Author Topic: Leader LBO-514A strange tail on square wave.  (Read 12307 times)

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Offline rf+tech

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Re: Leader LBO-514A strange tail on square wave.
« Reply #50 on: March 03, 2016, 09:52:04 pm »
Good enough to get a visual orientation. The capacitor marked 681 corresponds to C1203 in channel 1. From there VC1203 and VC1204 were sorted out by other clues.

This is the attenuator for ranges .5 1 and 2.

What I'm proposing will be highly sacrilegious to some purists, so it may be better for some people to turn their heads and look way.  :scared:

A good practice before making adjustments is to make a small vertical mark over the edge and side of each trimmer capacitor to identify the original position. A 0.5mm pencil is good. Wherever accessible, mark all of the capacitors.

Using the only square wave source available, the built-in calibrator, let's see what can be accomplished with the attenuator compensation. Connect the calibrator signal to the channel 1 input with just a piece of wire. Set the range to .5 volts per division. This should produce about a 2 division vertical signal.

Using the little plastic tweaker that came with the new probes, try adjusting VC1203 to see what improvement can be made to the overshoot. Work back and forth between VC1203 and VC1204 to obtain the best looking square corner, free of overshoot or rounding, just as is done for scope probes.

A note about VC1204, this control can go through multiple revolutions so it will be necessary to keep track of the turns.

In the event no improvement can be obtained, then it would be best to reset these trimmers to their original positions.

Ideally - these adjustments should be done as macboy described. Let's just see what can be done with bailing wire and ten-penny nails, as it were.

Edited to add:

VC1202, VC1204 and VC1206 establish the scope's input capacitance. In practice, these are all set to the same value with a capacitance meter.
Therefore, it is best not to alter VC1204 at this time.

RF+ Tech
« Last Edit: March 03, 2016, 10:14:11 pm by rf+tech »
 

Offline frrobert

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Re: Leader LBO-514A strange tail on square wave.
« Reply #51 on: March 03, 2016, 11:11:55 pm »
I only worked on channel 1 .  I have not tried this on channel 2

I adjusted what I believe is VC1203.  The components block items but I think I got the right one.

I adjusted just VC1203.  I connected BNC to CAL with just a wire and I adjusted VC1203 with Volts/Div set at 0.5 on channel 1.  I was able to adjust the wave to a flat line. 

I went the range with my probe at 1x and the on scope and the spikes are now only on 0.2, 0.1, and 0.05

 

Offline rf+tech

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Re: Leader LBO-514A strange tail on square wave.
« Reply #52 on: March 03, 2016, 11:48:49 pm »
Excellent, this is what I was expecting.

VC1201 is right under the switch shaft. This compensates ranges .05 .1 and .2 Set the range switch to .2 and adjust VC1201 for the best square corner.

VC1205 is on the opposite side of the switch shaft. This compensates ranges 5 and 10. The square wave is going to be quite small on the 5 Volt range. Pull out the Gain X5 knob to get a taller signal and adjust VC1205.

Channel 2 is done the same way. Once the attenuator compensation is set, the scope probes should be readjusted in 10X. The remaining overshoot seen in 1X is just a trade-off and is seen with virtually every probe in 1X.
 

Offline frrobert

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Re: Leader LBO-514A strange tail on square wave.
« Reply #53 on: March 04, 2016, 01:11:50 am »
The adjustments helped tremendously.  Still not perfect but a 100 times better and I  think as old as it is perfect is not going to happen.

One weird thing on channel 1 at 0.05 the line is fuzzy and fatter than channel 2.  See attached picture.

The other thing I noticed when I adjusted with the probes at 10X I can't really keep a flat top on the wave at 0.005.  It wants to curve one direction or the other. The other ranges seem ok.  Again I just wonder with the age of the scope and not having the proper tools that may just be a good as it gets.

Also when I pulled out the gain things got a bit fuzzy.  Is it ok to try to clean the gain switch with Detox if I try to avoid the pot in front?

Thanks again for all your help
 

Offline rf+tech

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Re: Leader LBO-514A strange tail on square wave.
« Reply #54 on: March 04, 2016, 03:07:21 am »
The scope's age really isn't the factor, it's making do with less than proper technique and equipment. The difference really is like night and day. Well done, frrobert.

With the metal covers off, there can be some stray electric field influences as the shielding is incomplete. If replacing the covers has no effect, then the interference is internal. I noted in the photo that the scope ground clip is not attached to the ground post (left of the calibrator), this can also make a difference in the fuzziness. By all means, work some Deoxit into the BNC connectors to ensure the grounding of the probe to the scope is effective. And yes, Deoxit is good to use on all the switches and controls. Its pretty much the first step in everyone's SOP when restoring any old test equipment.

At .005 Volts/div the gain of the preamplifiers magnifies the effects of turning the probe compensation control even the tiniest bit. Annoying, but quite normal. Adding even more gain via the 5X switch just magnifies everything that much more. Just another trade-off.

At some point in the future should you acquire a decent function generator and a capacitance meter, the internal compensation can be redone to obtain the best possible performance from this scope. Even though this scope is old, it can still perform like new with a good readjustment.  :-/O

Since the forum lacks a proper beer smiley, I've attached one just for the occasion.

Cheers, RF+ Tech
 

Online tautech

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Re: Leader LBO-514A strange tail on square wave.
« Reply #55 on: March 04, 2016, 03:50:58 am »
Since the forum lacks a proper beer smiley, I've attached one just for the occasion.
8)
Well done guys. Nicely documented.  :-+

@ frrobert
Could you post a good thread closing shot of the display with say nearly full amplitude of a Cal waveform and say only 1 or 2 cycles, ie. 500us/div. We'd all love to share in your success.  ;)
Avid Rabid Hobbyist
 

Offline frrobert

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Re: Leader LBO-514A strange tail on square wave.
« Reply #56 on: March 04, 2016, 12:55:20 pm »
Thank you to everyone for all your help.  The fuzziness did decrease when I added the cover.

As requested the calibration wave form at 0.01 Volts/Div and 0.5 ms/Div is attached.
 

Online tautech

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Re: Leader LBO-514A strange tail on square wave.
« Reply #57 on: March 04, 2016, 07:23:44 pm »
Thank you to everyone for all your help.  The fuzziness did decrease when I added the cover.

As requested the calibration wave form at 0.01 Volts/Div and 0.5 ms/Div is attached.
Nice, thanks.

Note the Cal square waveform doesn't quite line up with the graticules as it should.
This could be one of two things.

Either the Cal waveform is not an exact 1 KHz or the scope timebase needs adjusting.  :-\
Either is entirely likely but the Cal frequency is more so.

Neither are a big issue, it'd just be nice to know which is slightly out so if when you have to do frequency calcs from on screen measurements you'll know if you have to incorporate any error.

For a quick sanity check, with just a DMM frequency counter you could check the Cal waveform is 1 KHz.
Some Cal outputs are adjustable and if you trust your DMM or can have it verified as accurate you might be able to do the Cal frequency tweak yourself.
Adjusting the scope timebase is somewhat more involved but if you have access to a good fast FG or AWG it can be down quite well enough for the hobbyist.
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Offline alsetalokin4017

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Re: Leader LBO-514A strange tail on square wave.
« Reply #58 on: March 04, 2016, 09:47:37 pm »
First check to make sure the "Variable" horizontal timebase knob is in the "Cal'd" position......
The easiest person to fool is yourself. -- Richard Feynman
 

Offline semibogus

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Re: Leader LBO-514A strange tail on square wave.
« Reply #59 on: August 23, 2016, 08:07:02 pm »
By any chance does anyone have a PDF of the USER manual for LBO514A.   I have a scope but no clue as to what to do with it.    What I want to do is Frequency count to adjust antennas for RF toys.   

Ian
 


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