Kenwood CS-6040 'scope trouble (solved!)

Even though I use primarly digital oscilloscope nowadays, some things, like x-y in my experience works better in analog 'scopes.
My favoure analog 'scope, kenwdood cs-6040 is giving some annoying symptoms.

First, is ground is not staying grounded. I mean I start by setting vertical position for ground reference in center of screen. Then I change volts/div while still on ground setting.
What happens, is that ground reference, previously being perfectly centered, now goes up by 1/10 div or so, changing bit each time changing volts/div. I can take small video to better explain this behaviour. There is nothing gonnected, since I'm just changing ground position. What makes this probelm worse, is that highest resolution position actually swings ground position DOWN by 2/10 div. These symptoms are on channel 1. channel 2 acts same way, except no negative offset in finest reolution.

This change of vertical position offset during ground setting also effects actual waveform, which makes matters worse, but It is already big deal that ground won't stay put.
personally, I think there is something wrong in vertical amplifier section, grounding section/part.

Also, I gotta add, that volts/div knobs are rotary encoders, not traditional knobs with limits. These just roll around, and relays click. This makes me think that this repair can be challenging, but this 'scope is 4 channel, 150Mhz with cursors, so obviously I'd like this in working order.

PS: service manual is available

There is often a balance control for that.
I am not familiar with the CS-6040, but look here at the pic of my Telequipment D75
https://www.electro-tech-online.com/threads/how-rubbish-is-your-scope.141159/#post-1182331

Look at the Y amplifier module on the left, look at the Y shift controls. In the centre of each know is a screwdriver slot. The front panel is marked in red as DC BAL.
This is a pot for adjusting the DC balance to sort the problem which you are seeing on your scope.

Look for a similar control on your scope.

JimB

Humm, that knob seems to effect gain. If I read with 1:10 probe 1v p-p signal (that test signal used for compensating), that knob effects output so 100mV p-p is lower.

But, I read the manual (...), well, service, and there was method of dealing with phenoma. Adjust trimmer vr103 when changing between 1mV and 2mV range. Adjust trimmer when changing between 2mV and 5mV range. Now the line stays where wanted.

Thanks, this helped a lot!

fezder said:

Humm, that knob seems to effect gain. If I read with 1:10 probe 1v p-p signal (that test signal used for compensating), that knob effects output so 100mV p-p is lower.

Click to expand...

You are looking in the wrong place.
There is a sensitivity adjustment in the front panel.
There is a balance adjustment in the centre of the Y shift knob.

fezder said:

But, I read the manual (...),
Now the line stays where wanted.
Thanks, this helped a lot!

Click to expand...

You're welcome.

JimB

This is the front panel, only adjustmet knob I found is above volts/div:
https://i.ebayimg.com/images/g/dtkAAOSwIspaeNmY/s-l1600.jpg

Picture borrowed from google

We are talking at cross purposes.
I was using the D75 as an example, and showing where the balance adjustment is on that scope.

JimB

fezder said:

What happens, is that ground reference, previously being perfectly centered, now goes up by 1/10 div or so, changing bit each time changing volts/div. I can take small video to better explain this behaviour. There is nothing gonnected, since I'm just changing ground position. What makes this probelm worse, is that highest resolution position actually swings ground position DOWN by 2/10 div. These symptoms are on channel 1. channel 2 acts same way, except no negative offset in finest reolution.

Click to expand...

you should try to find a calibration manual for the scope. it sounds like the DC Balance is out of adjustment in the vertical amplifier. DC balance is only one of a number of adjustments that are checked during calibration. some adjustments interact with others, which is one reason there are specific procedures used during calibration. you probably don't have access to many of the tools (voltage and frequency standards especially). if you were able to trim out the DC shifts you were seeing, good, but it might not be a bad idea to get at least a cross check against a scope that is reasonably well calibrated.

Yeah, I noticed that at least 5mV p-p square wave signal generator I'm missing for adjusting 1mV gain. Sercice manual has all calibration stuff explained, what trimmer does what etc.
Best scpope to act as check reference I got now is perhaps rigol DS1054Z (upgraded to DS1104Z), but being digital, It's somewhat different.

Ok, this today I noticed that this fault is still up, but there's more into it; it appears to be worst when scope is just started, but after while, It's as good as I calibrated it. This gets me thinking that there could be some thermal issues. Any thoughts? I really wouldn't want to keep this scope powered all day long, neither powering it on long before actually using it...