Leader LBO-514A strange tail on square wave.

[rf+tech]

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?

[tautech]

The tail issue has not changed at all.

That would piss me off. 

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? 

[frrobert]

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.

[rf+tech]

@tautech:

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

[tautech]

@tautech:

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

 

[frrobert]

Yes, I had the probes in 10x when I adjusted them.

Sorry I forgot to mention that in the last post.

[rf+tech]

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.

[frrobert]

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.

[rf+tech]

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.

[frrobert]

I tried the fingertip idea and the display got "fuzzy"  I am not sure of the right term but no other real change.

[rf+tech]

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.

RF+ Tech

[frrobert]

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.

[rf+tech]

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.

RF+ Tech

[SAUL BRITTO]

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.

[SAUL BRITTO]

I remember now.It was a BF245. BF254 is a npn Tr.

[frrobert]

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.

[rf+tech]

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.

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

[frrobert]

The stripes look like brown red red gold.  Attached is a photo.

[rf+tech]

I just realized I screwed up the math on this series-parallel resistor network - and this is one of the most basic there is.  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 does.

RF+ Tech

[macboy]

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 t_RISE (ns) ~= 0.35 / BW(MHz)

[frrobert]

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.

[rf+tech]

@macboy: At the start of this thread, the OP has stated:

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

[frrobert]

@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.

[rf+tech]

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.

[frrobert]

Hope the photos are clear enough.