Author Topic: Leader LBO-514A strange tail on square wave.  (Read 12150 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
 

Offline tautech

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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 »
 

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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Offline 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 »
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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?
 

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.
 

Offline rf+tech

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Re: Leader LBO-514A strange tail on square wave.
« Reply #25 on: March 01, 2016, 11:32:35 pm »
Good progress.

What does the calibrator signal look like with a jumper wire connected to channel 1?

Does the calibrator amplitude change with each range?
 

Offline tautech

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Re: Leader LBO-514A strange tail on square wave.
« Reply #26 on: March 01, 2016, 11:33:19 pm »
The tail issue has not changed at all.
That would piss me off.  :rant:

P6100 probes are pretty good and I've not come across your problem before.  :-//

Your are trying to compensate them in 10x mode aren't you?
Wonder if there's a problem with your Cal output?  :-\
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Offline frrobert

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Re: Leader LBO-514A strange tail on square wave.
« Reply #27 on: March 01, 2016, 11:53:37 pm »
I tried the jumper wire and I still have tails.

I also scoped a simple led circuit and it also has tails so the issue is not with the calibrator.

Well the scope was free and I am learning a lot so my frustration level is pretty low.  The last year these were made I think was 1982 so for a scope that is at least 34 years old, I guess you can't expect much.  I just looked up one of the chips which is an amp and found a data sheet on it from 1975.

But I am keeping my eye out for a replacement.
 

Offline rf+tech

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Re: Leader LBO-514A strange tail on square wave.
« Reply #28 on: March 01, 2016, 11:57:01 pm »
@tautech:
Quote
Your are trying to compensate them in 10x mode aren't you?

That's exactly where I'm leading the OP  ;)

Sometimes a lesson is better learned by leading someone to the conclusion rather than telling.  :-+

@frrobert: you do have the probes set to X10 when attempting to adjust the compensation?

As you have just seen, X1 behaves the same as a piece of wire.

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Offline tautech

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Re: Leader LBO-514A strange tail on square wave.
« Reply #29 on: March 02, 2016, 12:01:58 am »
@tautech:
Quote
Your are trying to compensate them in 10x mode aren't you?

That's exactly where I'm leading the OP  ;)

Sometimes a lesson is better learned by leading someone to the conclusion rather than telling.  :-+

@frrobert: you do have the probes set to X10 when attempting to adjust the compensation?

As you have just seen, X1 behaves the same as a piece of wire.

RF+ Tech
;D
 ;)
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Offline frrobert

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Re: Leader LBO-514A strange tail on square wave.
« Reply #30 on: March 02, 2016, 12:06:47 am »
Yes, I had the probes in 10x when I adjusted them.

Sorry I forgot to mention that in the last post.
 

Offline rf+tech

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Re: Leader LBO-514A strange tail on square wave.
« Reply #31 on: March 02, 2016, 12:31:41 am »
Thanks for clarifying.

Next set of questions:
Is there any noticeable difference in the spike when switching between X1 and X10?
Does the signal amplitude change by a factor of 10?
Do the .02 and .05 ranges still look normal?

For the life of me, I have seen this condition years ago where a probe could be adjusted for a very nice flat top, except for that pesky leading edge spike, but the solution evades me at the moment.
 

Offline frrobert

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Re: Leader LBO-514A strange tail on square wave.
« Reply #32 on: March 02, 2016, 12:48:25 am »
When I switch to 1X the spike is much higher.

The signal amplitude does seem to change by a factor of of 10

Sorry, I misspoke before it is the .01 and .02 ranges that look normal and yes they still look normal.
 

Offline rf+tech

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Re: Leader LBO-514A strange tail on square wave.
« Reply #33 on: March 02, 2016, 01:18:17 am »
Got it. Since .005 is very high gain, it's probably not possible to see the top or bottom of the calibrator signal so .005 is probably okay. The significant detail is that the three lowest ranges are all direct input, without an attenuator and compensation capacitors.

So this leaves us with all three attenuators affected. What would really help is another scope probe as a cross check. If you can measure capacitance, it would be good to check the input capacitance on all ranges, between the BNC center pin and outer shell.

Lacking a capacitance meter, one can try using fingertips to perturb the attenuator compensation capacitors, to see what effect there may be. The trimmers are piston type and have screwdriver slots in the top and four of them are visible next to the rear range switch deck. It is best not to turn them, but rather place a fingertip on the side. When you find the correct trimmer for a given range, it should have an obvious effect on the signal. See if you can improve the spike by adding capacitance via a fingertip. The voltage here is very insignificant but it is still a good idea to keep one hand in your pocket when doing this.
 

Offline frrobert

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Re: Leader LBO-514A strange tail on square wave.
« Reply #34 on: March 02, 2016, 01:48:20 am »
I tried the fingertip idea and the display got "fuzzy"  I am not sure of the right term but no other real change.
 

Offline rf+tech

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Re: Leader LBO-514A strange tail on square wave.
« Reply #35 on: March 02, 2016, 03:59:08 am »
Hold the presses, I just made a sanity test on two scopes with the calibrator signal connected by a piece of wire. And saw only the tiniest hint of a spike.

Since both channels behave the same, the problem must be in the vertical deflection amplifier after the channel preamplifiers. I've attached the schematic and placed a red box around the area to be examined. There are two compensation adjustments, VC201 for medium frequencies and VR201 for high frequencies. Take a look at this area to see if either of these parts are damaged or missing. The dashed circles around transistors Q203 and Q204 may indicate heat sinks.

A nice large photo of this area would be great.

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Offline frrobert

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Re: Leader LBO-514A strange tail on square wave.
« Reply #36 on: March 02, 2016, 12:48:46 pm »
I have attached photos.  Many of the joints on the front side of the pcb seem to have a brown residue around the solder but not on those two adjustments.
 

Offline rf+tech

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Re: Leader LBO-514A strange tail on square wave.
« Reply #37 on: March 03, 2016, 01:36:39 am »
Thanks for the photos, frrobert. Already I see some things to look at. As for the brown spots, these are not a concern.

Those big 6.8 kOhm resistors should be measured to see if they have changed value. Also measure across R210 (1.2 kOhm). Due to the series combination of R211 and R212, in parallel with R210, the expected measurement would be:

(1/1500) + (1/1500) = .00133
(1/1200) + .00133 = .00216
1/.00216 = 461.5 Ohms

Then turn the scope on and measure the voltages at P204 on the vertical board.

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Offline SAUL BRITTO

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Re: Leader LBO-514A strange tail on square wave.
« Reply #38 on: March 03, 2016, 04:51:36 am »
Three years ago I had a similar experience with a ICEL oscilloscope brand, very popular in my country,it was in the same condiction.In my case was a FET on vertical input I don't remember if BF254 or BF245.
« Last Edit: March 03, 2016, 04:54:49 am by SAUL BRITTO »
Thank You, for all earth.
 

Offline SAUL BRITTO

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Re: Leader LBO-514A strange tail on square wave.
« Reply #39 on: March 03, 2016, 04:57:01 am »
I remember now.It was a BF245. BF254 is a npn Tr.
Thank You, for all earth.
 

Offline frrobert

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Re: Leader LBO-514A strange tail on square wave.
« Reply #40 on: March 03, 2016, 12:19:59 pm »
RF+ Tech

Here are the resistor values

r213 6.58 kohms
r214  6.58 kohms
r210 1925 ohms

I still need to take the voltage readings for P204
I think I found the connector and if I am reading the schematic right

I should have a +15V on one terminal, a -15V on another and +250 on another.  My question before I do this is where is the best place to put the negative end of my multimeter?  I don't want to fry me or the scope.

Thanks for all your help.
 

Offline rf+tech

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Re: Leader LBO-514A strange tail on square wave.
« Reply #41 on: March 03, 2016, 03:20:25 pm »
frrobert,

The best place to put the negative end of the multimeter is the metal chassis. The outer shell of the input BNC is also good.

Quote
r210 1925 ohms
This measurement is over four times the expected value so it appears these resistor values may have risen. If these resistors have been running hot, it is not uncommon for the color code bands to look discolored.

If you are not sure how to read color codes, R210 should be brown-red-red-gold, R211 and R212 should both be brown-green-red-gold. Please post a closeup photo, with good light to see the color bands on R210, R211 and R212.

R213 and R214 are within +/-5% spec.

RF+ Tech
 

Offline frrobert

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Re: Leader LBO-514A strange tail on square wave.
« Reply #42 on: March 03, 2016, 04:12:02 pm »
The stripes look like brown red red gold.  Attached is a photo.
 

Offline rf+tech

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Re: Leader LBO-514A strange tail on square wave.
« Reply #43 on: March 03, 2016, 05:02:05 pm »
I just realized I screwed up the math on this series-parallel resistor network - and this is one of the most basic there is.  |O Trying to break it down into a written explanation got my mind inverted.  ::)

Using the schematic values:
1.5k + 1.5k = 3.0k for the series combination of R211 and R212.

To solve a parallel resistor network, we add the reciprocals of the individual resistor values:
(1/3) + (1/1.2) = .333 + .833 = 1.167
And then find the reciprocal of the above result:
1/1.167 = 0.857 kOhms

Using the actual color code values of 1.2 kOhms:
1.2k + 1.2k = 2.4k

In parallel with 1.2k:
(1/2.4) + (1/1.2) = .417 + .833 =1.25
And the reciprocal of the above:
1/1.25 = .8 kOhms.

Still, the measurement of 1925 Ohms is over twice what is expected. With this much change, the shunt peaking effect of the capacitors in the network will be excessive. Short of unsoldering one end of each resistor to measure individually, it is possible to tack solder some extra resistance across this network. By temporarily reducing the total resistance to 800 Ohms, we can see what effect it has on the overshoot spike. This will be much easier than removing the pc board to unsolder resistor leads.

By rearranging the parallel resistor equation:
(1/0.80) - (1/1.925) = .703
1/.703 = 1.369 kOhms

Using two 2.7 kOhm resistors (red-violet-red-gold) in parallel will get pretty close at 1.35 kOhms. Let's see what this :-/O does.

RF+ Tech
 

Offline macboy

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Re: Leader LBO-514A strange tail on square wave.
« Reply #44 on: March 03, 2016, 05:04:43 pm »
This is an issue of high frequency compensation. The low frequency compensation adjustment on the probe can't help here.

You won't know if the scope or the probe (or both) is out of adjustment unless you can connect a fast, well behaved pulse generator to the scope with proper 50 ohm cable and terminated with 50 ohm at the scope. Then you can check for overshoot on the waveform. If it is there, then the high frequency compensation of the scope needs to be adjusted. There are often 2 adjustments (mid and high) and usually 2 or 3 sets of these to cover the various attenuator ranges.

After the scope is adjusted for a nice flat top pulse response then you can check the response with the probes. Some probes can be adjusted, usually higher frequency ones like 200 + MHz. I doubt that your very cheap 100 MHz probes can be adjusted. I have some relatively inexpensive Velleman 250 MHz probes that have mid and high frequency adjustments. All of my Tek branded 10x probes have these also (but you need to partly disassemble the compensation box to get at the trimmers).

The proper way to adjust the probes is to adjust the LF compensation for the flattest back 2/3 or so of the pulse, disregarding any peaking or rounding at the front of the pulse. The front of the pulse is adjusted with the mid and high frequency trimmers (if it has them). Note that a separate pulse generator is almost always required to do this properly since the Cal output of most scopes is far too slow (rise time) to generate the high frequency components that are needed. The pulse generator should have a rise time of at least half (twice as fast) of that of the scope. A 100 MHz scope has a ~3.5 ns rise time. Rule of thumb is scope tRISE (ns) ~= 0.35 / BW(MHz)
 

Offline frrobert

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Re: Leader LBO-514A strange tail on square wave.
« Reply #45 on: March 03, 2016, 06:03:08 pm »
RF+ Tech,

It must be a full moon or something.

I just double checked the resistance on R210 and it is now reading 799 ohms right where it should be. 

I am not sure what happened.  I don't know if it was me or my Fluke meter but something had a brain hiccup.

Sorry about that.
 

Offline rf+tech

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Re: Leader LBO-514A strange tail on square wave.
« Reply #46 on: March 03, 2016, 06:19:28 pm »
@macboy: At the start of this thread, the OP has stated:

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

With that in mind, I have avoided getting him in over his head and overwhelming him with details he may not comprehend.

At the top of the second page, we have determined that the overshoot problem exists even with a piece of wire to connect the calibrator signal to the input. I posted that two test cases in my lab show only the tiniest hint of overshoot. What does that tell you?

The probes can be compensated for a very good flat top, exclusive of the overshoot. Look at the additional photos posted with the compensation adjusted to either extreme. What does that tell you?

You are correct that this is an HF compensation problem and I am no stranger to the adjustment procedure. As the OP is a newbie, we don't know the extent of his skill nor what additional test equipment he may have, so we work with the very basics. We have potentially identified a compensation problem in the vertical deflection amplifier in the common emitter circuit of the output stage. Both the vertical preamp and the vertical amplifier schematics have been posted, with annotations of the suspect areas. One must conclude from your post that you have not looked at these nor read the entire thread.

Informed suggestions based on the evidence presented in this thread thus far are most welcome.

** Just saw frrobert's latest post and the above suspect has been released from custody.  ;)

So now we return our focus to the vertical preamps.

@frrobert - what additional test equipment do you have available, that might be of value in resolving this problem?

RF+ Tech
 

Offline frrobert

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Re: Leader LBO-514A strange tail on square wave.
« Reply #47 on: March 03, 2016, 06:50:39 pm »

@frrobert - what additional test equipment do you have available, that might be of value in resolving this problem?

RF+ Tech

I just stated with electronics this January, as a way to develop real world examples for a technical math class I teach; so at this point all I have is multimeters. 

It is starting to sound like I may not have the right tools for the job and that is ok.  I didn't pay anything for the scope and I am learning a lot from this exercise. 

With all that said if I can fix it great, if I can't that is ok too.
 

Offline rf+tech

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Re: Leader LBO-514A strange tail on square wave.
« Reply #48 on: March 03, 2016, 07:03:35 pm »
Okay, we'll continue to work with the basics. Please post a photo of the eight piston trimmer capacitors, next to the range switch decks. At an angle is best, to see if someone may have "tightened those loose screws." These are the compensation capacitors for the three attenuators.

What I'm interested in is seeing the relative height of each adjustment, in relation to the fixed body of the capacitors. If it requires several photos, it would be nice to see the silk screen identifiers so the physical locations can be related to the schematic.
 

Offline frrobert

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Re: Leader LBO-514A strange tail on square wave.
« Reply #49 on: March 03, 2016, 08:40:18 pm »
Hope the photos are clear enough.
 

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
 

Offline 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.
 

Offline 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.
Avid Rabid Hobbyist
 

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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