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LBO-514A Oscilloscope no TRACE

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mich82

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I bought Leader LBO-514A Oscilloscope, it powers on but no Trace at all. I opened it and checked the PSU voltages.
Right now, all the required voltages for Scope to work are present (ie -15,+15, 250v, 1750V HV) but still no trace . I only see a DOTTED Light which flash ONCE upon switching off the Oscilloscope.
What could be the problem? PLEASE HELP ME FIX my LBO-514A Oscilloscope. Thanks.
 

Your high voltage measurements sound about right. Do you see both +250 and -250v? The beam gets a negative polarity.

To see the dotted line at power-down suggests you have a beam but it is driven way off the screen when powered. The vertical or horizontal plates send an abnormal voltage, whereas they ought to send a waveform which makes the beam go Left-Right & up/down.

Things to try:

a) Does your scope have a vertical adjust and horizontal adjust? After power-off, try turning these knobs quickly, and watch if it affects the beam.

b) There are settings which might prevent starting a sweep. (Example, threshold voltage.) Choose every setting that causes sweeps repeatedly. Apply various signals.

c) Maximize beam intensity. Change the focus. Look for any bright patch at the edge of the screen, and the side of the crt.

d) Turn off horizontal sweep. (Normally the beam becomes stationary although it can respond vertically). Apply signals to the horizontal input jack. Maximize horizontal sensitivity. Watch to see if sufficient voltage brings the beam onto the screen.
 

Thanks, BradtheRad. I now get a Dot beam when I disconnect the CRT tube pin 3 cable from the H.V psu circuit, the beam only disappears when I put it back. With the Dot beam on the screen, all the control knobs work fine but I can not get the beam to sweep horizontally on the screen not matter the settings used .
Strangely, CRT pin 3 is giving 1480 V in the disconnected G1 cable, while pin on the H.V psu circuit is also giving out about the same HV voltage.
 

Strangely, CRT pin 3 is giving 1480 V in the disconnected G1 cable, while pin on the H.V psu circuit is also giving out about the same HV voltage.

Combined your symptoms suggest some kind of improper connection between hi-voltage generator and one or two horizontal plates (or else the horizontal sweep generator.) Such a thing is unlikely and more troubleshooting is needed.

As a result the hi-voltage gets lost in the horizontal drive, or vice versa. When you separate them, one of them starts to work but not the other.

By the way, the electron beam should be negative polarity relative to chassis, or ground. (My scope hi-voltage is -1000 VDC, yours should be -1480 or -1750).

Things to look for:

* improper wire connections

* high voltage wire touching components

* build-up of carbonized dust and lint improperly carrying hi V to horizontal drive

* blobs of solder on circuit board, bridging across copper traces

* misplaced components

* component leads bent so they touch other components

My own scope gives me recurrent problems with arcing of the high-V circuit. It occurred first inside the main power transformer, then in each extra transformer which I installed to create 6.3V heater voltage for the tubes. I want to make a substitute hi-V source (using Cockcroft-Walton voltage multiplier), but I'm not certain it would succeed.

If you have sufficient knowledge, you can try making a hi-voltage source for your electron beam, and disconnect the internal one.
 

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