BK precision 2120 scope

[cdntech]

Hello all! I have been lurking for sometime now, and finally decided to make myself an account. I am fairly new to electronics, and recently purchased a BK precision 2120 oscilloscope. I am having an issue with the display...I have adjusted the trace rotation as best as possible, but on the lower end of the CRT the trace seems to dip at the ends while at the top of the CRT the trace is perfectly straight. I have the service manual, but after reading it I am still unsure as to what it could be?

I have included some pics below. Thanks in advance!

[David Hess]

There may be a CRT geometry adjustment or two inside to correct for CRT geometry errors.  The vertical and horizontal shield and common mode plate voltages can also affect geometry so these might be adjustable.

The geometry error shown is noticeable but probably within the specifications.

I checked the manual and VR804 may have an effect on the geometry.  Either check it according to the calibration instructions are just adjust it to see what effect it has.  Only a voltmeter is required.

[cdntech]

Right on, thanks! I will check that out.

[alank2]

It goes without saying, but be careful with the cover off, there are places where there is +2000 volts exposed.

[med6753]

Welcome to the blog and fellow BK 2120 owner here. As mentioned there is no geometry adjustment but I also suggest checking all the supply voltages although you can't check/adjust the -2KV HV unless you have a high voltage probe.

Your trace distortion MIGHT be due to a strong magnetic field (I.E. a power transformer) nearby. Try moving the scope to a different location and check the traces. If they straighten out you have your answer. I found out quite by accident that this CRT is very prone to distortion due to nearby magnetic fields. It's because the neck of the CRT is shielded but the bell is not. And the metal case seems to provide little protection if the magnetic field is strong enough.

I've done a teardown of the 2120 which you can find here.....
https://www.eevblog.com/forum/testgear/teardown-bk-2120-scope/msg923693/#msg923693

There's also some common issues with these scopes which I can help you with.

 

[cdntech]

Thanks for the info! I don't have very much money invested into the scope which is nice...I picked it up along with a wavetek 110 function gen for 120 bucks. Pretty good for my first scope!

As for the distorted trace...I don't believe there is any interference coming from near by. All other equipment on my bench was switched off at the time that I snapped the picture.
 The other quirks I have noticed are the noisy rotary switches which probably need a good clean! One day when time allows I may tear it down.

When I move the trace to the top of the CRT it is level. It's only on the lower half of the CRT where the trace is skewed. It's more of a curiosity thing than anything else...it's probably more than accurate for what I'm using it for.

Anything else to watch out for?

[med6753]

Anything else to watch out for?

Other than the tendency of the volts/div switches to get heavily tarnished....which apparently you have....the focus voltage divider resistors tend to open up and you lose focus. In the teardown thread I outline which resistors are the usual suspects.