Leader LBO-325 CRT control issues + more

[Matt Hatter]

I've got an old Leader LBO-325 that I picked up a few years ago as my first scope, but got busy with life and didn't use much.

Fast forward to now and I am a Physics Major that has been thrown back into the thrills of hobby electronics. I pulled out the cute 'ol girl, but I cannot get any of the CRT adjustment VR's to function sans the backlight. I initially had some response from the Focus pot, but it quickly stopped working (And on the most out of focus setting to make matter worse). I really am quite the noob on how to properly set and utilize this scope, but I still believe there is something else going on.

Here she is:


Here I'm probing the calibration post, which should be 0.5v peak to peak. You can really tell just how out of focus it is 


And here is the best close up I could get of the display, just incase it is of any use.


You can just barely make out the faintest hints of the vertical part of the trace. I really can't say if it is absent due to the focus settings or other possible issues.
I really am quite fond of this tiny little scope and would love to get her back into working order again. Here is a link to the service manual: http://bee.mif.pg.gda.pl/ciasteczkowypotwor/Leader/LBO-324%20&%20325.pdf

Here are some additional reference photo's stolen from user HighVoltage's thread here: https://www.eevblog.com/forum/buysellwanted/fs-leader-lbo-325-60-mhz-mini-oscilloscope-(for-beginner)/msg1265385/#msg1265385



There are a handful of case screws missing that I did not remove, so someone has definitely been inside of this scope before. Are there any trouble spots that I should check first?
I know that Leader doesn't seem to be very ubiquitous, but hopefully most of my issues here are more general analogue scope issues.
Any advice or help is appreciated, especially since this old girl is my only really scope at the moment.
Thanks!

[George Edmonds]

Hi Matt

The problem with scopes of a certain age is the the High value resistors in the focus chain and other parts of the CRT high voltage supply go high resistance with time and applied voltage, it looks like you have exactly this problem.

George G6HIG Dover UK

[Matt Hatter]

Thanks George,

I flipped through the service manual and found what I believe to be the schematic for the CRT's high voltage supply as well as the Intensity circuitry.

Blanking / Intensity:



High Voltage:



Would you be able to steer me in the right direction in terms of identifying these resisters? I imagine it is also possible that the VR itself is bad. I'll have to open the scope up and try and measure the resistance tomorrow sometime. Though, I hope that it isn't the pot. They look to be custom jobs, and I get the impression tracking down suitable replacements would be quite the challenge.

[George Edmonds]

Hi Matt

As a starting point look at R7, R16, and R17 on the High Voltage schematic.

The problem is that without specialized equipment it is difficult to measure the voltages around the  high voltage supplies  and CRT base.  Small changes in the order of volts in the 1.9KV supply can cause focus and intensity problems.

Don't know how long this scope has been out of use, but make certain that the high voltage section and CRT base are clean also if the scope has been damp dry it out well first.

George G6HIG Dover UK

[shakalnokturn]

I can't really tell just from your photos it probably is focus only related, still before digging in to finding open resistors and other problems you may want to check:

Astigmatism: Going to X/Y mode with a low spot intensity does the spot look round or rather oblong?

HV regulation: You mention the displayed waveform should be 0.5V p-p. I can't tell from your settings if the display is higher or lower than expected?
If the waveform appears reduced both vertically and horizontally the HV/PDA may be too high, on contrary if the waveform appears zoomed both vertically and horizontally the PDA/HV could be too low.

It is normal to hardly see the rise and fall of a good square wave.

[Matt Hatter]

Hi Matt

As a starting point look at R7, R16, and R17 on the High Voltage schematic.

The problem is that without specialized equipment it is difficult to measure the voltages around the  high voltage supplies  and CRT base.  Small changes in the order of volts in the 1.9KV supply can cause focus and intensity problems.

Don't know how long this scope has been out of use, but make certain that the high voltage section and CRT base are clean also if the scope has been damp dry it out well first.

Thanks for the assistance, George! The scope hasn't been used for probably 6 or 7 years.


Shakalnokturn,

Here are some more photos. I assume the spot intensity refers to the beam intensity? My control is currently unresponsive, but I was still able to get two dots, as shown.

Accounting for the horribly out of focus beam, it does appear that the internal waveform is O.K. At .5V volts per division the waveform spans two divisions.

[shakalnokturn]

That looks good enough to say it isn't astigmatism related and PDA shouldn't be drastically off either.

The two portions that are common to brightness and focus are the "auto focus" control and the -1900V supply.

I would start by checking the +5V / +8V / +15V / +100V supply voltages then looking at unblanking control at TP8.
TP8 should react to changing intensity control.

If you don't have another scope around use the one you have to probe itself.

George is likely to be right as the HV sections on scopes are often prone to failure, but it is worth checking on the low voltage before diving into the hard to probe side.

[Matt Hatter]

That looks good enough to say it isn't astigmatism related and PDA shouldn't be drastically off either.

The two portions that are common to brightness and focus are the "auto focus" control and the -1900V supply.

I would start by checking the +5V / +8V / +15V / +100V supply voltages then looking at unblanking control at TP8.
TP8 should react to changing intensity control.

If you don't have another scope around use the one you have to probe itself.

George is likely to be right as the HV sections on scopes are often prone to failure, but it is worth checking on the low voltage before diving into the hard to probe side.

shakalnokturn,

Thanks! I appreciate you and George, both, sharing your knowledge. I'll check the supply voltages you specified and record them.
This is my only scope sadly. The front end of my scope is rated at 400v, but do you have any tips that might curb any stupid mistakes on my part? Proper probe grounding techniques, etc?

I have two Exams this week that I need to continue studying for, so I really shouldn't allow the scope to distract me till the weekend, but we will see if my resolution remains steadfast 

[ambrosia heart]

adjust the VR3 first

if unable to do so, replace these transistors used to control focusing. 


if the result is no good, replace transistors used to control auto focusing 

if the result is also no good,  replace components in high voltage board.

[George Edmonds]

Hi

NO NO NO NO.

FIRST REPLACE THE HIGH VALUE RESISTORS, or at least check them.  I have now been repairing scopes for about sixty years and now know exactly what I am doing.

The replacement resistors should be type VR25 of similar, modern standard resistors have a maximum working voltage of around 250VDC.

George G6HIG Dover UK

[Matt Hatter]

Ambrosia, thanks for the advice! I'll definitely check VR3 first.

I'll also be sure to check those high voltage resistors before I start replacing anything, George.

I had a few minutes today so I started poking around. For one, it is tight quarters in there. Second, I also discovered that my set of hand tools is not diminutive enough for adjusting the HV section VR's- at least without significant disassembly of the scope. It also looks like it will be hard going trying to probe anything in the scope while it is in place. The design is cleverly compact, but not very accessible.

This scope looks decent inside, but it could use a bit of a clean. Since it looks like I'll have to take quite a bit of the scope apart if those HV resistors need replaced, I'll probably go ahead and clean what I can while I'm inside.
 
Aside from the CRT tube itself and maybe a few large capacitors on the HV board, where do I need to watch out for?

[shakalnokturn]

+1 with the "NO NO NO NO" the shotgun approach may give a successful repair, however it is by no means correct troubleshooting, is time consuming (although proper troubleshooting can be too) worse, it's multiplying chances of damaging something, worst it's not teaching you much in the end.

For the stored energy dangers for yourself I think you have them all covered.
The other risk is for the scope itself if it's really packed try not to touch any solder side to metal frame while disassembling, low voltage capacitors could store enough energy to destroy components.

I'd expect that they'd make at least the test points accessible  for probing.
If you have no access solder a few wires to the points you want to probe then reassemble the boards. Don't leave wires floating around, tape them down to an insulating support.