Analogue oscilloscope repair. Help needed.

[Hexureus]

Hello,

I own a late 80's oscilloscope branded kiotto. It's a 2 channel 40mhz oscilloscope.
I have been trying to repair this oscilloscope for quite a while but have not been able to.
Information on screen appears well but not the trace, neither ch1 or ch2. The trace almost seems to have been stretched and appears as a big blur on the screen. By moving the trace up and down its noticeable that its there.

What I have done until now was measure all voltages and they seem fine, probed all boards for shorted diode's and cap's but didn't find any bad one.

I was wondering if anyone had a similar problem and found a solution. If searched the web for weeks and I didn't find anyone with a similar problem.

Attached is the schematic's and a link to the video showing the problem.

Thank you.

https://youtu.be/sHL0oiO7_Mc

 

[BradtheRad]

It looks as though the beam is shooting somewhere else than the screen. It's still bright, however. As a result all you see is reflected glare onscreen.

The focus knob seems to change something.

It's possible the vertical and horizontal drive circuits are dead. However then it is a question where the beam would shoot by default, if that is the case. So I could be wrong.

 

[Hexureus]

It looks as though the beam is shooting somewhere else than the screen. It's still bright, however. As a result all you see is reflected glare onscreen.

The focus knob seems to change something.

It's possible the vertical and horizontal drive circuits are dead. However then it is a question where the beam would shoot by default, if that is the case. So I could be wrong.

Hi and thank you for the reply.
The knob I'm moving on the video is the vertical trace position,Ii don't know if you noticed on the video but the screen gets darker because of the trace going up and down. The trace seems to be very stretched vertically. A friend told me that it could be a problem with an opamp, I just don't know where to look.
On the video I also move the time/div and volt/div knobs to show that they don't influence the trace, at least not enough I can notice.

Edit, No signal is being fed into either channel.

 

[BradtheRad]

Have you opened it up to look at the CRT? You might be able to see a spot of light somewhere, perhaps on the inner wall of the tube.

Could there be a coil of wire around the neck of the tube? And it's come loose, deflecting the beam?

No signal is being fed into either channel.

Try a few different DC levels. It may bring the beam onscreen.

Another thing to try is, turn off the horizontal sweep, feed different DC signals to the horizontal input, and see if it brings the beam onscreen.

 

[Hexureus]

Hi, thank you fer the reply.

The CRT tube is all covered inside.
There is a coil under the tube more or so in the mid, the coil is ok.
I have tried to feed multiple DC voltages and the only difference is that the ultra wide trace (blur) goes up as voltage increases.
Just wondering, if the problem was in the CRT circuitry shouldn't the data displayed on screen be also affected?

Thank you.

 

[E-design]

I think you can assume that the vertical and horizontal main amps function normally as the character information (green paths) shows up on the screen as it should. The vertical input pre-amp signals and sweep signals to the horizontal amps are selected by diode gates. You will need another oscilloscope to verify the waveforms fed in at these points. There may be a massive DC offset due to a fault in one of these circuits that locks the beam hard to one side. From the video, it appears that the problem may be in the horizontal sweep gen (since it looks like you can move the beam vertically) or horizontal control board (10) that controls horizontal position.

What happens when you put it in X-Y mode?

 

[D.A.(Tony)Stewart]

You can expect at least 1 wet electrolytic cap , if not all have high ESR and if circuit depends on ESR, functions will fail.

If you can test for C value and ESR, replace all, large caps.

If you cant verify voltage on every test point, this is the easiest plan.

Wet caps age rapidly when not used > 3 yrs for ESR, which if operated can reduce C.

label each cap location and test after. If it still fails then secondary damage may have occurred if you verify a flawed cap with LCR meter type test or pulse test with good cap of same value to get 50% step size

 

[Hexureus]

I think you can assume that the vertical and horizontal main amps function normally as the character information (green paths) shows up on the screen as it should. The vertical input pre-amp signals and sweep signals to the horizontal amps are selected by diode gates. You will need another oscilloscope to verify the waveforms fed in at these points. There may be a massive DC offset due to a fault in one of these circuits that locks the beam hard to one side. From the video, it appears that the problem may be in the horizontal sweep gen (since it looks like you can move the beam vertically) or horizontal control board (10) that controls horizontal position.

What happens when you put it in X-Y mode?

Hi,
Thank you for the reply.

https://youtu.be/sHL0oiO7_Mc?t=25s
In this link goes to the video on the OP at 25s mark, when the information on the screen disappears that's the x/y mode.

I would like to add that I noticed sometimes when I'm messing around with the time/div knob, for some brief milliseconds the trace appears.

 

[D.A.(Tony)Stewart]

Looks like the Z blanking is working too well...maybe

 

[E-design]

I would like to add that I noticed sometimes when I'm messing around with the time/div knob, for some brief milliseconds the trace appears.

It sounds like something is preventing the sweep generator from starting. You will most probably need another scope to aid in tracing this fault.