Help me fix this Oscilloscope (kikusui)

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
 
SY said:
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.

jackinnj said:
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
 
thomas997 said:

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.
 
SkinnyBoy said:
alrighty... my cro doesn't display a line... rather a dot, of varying amptitude... amplitude? amp something.... anyhoo. is it something I need to adjust? even though I have adjusted everything.. lol or is it a fault? THANX!!! tell me what control SHOULD fix this.. if there is one.. thanx :)

Can you post a pic?

Dot is moving horizontally at all?
 
It seems to be a more general age related disease of this scope model as it was described several times in the web and my suffered from it as well by a sudden. Leaving it running for an hour or so (warming up) cures the fault mostly until it becomes worse.


Root cause is a solder joint on the delay line board that has gone bad (so called "cold joint"). It's not clearly visible by naked eye. It was reported that re-soldering the delay line board cured the fault. In my case pressing on the delay line coax cable or slightly bending the delay line board created some strain which re-established the base line without oscillation.


Unfortunately shortly after I got another problem with the vertical section.

VERT MODE switch functions apparently don't work as they should and I also can't change the vertical position of the sweeps anymore. Neither in channel 1 nor channel 2, while it works fine in channel 3. Also, channel 2 signal reversal (CH2 REV) doesn't work anymore, also X/Y mode doesn't work and a few other anomalies. Looks that the switching logic of the vertical section is screwed up by a faulty locic IC so that the high/low switching signals are incorrectly sent. At least that was my diagnosis, but I could be mistaken. The bad thing is that the switching logic involves 10 logic ICs on the VERT switching board and I am not knowledgeable enough to figure out, which of them is the culprit or if it is some other reason. It must be a problem fully unrelated to the first one (oscillating signal on upper part of screen). But nevertheless, the scope is unusable now for me.



Will try if someone who is capable to fix it, would be interested to make it up again for himself. It's a nice 100 MHz unit, if it works.

I got in the meantime a new scope already.



regards,
Reinhard
 
Last edited:
Kikusui oscilloscope COS6100 / oscillation in the upper part of the screen


ADDENDUM:

"delay line board" in the previous post should read "vertical output amp board" (A4). If oscillation occurs (usually if the trace position is in the lower half of the screen), it's caused by a capacitive load at one of the vertical outputs Y+ / Y- which originates from a bad solder joint in the vertical amp or a bad connection of one (or both) the small plugs at the vertical Y+ and Y- contact pins to the CRT.
 
Last edited: