Leader LBO 508 Oscilloscope, no CRT

[RickEev]

The Leader oscilloscope was a gift from a friend of a friend.  I was hoping to use it straight away, but this oscilloscope has become a project unto itself.  I have replaced some of the big (non-oil) capacitors and that fixed the problem of the screen slowly dissolving away after being turned on for 20 minutes.

I was able to get a 1Khz square wave to display; I then walked away for a while, and when I came back, I had no CRT display.  I checked the T1 to T5 connections and T1 to T4 were spot on, but T5 was registering  ~300V instead of 1500V.  Nothing really stood out (like smoking or smelling parts), but I did notice some cracks in the varistors.

I assume this is not a "good thing" and plan to replace them, but with what?  The Marcon part numbers are close to the current varistor codes, but they have their differences.  From my research (I first thought they were ceramic capacitors), the TNR appears to be a MOV type,  681 is 680 volts, K is 10%, and then things get vague.  I assume 015A is some multiple of 1.5 amps, and maybe 70116 is a date.

My question is: Can I replace these with another MOV with the correct voltage or is TNR some special type of varistor?  Is that really 1.5 amps and is there any other info I need?


As you probably have noticed, I am a newbie to the world of oscilloscopes and soldering.  This is really meant be a learning experience and if I get a working scope at the end, then so much the better.  I haven't lost patience with this scope, yet.

My future plans for this oscilloscope is to replace all the electrolytic capacitors, big and small, in the power supply (I am tired of futzing around here).  The only other power supply components that have heat issues are a couple of screwed in ?transistors? (see photos) and maybe a resistor or two.  Are the transistors easily replaceable or am I wasting my time here? 

Do you have any suggestions for the various boards (power supply, preamp and trig sweep)?  Electrolytic cap replacement always seems popular or should I just deal with them when a specific issue shows up?

Thx for any help.

[petertronics]

Hello there

I had a simmilar problem with my lbo 508  . (https://www.eevblog.com/forum/repair/leader-lbo-580-repair/msg1753652/#msg1753652 for further reference like service manual). I would also recomend that you check the HV oil caps and replace them if needed.

[RickEev]

I saw your thread and was hoping the hv oil caps weren't the problem.  I'll look into them.  I think I'll head over to the beginners section for the varistor info and see what they have to say.

It looks like you replaced some of the electrolytic caps on your scope, too.  On my scope, one had spewd and other others were bulging.  I like things that say: Fix me.  I also like your fix for the HV oil cap.  It's nice to see clean work done on these old tools.

Cheers