Help me fix this Oscilloscope (kikusui)

[thomas997]

I bought a Kikusui COS6100 oscilloscope and it was working fairly well for a while (some minor HV supply problems that probably cannot be discussed here).

Anyway one day I turn it on and it has this weird sorta bloom effect. For some reason it would only happen if the trace was moved to the bottom half of the screen.

I suspect its some kinda interference issue as it is affected by the 20MHz bandwidth limit button (?).

So, today I was messing around with some pots on the CRT board and one of them seemed to fix the problem. But it actually just shifted the problem from the top half of the screen to the bottom (so now the trace displays fine when at the top half, and when moved down it messes up).

Here are some pictures of me moving the trace to the "bad area" on the screen (the trace isnt as blurry as it looks ):

First pic (good)
second
third
fourth
fifth (worst)

It is not affected by timebase, voltage setting, channel, brightness, probe setting, etc.

thanks

[kilowattski]

I would say that you have some sort of a vertical amplifier problem.

[SY]

That looks like something in the vertical amp is oscillating.

[thomas997]

Any idea where the amp is?

Im assuming its the board with the pots I was messing around with.

If only I had a schematic...

[kilowattski]

Call the manufacturer and ask for one.

[SkinnyBoy]

I never thought about asking for help to fix my cro... anyone wanna help? it doesn't actually display anything...

[SY]

Don't even think about doing this blind. Get a service manual. You'll need one anyway to do the calibration.

[jackinnj]

broken oscilloscopes don't go to heaven, they go to Ebay.

[thomas997]

Originally posted by SY
Don't even think about doing this blind. Get a service manual. You'll need one anyway to do the calibration.

Yeah I probably messed up a bunch of calibration settings... but I dont want to pay ~50$ for a manual off ebay.

Originally posted by jackinnj
broken oscilloscopes don't go to heaven, they go to Ebay.

Yeah I got this one off ebay



Anyway the 5 or so variable resistors on the vertical board seemed to "fix" this problem (only messes up now when the trace is wayyy off the bottom of the screen). No idea why it would come up like that and need to be adjusted so much.


btw is it normal when viewing a square wave, for it to turn into a weird rounded off wave when changing the probe from 1x to 10x (the traces shown in my pictures)?

1x on the probe is rated for 7MHz, 1MOhm, 95pF.
10x is rated 100MHz, 10MOhm, 16pF.

thanks

[jackinnj]

Originally posted by thomas997

btw is it normal when viewing a square wave, for it to turn into a weird rounded off wave when changing the probe from 1x to 10x (the traces shown in my pictures)?

1x on the probe is rated for 7MHz, 1MOhm, 95pF.
10x is rated 100MHz, 10MOhm, 16pF.

thanks

there should be an adjustment trimmer on the probe -- you connect the probe to the square wave source (on the scope) and adjust so that the edges are sharp.