Help lowering the trace on an LBO-522 oscilloscope 20MHz

[dukebubs]

After knowing about them for a while, I was lucky to find a nice analog oscilloscope at my local thrift store and I was hoping to get into the hobby. Unfortunately, the trace seems to be stuck near the top of the screen.

I reached out on another board and a veteran hobbyist tried to help me trouble shoot it. We werent able to get much to work. He talked me through tweaking these adjustment points (as seen in the schematic), but nothing really worked.

 This lead him to believe that the 'chip' on the back of the CRT might be damaged, but were really grasping for straws as to what the cause may be now. If anyone has any ideas, Id love to try them. Thanks in advance

[Wallace Gasiewicz]

Is the trace stuck where it is in all ranges?
 And on both channels?

[dukebubs]

this is the lowest the trace will go. adjusting the knobs will make it rise off of the screen. and yes, both channels give the same result

[Andy Watson]

Is the channel 2 attenuator knob pushed in? In the photo it looks like it is pulled-out in the X10 position.
Also, if the trace is really stuck at the top of the screen, it is probably beyond a little tweaking - something more serious is a foot.

[dukebubs]

that was just an older picture. Ive tried every combination of settings, as well as messing with the board inside. I also believe it was something more serious but Id like to track down what it is and see if its something that can be fixed

[R3P]

Hi EEVblog people, I was trying to assist dukebubs with his oscilloscope before they posted on here (at my suggestion). I'll try provide a summary of what we checked together.

Firstly, we attempted basic calibration of the oscilloscope, without much success - the trace was always off-screen. I guided them through safely opening it up (small brief on the dangers of CRTs and oscilloscope circuits) and directed him toward some of the adjustment pots on the vertical amp board. The boards and adjustments in the manual didn't line up exactly with the physical version of the board (possibly a different revision?) and at some point before bubs purchased it there appears to have been some additions made to the circuit on the back with additional pots and resistors wired in - they didn't appear in the service manual. The display does show a portion of a trace, and some of the pots have influenced it, but nothing significant.

Here's the manual https://www.download-service-manuals.com/download.php?filename=Leader--LBO-522--user_and_schematics--ID3791.pdf and the pots that we tried adjusting were all on the vertical amp board (page 24 of the pdf file). We started with the vertical position pots, then the vertical gain 1x and 10x pots. After that several other pots were tried, including the two additional ones that someone hand-soldered in place, those hand soldered ones appear to be sort of compensation for initial voltage spike compensation (the start of the square wave was influenced by it).

We didn't probe anything with a multimeter yet, but as the pots didn't line up with the manual exactly, I might have strugged to give good, safe direction on where and how.

[srb1954]

We didn't probe anything with a multimeter yet, but as the pots didn't line up with the manual exactly, I might have strugged to give good, safe direction on where and how.

Time to get the multimeter out and firstly check that the power supply voltages are all OK.

Then look at the the Y-deflection plate connections - both should be approximately equal voltage for the trace to be centred on the screen. Work backwards through each stage of the Y amplifier - since this is a differential amplifier the collectors of the corresponding transistors should again be approximately equal voltages.

On the schematic there appears to be a test jumper (J22-J23) at the input of the final Y amp that allows disconnection of the output of the beam switching IC (IC201) to allow separate testing of the Y amp. Removal of this will force the input on the final Y amp to zero and the trace should become centred on the screen if the final Y amp is OK. If this does centre the trace it suggests that the fault is in the beam switching IC and you may have trouble getting a replacement for this likely custom IC.

[R3P]

Thanks for the guidance, I'll try to continue assisting via a video call with them soon and get back here with the results.

[Wallace Gasiewicz]

Sometimes the pins on the back of the tube get corroded.The little board that you are calling a chip seems to be making the connection there.
If it has a socket, take it off and clean it. Make sure that the solder joint on it are OK. And make sure the pins on the tube are clean.

[wasedadoc]

Sometimes the pins on the back of the tube get corroded.The little board that you are calling a chip seems to be making the connection there.
If it has a socket, take it off and clean it. Make sure that the solder joint on it are OK. And make sure the pins on the tube are clean.

When taking the socket off the end of the tube, be sure to pull it straight back.  Not at an angle as that could crack the glass around a tube pin.

[Wallace Gasiewicz]

Maybe even use solvent if it is a little stuck