Leader LBO 505 Scope Repair

[Juan_Ctan02]

Hello!

I recently got my hands on a Leader brand LBO-505 oscilloscope, that does not work. I have spent the past two weeks attempting to repair it, but I am having a lot of trouble isolating the fault.


Symptoms from the outside are no trace on the screen, regardless of button/input combination. The scope powers on just fine, and does not seem to overheat or produce smoke/smell.


First of all, I stripped the case away, did a little visual inspection and aside from some dust here and there, nothing seemed visually damaged or otherwise faulty. No unconnected cables, no burn marks, no obviously busted electrolytic caps, etc. I did notice something that looks a bit like some slight, slight bulging on two of the four bigger caps on the power supply side however.

Next, "Thou shalt test voltages" and thy voltages I did test. I tested all of the testpoints shown in the service manual with a multimeter, and every singe one of them gave somewhat correct measurements. If anything, all of them seem to be a slight % too high like +45V instead of +42, -7.8 instead of -7.6, +330V instead of +320V etc, but none are way out of line. I even tested the HV line, which according to the manual should be between -1800V and -1320V, and got somewhere around -1530V.

This confused me, since I thought that all voltages being fine, it would most likely be the HV circuit failing to supply enough kV. However, it seems like the HV circuit is fine, but there is still no trace at all on the screen. This lead me to think that maybe the CRT was busted, and hence not repairable, however, when I powered it on with all the lights in the room off, in almost total darkness, I noticed that there was indeed a trace!

This trace looks like a vertical blur that sweeps from left to right. From this, I was able to verify that the horizontal controls work fine, both position and timescale. However, none of the vertical controls did anything. The same faint vertical blur came out regardless of volts/div settings or vertical position settings.

Both channels exhibit the same exact behaviour so I believe it is unlikely that the failure is in the vertical preamps. I have been probing around on the vertical amp board but that is stretching my troubleshooting skills a little.


Anyone has any tips on what to test / where to look / what to probe ? Or other ideas as to where the failure might be? Only thing I can think of trying now is replacing those seemingly bulging caps on the supply, but I would have to de-solder like 20-30 unique wires to be able to access them, and all of the supply voltages seem mostly fine.

[GerritMax]

Might be silly but does it have a brighness knob and if so does that make any diference

[Juan_Ctan02]

Yeah it has an intensity knob, which effectively changes the brightness of the blur from barely visible in the dark to completely invisible :/

A guess I have is that since the vertical blur is spread out from top to bottom, not enough electrons hit the screen at any one point to produce any significant brightness

[fzabkar]

These scopes were plagued by dry solder joints at the through-holes. I painstakingly repaired my LBO-514 by tediously inserting a pin in every through-hole to reliably interconnect both sides of the PCB. I assume your model would be affected by the same cancer.

[Juan_Ctan02]

As far as I can tell from visual inspection and the diagrams in the manual, all the PCBs seem to be single sided. What do you mean by dry solder joints?

[tunk]

https://en.wikipedia.org/wiki/Soldering#Soldering_defects