Analog oscilloscope : Half screen working

[SreejitS]

Hello all !!

Can someone suggest what could be wrong with this oscilloscope? Only the left half of the screen works. I heard some relay clicks and then this behavior started. The scope is an old ESCORT EAS-201 .

[JWH]

Could easily be a failed electrolytic capacitor in the power supply. Check them all.

[FlyingHacker]

Perhaps one of the transistors that drives the horizontal deflection is bad.

[PA0PBZ]

Perhaps one of the transistors that drives the horizontal deflection is bad.

Yes, you can see it 'giving up' in the middle of the screen. Either the driver or the power supply for the driver.

[tautech]

Could easily be a failed electrolytic capacitor in the power supply. Check them all.

Could be but they don't usually have this type if effect on the display, however as you say the first thing to check is always the PSU, then you have a sound starting point for the final diagnosis.

Perhaps one of the transistors that drives the horizontal deflection is bad.

More than likely IME and if so need be replaced with another and better "video output" transistor with better specs.
BF259 has been my favourite for CRT plate drivers.


This scope looks like a rebrand, anybody know its OEM ?

Escort Instruments, Taiwan

For the OP to get started and get some insight to a possible repair this Tek guide is a good starting point:
http://www.sphere.bc.ca/test/tek-parts/troubleshooting-scopes.pdf

[JFJ]

Can the trace be expanded vertically to fill the display (or moved over the full height of the display)?

If so, then problem is unlikely to be due to a simple power supply fault, as both the X and Y axis drivers are likely to be powered from the same supply rails.

[FlyingHacker]

Perhaps one of the transistors that drives the horizontal deflection is bad.

Or at least something in the horizontal difflection circuit. Could be a capacitor giving up or shorted in that cuircuit as well.

[David Hess]

Check to see if the vertical deflection accuracy is correct.

It may be that one of the horizontal deflection plate leads fell off of the CRT.  They should be checked.

[FlyingHacker]

Check to see if the vertical deflection accuracy is correct.

It may be that one of the horizontal deflection plate leads fell off of the CRT.  They should be checked.

Just curious, are those two related? In other words, if the vertical accuracy is or isn't correct does that tell you something about the horizontal deflection leads? I know you are an expert with scope repair. So I am just trying to figure out your troubleshooting thought process to improve my own.

Certainly a good point that the leads could have fallen off! I was going to say measure the voltage range on both leads left and right and see how they compare.

[David Hess]

Check to see if the vertical deflection accuracy is correct.

It may be that one of the horizontal deflection plate leads fell off of the CRT.  They should be checked.

Just curious, are those two related? In other words, if the vertical accuracy is or isn't correct does that tell you something about the horizontal deflection leads? I know you are an expert with scope repair. So I am just trying to figure out your troubleshooting thought process to improve my own.

Certainly a good point that the leads could have fallen off! I was going to say measure the voltage range on both leads left and right and see how they compare.

Thank you for your kind words.  I do have a lot of experience with analog oscilloscopes.

No, they are not related and disconnecting one horizontal deflection plate should not significantly affect vertical deflection; these are just very easy initial checks which can be made.  I did not notice any non-linearity in the video or I would have asked different questions.  When the test results are weird, record the amount of weird.

If both the horizontal and vertical deflection are about 1/2 of what they should be, then that indicates either a high cathode acceleration voltage or possibly a low or missing PDA (post deflection acceleration) voltage.

The puzzling thing shown in the video is that the sweep is short covering only half of the horizontal width but the positioning is not centered which would be expected if there was a deflection sensitivity problem.  That may indicate a problem with the sweep itself instead of deflection.  Update: a disconnected horizontal deflection plate can explain this when the disconnected plate charges to equilibrium.

To try to resolve this without invasive diagnosis, I would check the horizontal and vertical deflection in X-Y mode so the sweep is not involved.  I cannot tell from the video if the EAS-201 has a B timebase but if so, I would check its sweep length also.

It would be helpful to see a triangle wave displayed so that linearity errors would be visible as the position controls are adjusted.

[SreejitS]

Can the trace be expanded vertically to fill the display (or moved over the full height of the display)?

There is no problem in expanding vertically and the trace moves over full height of the display.

[james_s]

Check to see if the vertical deflection accuracy is correct.

It may be that one of the horizontal deflection plate leads fell off of the CRT.  They should be checked.

That's what I was going to suggest, I've actually seen nearly that exact symptom before and it was a broken wire to one of the deflection plates. Whatever the case this is likely to be something fairly simple, the deflection amplifiers tend to be pretty similar across analog scopes, the horizontal in particular is usually not too fancy because it only has to operate up to the maximum sweep speed.

[FlyingHacker]

When the test results are weird, record the amount of weird.

Bob Pease, no?

His Troulbeshooting Analog Circuits has been a valuable resource in my quest for knowledge.

[David Hess]

Check to see if the vertical deflection accuracy is correct.

It may be that one of the horizontal deflection plate leads fell off of the CRT.  They should be checked.

That's what I was going to suggest, I've actually seen nearly that exact symptom before and it was a broken wire to one of the deflection plates. Whatever the case this is likely to be something fairly simple, the deflection amplifiers tend to be pretty similar across analog scopes, the horizontal in particular is usually not too fancy because it only has to operate up to the maximum sweep speed.

I have never run across one myself but a broken deflection plate connection has been reported a couple of times on the TekScopes@yahoogroups.com email list.  As far as diagnosing a problem, when you hear hoofbeats, think of horses not zebras (common before rare causes) and do simple and noninvasive things before complex and risky things.  A broken deflection plate connection is very simple and noninvasive so it should be checked early.

Horizontal amplifier stages are a little deceptive.  They can be remarkably slow, the horizontal amplifier in a 500 MHz 7904 with a 500ps/div sweep only has a bandwidth of 1 MHz, but they do require high linearity so typically the final high voltage stage uses shunt feedback with a constant current load.  Some of them are remarkably complex like I recently discovered with the Tektronix 475.

When the test results are weird, record the amount of weird.

Bob Pease, no?

That is correct.  I only got to hear him lecture once but it was fun.

His Troulbeshooting Analog Circuits has been a valuable resource in my quest for knowledge.

It is a great book and worth reading periodically.  Also check out:

The Art and Science of Analog Circuit Design - Edited by Jim Williams
Analog Circuit Design, Art, Science, and Personalities - Edited by Jim Williams

[SreejitS]

Well, The transistors in the horizontal amp is at fault ie. it does not vary when the horizontal knob is turned. I've found a similar thread here :
https://www.eevblog.com/forum/repair/have-a-screen-help/25/

My scope also has same pcb. I guess have to swap 'em out.

[David Hess]

Well, The transistors in the horizontal amp is at fault ie. it does not vary when the horizontal knob is turned. I've found a similar thread here :
https://www.eevblog.com/forum/repair/have-a-screen-help/25/

My scope also has same pcb. I guess have to swap 'em out.

That is a bummer.  If you have the schematic and maybe even if you do not, it should be repairable.