Author Topic: Leader LBO-514A strange tail on square wave.  (Read 12382 times)

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Offline frrobert

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Leader LBO-514A strange tail on square wave.
« on: February 29, 2016, 12:52:53 am »
I was gifted an LBO-514A last week.

I am new to electronics and this is my first scope.

My first issue was the volts/div knobs were not working correctly.  I got into the scope cleaned the switches with contact cleaner and they are working much better.

I purchased probes and adjusted my attenuation on them with the calibration square wave so the tops are level but the edge on one side has a tail.    If the image is not showing here is link to the image https://uploadly.com/u35ga5j7

Being a newbie, I am not sure where to go from here.  Thank you for any help
 

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Re: Leader LBO-514A strange tail on square wave.
« Reply #1 on: February 29, 2016, 01:16:40 am »
By the image you've posted the probes aren't correctly compensated.
Some probes have 2 adjustments, one on the probe body and the other on the termination where they connect to the scope.
If by any chance you've bought some whose compensation range adjustment falls outside of the input capacitance of you scope, you'll never be able to get them perfectly compensated.
So check the adjustment range of the probes covers the scope input capacitance, the value of which is normally printed on the front panel close to the inputs.
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Offline frrobert

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Re: Leader LBO-514A strange tail on square wave.
« Reply #2 on: February 29, 2016, 03:14:29 am »
Thanks for the information. 

My probes only one adjustment, so it looks like these probes may not work.

The scope does not have the input capacitance on the front panel and I can not find specs for it on Google so I guess I will keep looking.

Thanks again.

 

Offline rf+tech

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Re: Leader LBO-514A strange tail on square wave.
« Reply #3 on: February 29, 2016, 03:27:41 am »
frrobert,

The LBO-514A manual states the input capacitance is 30 pf. When turning the probe's compensation adjustment (trimmer capacitor), the spike amplitude should decrease, vanish, and the leading corner become rounded. The point where the corner is square, no rounding or spike, is the proper setting. A very tiny spike followed by a tiny amount of ripple is acceptable at the leading edge, if a perfectly square corner cannot be obtained.

If the probe adjustment does not exhibit this behavior, then the probe most likely does not have sufficient capacitance range. If the probe is marked with say 20 pf or 15 pf, that's the problem. You'll need probes designed to work into a 30 pf input capacitance.

Edit: LBO-514A manual download link:
http://www.download-service-manuals.com/download.php?file=Leader-437.pdf&SID=eovdc7q54r9ksq7a95k4g5g9i0

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« Last Edit: February 29, 2016, 03:29:44 am by rf+tech »
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Offline Andy Watson

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Re: Leader LBO-514A strange tail on square wave.
« Reply #4 on: February 29, 2016, 03:33:26 am »
My probes only one adjustment, so it looks like these probes may not work.
Could you post images of the waveforms with the probe compensation set at both extremes of its adjustment?
 

Offline frrobert

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Re: Leader LBO-514A strange tail on square wave.
« Reply #5 on: February 29, 2016, 04:21:51 am »
Here are the specs for the probes I bought.

* Input capacitance: 10×(14-18P)?1×?120P?
* Compensation range : 8 pF - 35 pF

Attached are the pictures of the waveform at each extreme.

Thanks for all the help.
 

Offline T3sl4co1l

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Re: Leader LBO-514A strange tail on square wave.
« Reply #6 on: February 29, 2016, 04:41:20 am »
An internal compensation capacitor is out of adjustment. Does it vary on different ranges too?

Tim
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Online tautech

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Re: Leader LBO-514A strange tail on square wave.
« Reply #7 on: February 29, 2016, 04:44:17 am »
Hmmmm, as you say that don't look right.  :-//

You've stated you're using the scopes probe Cal output but where have you connected the probe reference lead?

Any chance you can check the probe Cal square wave for overshoot on another scope?

Or show us the probe Cal waveform with just a plain wire connection?

Might be just your probes are near their max compensation range.  :-\
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Offline SAUL BRITTO

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Re: Leader LBO-514A strange tail on square wave.
« Reply #8 on: February 29, 2016, 06:23:20 am »
Look if anybody tried to fix this scope and forgot to put back some shielding on V attenuator.
« Last Edit: February 29, 2016, 06:29:30 am by SAUL BRITTO »
Thank You, for all earth.
 

Offline frrobert

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Re: Leader LBO-514A strange tail on square wave.
« Reply #9 on: February 29, 2016, 11:46:12 am »
An internal compensation capacitor is out of adjustment. Does it vary on different ranges too?

Tim

If you mean does the tail show on some ranges but not all the answer is yes.
 

Offline T3sl4co1l

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Re: Leader LBO-514A strange tail on square wave.
« Reply #10 on: February 29, 2016, 11:56:32 am »
An internal compensation capacitor is out of adjustment. Does it vary on different ranges too?

Tim

If you mean does the tail show on some ranges but not all the answer is yes.

Ah, then only those ranges will be affected.  Which likely means the oscilloscope uses a series of attenuators to set the volts/div, and only one or a few (but not all) of them are out of whack.

Tim
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Offline frrobert

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Re: Leader LBO-514A strange tail on square wave.
« Reply #11 on: February 29, 2016, 02:21:48 pm »
Thank you everyone for the help.

So if I understand correctly is a scope issue and not a probe issue.  Is it something a newbie could fix or is a complicated fix?
 

Offline rf+tech

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Re: Leader LBO-514A strange tail on square wave.
« Reply #12 on: February 29, 2016, 03:30:17 pm »
frrobert,

Quote
If you mean does the tail show on some ranges but not all the answer is yes.

This is a good clue. Knowing which ranges are affected can point to the specific attenuator section.

The attached schematic has been annotated to show the relationship between the volts/div settings and the range switch contacts. The corresponding attenuator sections, 1/10, 1/100 and 1/1000, are shown below the range switch. Note that the first three ranges (.005, .01 and .02) have no attenuator.

Narrowing down which range will get us closer to the cause. What about channel 2, does this show the same problem?

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Offline frrobert

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Re: Leader LBO-514A strange tail on square wave.
« Reply #13 on: February 29, 2016, 03:46:55 pm »
Same problem on both channels.
 

Offline alsetalokin4017

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Re: Leader LBO-514A strange tail on square wave.
« Reply #14 on: February 29, 2016, 06:01:38 pm »
Specifically, what probes did you purchase?

The easiest person to fool is yourself. -- Richard Feynman
 

Offline rf+tech

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Re: Leader LBO-514A strange tail on square wave.
« Reply #15 on: February 29, 2016, 06:29:51 pm »
...and specifically, what ranges affected?
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Offline frrobert

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Re: Leader LBO-514A strange tail on square wave.
« Reply #16 on: February 29, 2016, 07:08:55 pm »

The probes I purchased were
http://www.amazon.com/Signstek-Pack-P6100-Oscilloscope-Probe/dp/B00ENF272C/ref=sr_1_3?s=industrial&ie=UTF8&qid=1456772259&sr=1-3&keywords=oscilloscope+probes

I am away right now and I do not remember the ranges.  I will have to check when I get back.
 

Offline anachrocomputer

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Re: Leader LBO-514A strange tail on square wave.
« Reply #17 on: February 29, 2016, 10:16:16 pm »
I recently got some P6100 probes to use with the Tek 5440/5A48 plug-ins. I was able to adjust them OK there: https://www.flickr.com/photos/anachrocomputer/24904574900/
 

Offline frrobert

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Re: Leader LBO-514A strange tail on square wave.
« Reply #18 on: March 01, 2016, 02:23:13 am »
...and specifically, what ranges affected?

The only ranges not affected are 0.01 and 0.02.  .005 is affected and .1 and above are affected.

« Last Edit: March 03, 2016, 11:11:07 pm by frrobert »
 

Offline rf+tech

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Re: Leader LBO-514A strange tail on square wave.
« Reply #19 on: March 01, 2016, 03:04:35 pm »
frrobert,

Two important details stick out. In your first post:
Quote
My first issue was the volts/div knobs were not working correctly.

The pattern of affected ranges does not align with the steps between the attenuators. Point to consider: .005 is affected, while .01 and .02 are not. All three ranges share a single 100 Ohm resistor (R1201) straight through without attenuation. The only thing that changes between these three ranges is the mechanical position of the switch. Electrically, there is zero difference. The gain change is accomplished via the third section (S1203-3) in the middle of the schematic.

Also in your first post:
Quote
I got into the scope cleaned the switches with contact cleaner and they are working much better.

Given that the range switches were the initial problem, the data-points available so far imply the range switches are still suspect.

Please post a photo of the product used to clean the switches. Some contact cleaners just aren't effective.

As it will be necessary to open the scope for some troubleshooting, a few clear closeup photos of the range switches will be helpful.

First troubleshooting step should be to measure resistance from the channel 1 BNC center pin to the output side of the range switch. The output will be the wire that connects the range switch to the pcb, the junction of R1208 and C1205.

Make certain the AC/DC switch is set to DC and the GND switch is not set to the GND position. Measure the resistance with the range switch set to .005, .01 and .02. Each range should measure exactly the same, within 5% of 100 Ohms.

Repeat this same test for channel 2.

Post back with the measurement results and photos, once you have the time to do so.

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Offline frrobert

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Re: Leader LBO-514A strange tail on square wave.
« Reply #20 on: March 01, 2016, 04:00:03 pm »
The pictures are attached.  The cleaner is Deoxit D5.

The switch is mounted to a pcb which is mounted to another pcb  and the backside of that pcb is protected by a metal shield..  I am unsure on how to get in there to take a resistance measurement. and what to connect to.

 

Offline rf+tech

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Re: Leader LBO-514A strange tail on square wave.
« Reply #21 on: March 01, 2016, 08:01:07 pm »
frrobert,

Well, it's hard to beat Deoxit. Excellent choice.

The very last photo provides a clue where to measure. Look at how this contact sits on the inner ring, it is always in contact throughout each range. This switch deck corresponds to S1203-2, the output side of the range switch. Just above is a red capacitor and the "05" silk screen marking means this is very likely to be C1205. Each side of C1205 connects to one side of R1208, a 1 kOhm resistor (brown black red gold). Locate R1208 and make the resistance measurement from the end closest to the range switch, and the center pin of channel 1 BNC on the front panel.

We want to check resistance in the signal path all the way from the input, up to this point at R1208. If there is a problem with any of the contacts on either switch deck, this would show up as different resistance readings for different ranges.

There is something odd here that does not add up. The photos show the first two range switch decks and the common shaft. Where's the third switch deck? The shaft should mechanically couple all three together. Maybe the schematic I'm working from doesn't completely match up with this particular scope.

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Offline frrobert

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Re: Leader LBO-514A strange tail on square wave.
« Reply #22 on: March 01, 2016, 09:02:49 pm »
I found the resister and the equivalent one for channel 2 so I checked both channels.

Here are the readings for all the volts/div positions for each switch on both channels


Channel 1
0.005  100.1 ohms
0.01     100.1 ohms
0.02     100.1 ohms
0.05   .9 mega ohms
.1        .9 mega ohms
.2         out of range
.5      .9 mega ohms
1        .99 mega ohms
2        .99 mega ohms
5         1 mega ohms
10       1 mega ohm

Channel 2
0.005  100.0 ohms
0.01     100.1 ohms
0.02     100.1 ohms
0.05   out of range
.1 out of range
.2         .9 mega ohms
.5      .99 mega ohms
1        .99 mega ohms
2        1 mega ohms
5         out of range
10       1 mega ohm
 

Offline rf+tech

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Re: Leader LBO-514A strange tail on square wave.
« Reply #23 on: March 01, 2016, 10:50:13 pm »
Well, there you go - some of the switch contacts are open.

Time for a little more Deoxit and twist the knobs from end to end five or six times.
Test the open circuit (out of range) positions again to see if they're working:
- channel 1 0.2  should read .9 megohm
- channel 2 0.05 should read .9 megohm
- channel 2 0.1  should read .9 megohm
- channel 2 5.0  should read 1 megohm

If some now test good but some still don't , twist the knobs some more. Then some more, lots more and test them again.

If Deoxit and twisting makes no difference at all, then cracked solder joints on the backside of the pcb where the switch decks attach are a very likely cause.
This can be tested by flexing the switch decks a bit while watching the meter. If the meter suddenly shows a good reading and changes back to open circuit (intermittent), you can be pretty certain the solder joints need to be inspected.

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Offline frrobert

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Re: Leader LBO-514A strange tail on square wave.
« Reply #24 on: March 01, 2016, 11:23:27 pm »
The third shot of Detox cleared the out of range issues.

The tail issue has not changed at all.
 


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