Scope diagnosis before beginning project

[clh333]

I purchased a Leader LBO 502 scope from eBay hoping to use it for diagnosis of a problem with an old computer's motherboard. I am not knowledgeable about scopes but I have a reference that I am using to find my way around. The reference describes (and the scope manual also describes) using the internal calibration signal to generate a square wave pattern on the display. Following the prescribed procedure I am able to produce a pattern, but instead of a square wave the signal looks like two parallel dashed lines. In other words, the vertical traces are non-existent or at least too faint to discern, even with full brightness. Modifying the probe's compensator does nothing to resolve this.

I suspect this indicates some defect in the scope and am wondering whether to pursue repairing it or to scrap the scope. There are few resources in my area for competent electronics repair; I wouldn't know where to look for help. I don't have a spare scope for comparison nor does anyone I know, and obviously my motherboard diagnosis is going nowhere until I get this issue settled.

Any suggestions would be greatly appreciated.
Thanks.

 

[Reloadron]

The cal out should be a 0.5 volt peak to peak 1KHz square wave. If you adjust your horizontal time base for 1 mSec/Div you should see the square waves. Changing to .5 mSec div you should see about 5 repetitions of the square wave. You want your vertical volts division set around .2 V/Div to see about 5 divisions of vertical amplitude. Do you have the manual for your scope?

Ron

 

[JimB]

I suspect this indicates some defect in the scope

What you are seeing is quite normal.
The risetime of the edge is so fast compared with the frequency of the waveform that you are displaying that the edges do not show very well on the screen.
If you speed up the timebase, so that you are just looking at the rising (or falling) edge, you will need to get the triggering right to do this, then you will see the edges. You may also have to turn up the brightness to see them.

JimB

 

[zeighty]

First time poster... Reloadron is obviously the expert here (that's a lot of posts/messages) but I thought I would toss in my 2 cents worth.
My Hitachi v-650f does this; however, I can see faint verticals if I dim the room lights and turn the intensity up all the way. I don't know that I have seen an analog scope that shows solid bright traces on square waves.

 

[clh333]

The cal out should be a 0.5 volt peak to peak 1KHz square wave. If you adjust your horizontal time base for 1 mSec/Div you should see the square waves. Changing to .5 mSec div you should see about 5 repetitions of the square wave. You want your vertical volts division set around .2 V/Div to see about 5 divisions of vertical amplitude. Do you have the manual for your scope?

Ron

Thanks for your reply.

I have the manual, a PDF of the original, which states that the standard signal output is a square wave, 0.5V p-p at 1 kHz. The procedure which I am following is as follows: Trig Slope = Pos, Trig Mode = Normal, Trig Source = Internal, Vertical Axis Coupling set to AC, Trigger Level set to Auto, Horizontal and Vertical Trimmers (inner knobs) set to Calibrated, Horizontal Time/CM set to 0.2 mS, Vertical Volts/CM set to 0.02, image adjusted to center of the display.

At this setting I see 3 wave tops and 2 wave bottoms, clearly defined, about 1 division (CM) apart vertically, each wave. The horizontal lines are nice and flat; the top traces are about 2.2 CM long, the bottom ones about 2.6 CM long. There is a very faint vertical trace visible, but not nearly so strong as the horizontal image.

Every reference I have shows a symmetrically shaped and equally strong vertical and horizontal line. That's what has been confusing me. Is this normal, or should the image look like a "square wave"?

 

[Nigel Goodwin]

Every reference I have shows a symmetrically shaped and equally strong vertical and horizontal line. That's what has been confusing me. Is this normal, or should the image look like a "square wave"?

It's normal, and the 'better' the square wave is the less visible the vertical lines will be.

 

[Reloadron]

Thanks for your reply.

I have the manual, a PDF of the original, which states that the standard signal output is a square wave, 0.5V p-p at 1 kHz. The procedure which I am following is as follows: Trig Slope = Pos, Trig Mode = Normal, Trig Source = Internal, Vertical Axis Coupling set to AC, Trigger Level set to Auto, Horizontal and Vertical Trimmers (inner knobs) set to Calibrated, Horizontal Time/CM set to 0.2 mS, Vertical Volts/CM set to 0.02, image adjusted to center of the display.

At this setting I see 3 wave tops and 2 wave bottoms, clearly defined, about 1 division (CM) apart vertically, each wave. The horizontal lines are nice and flat; the top traces are about 2.2 CM long, the bottom ones about 2.6 CM long. There is a very faint vertical trace visible, but not nearly so strong as the horizontal image.

Every reference I have shows a symmetrically shaped and equally strong vertical and horizontal line. That's what has been confusing me. Is this normal, or should the image look like a "square wave"?

Zeighty, thank you but I am not the expert. Just about all of the forum members can answer questions like this. Also, a members post count means somewhere between very little and nothing. Since I retired (the sole benefit to being old) I have more time for my forums.

CLH333, what you are seeing is likely normal as was mentioned. You won't easily see the vertical transitions. For example here is a cal square wave, for these pictures the scope intensity was really turned up to the point the tops and bottoms are "blooming" just to see the vertical transitions:

Keep in mind also that Cal Out signal is merely a reference signal. It is not a highly accurate signal to calibrate the scopes vertical or horizontal gain. If I were to reduce the intensity the vertical transitions would no longer be visible.

Ron

 

[clh333]

It's normal, and the 'better' the square wave is the less visible the vertical lines will be.

Thank you all for your assistance.

I guess that answers that. Now to find out why my old Kaypro is producing garbage characters on the screen at power-up.

-CH-

 

[MikeMl]

The signals on a modern computer motherboard switch a ~1GHz. Even my 400MHz scope cant display them. What is the bandwidth of yours? Your probes?

 

[clh333]

The signals on a modern computer motherboard switch a ~1GHz. Even my 400MHz scope cant display them. What is the bandwidth of yours? Your probes?

The Z80 processor runs at 2.5 mHz. My Leader LBO 502 has a bandwidth of 15 mHz. We'll have to give it a shot and see if it is capable of displaying the signals.
-CH-

 

[Nigel Goodwin]

The Z80 processor runs at 2.5 mHz. My Leader LBO 502 has a bandwidth of 15 mHz. We'll have to give it a shot and see if it is capable of displaying the signals.
-CH-

It will be fine, as far as it goes, but a scope (and certainly an analogue one) isn't all that useful for such purposes - basically it can only show if you've got pulses or not.

 

[clh333]

It will be fine, as far as it goes, but a scope (and certainly an analogue one) isn't all that useful for such purposes - basically it can only show if you've got pulses or not.

Thank you for your response. I must protest however that you are implying a level of expertise beyond my abilities. I purchased the scope, locally and for a small sum, a few years ago; out of curiosity more than need. It was a solution looking for a problem.

Recently I began cleaning out storage and came across a great cache of software, in the form of 5.25 and 3.5-inch disks, for CP/M, MS-DOS, C-64/128 and Amiga machines that I own or once owned. In order to judge its viability I set myself the task of resurrecting each of these machines. That led me to acquire a Kaypro 4 of approximately 1983 vintage.

Unfortunately the Kaypro had been modified - badly - by one of its previous owners and was incapable of booting. So the task has expanded to diagnose and repair the motherboard. I am looking for voltages and signals - essentially pulses - at various points on the board. I have reasonable confidence in the switching power supply and video (CRT) section. I haven't seen the FDS operate but it does light up.

Kaypro's TSM has limited troubleshooting suggestions. For this problem, i.e. screen full of garbage characters, their solution is to replace the motherboard.

So I know I'm on a fool's errand and the odds are against a successful outcome. On the other hand, I expect to learn from the experience. Indeed, I already have, and I thank you and the other respondents for your tutelage.

Charles Hudson
clh333

P.S. Knowing what I know now I would look for a dual-channel scope with higher bandwidth... and possibly other equipment as well.

 

[zeighty]

clh333 - I just noticed that you joined recently too. I am not sure what your plan is for the Kaypro trouble shooting, but I would be interested in your process and progress. Because I am new on the board I don't know if there is a forum specific to this kind of posts. Please consider posting your results as you go here or let me know where.

 

[JimB]

clh333 wrote:

So I know I'm on a fool's errand and the odds are against a successful outcome. On the other hand, I expect to learn from the experience. Indeed, I already have, and I thank you and the other respondents for your tutelage.

Sometimes it is good to go off on a personal journey of discovery.
Sometimes you just end up back where you started thinking "What was that all about?"
Sometimes you learn that you should never bother doing that again!
Sometimes you will learn that little nugget of knowledge which is very valuable and will expand your horizons greatly.

Good luck on your journey!


clh333 also wrote:

P.S. Knowing what I know now I would look for a dual-channel scope with higher bandwidth... and possibly other equipment as well.

One of those tyranies of "big boys toys", no matter how big the toy you will always find a need for an even bigger more expensive toy.
He who dies with the most toys in his possession wins.

JimB

 

[Nigel Goodwin]

P.S. Knowing what I know now I would look for a dual-channel scope with higher bandwidth... and possibly other equipment as well.

The scope is perfectly fine for the vast majority of purposes, and higher bandwidth isn't going to help trying to troubleshoot a computer motherboard.

For the screen full of random characters, it 'could' be something like the ROM that stores the character tables - my original MicroTan65 has a dead lowercase ROM in it.

 

[cowboybob]

Welcome cth333 and zeighty!

Used to sell/support Kaypros ( ComPaq, Osborne, etc., dating myself). Perhaps this will help: http://bitsavers.trailing-edge.com/pdf/kaypro/1484-D_KayproTechnicalManual_Dec84.pdf.

It may not cover your particular MB, since they were changed from time to time.

But most of the digital signals it provides (scope images) will be useful although, as Nigel notes, your problem is quite likely in the Character EPROM (U26, and good luck finding a working replacement).

Just out of curiosity, will the machine at least "sound" like it'll boot from a floppy?