of 5 numbers of 5 or 10 Meg 1/2 watt resistors in series, across the 1900V DC. then measure across bottom resistor and ground.
you get 20% of the value with meter error to be calculated and managed.
 

AA9GG

New Member
ok....found a bad transistor in the HV osc and replaced it, along with a resistor that was open. I think I have my HV now (I see the 4 neon bulbs they use for Vreg glowing when triggered), but I still get no trace display :(
I do get the blanking voltage....I'm wondering if the CRT is any good! I can see the filament glowing in a darkened room though.

Maybe I don't have a full 1900v? I'll try the R chain you described (I guess I should get around 375-400v?)

thanks....
 

AA9GG

New Member
Ok...wired up 5 1Meg ohm resistors in series, connected top end to HV and the bottom end to ground. Schematic says HV is around -1900 volts so I figured it should read around 400 volts or so. Measuring across the bottom resistor I get 0 volts. :( Guess I need to dig deeper into this.
 

AA9GG

New Member
Tweaking the HV pot, HV measures around -900v....I'm 1000v to low (should be -1900v)!!!! Hmmmm
 
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bountyhunter

Well-Known Member
Do you have a schematic? I am guessing the design generates HV with a transformer winding and maybe a diode multiplier to boost it up? That's how most low priced scopes go for getting 2kV. You may want to check the rectifier diodes and maybe the HV ceramic disc capacitors if there are some in the HV supply..
 
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AA9GG

New Member
Turns out that someone replaced the HV selenium diode with 3 standard rectifier diodes. I replaced them with a single HV fast-acting diode. The HV is now at about the -1900v it's suppose to be! The voltage rating of the diodes were ok, but the did not recover fast enough so the voltage was "folding" back. I also had to replace the HV osc transistor, as well as a vertical drive transistor. Now all I have to do is just some basic alignment. This scope is looking pretty promising now.
 

alemao

New Member
datasheet lbo 505

hi I'm in Brazil and I need your help if possible, I do not know you answer it, but I also have a problem I adiquiri an LBO 505 broke and I need the datasheet.
you could help me by sending me email lucas.paulochaves @ gmail.com
Thanks in advance

EXCUSE MY ENGLISH BUT IS WHY I'M USING GOOGLE TRANSLATOR
 

audiofox

New Member
LBO-505 Ocilliscope

Hello all,

I recently purchased an LBO-505 from a hamfest but was not lucky enough to get a manual with it. Above it sounded like there may be some manuals out there. just wondering if I could trouble someone out there to send me a copy some way-some how or lead me in the right direction. She seems to be working ok but would be nice to see the manual. Help would be greatly appreciated.

Sincerely,
audiofox
 

Bule

New Member
If you Google LBO-505 you will notice that someone is offering a free download of the manual.
 
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