Old analog two channel, 20MHz oscilloscope - Problem seems to affect both channels

[komrad2236]

Hi
I got this analog, 2 channel 20 MHz, very old oscilloscope.

Problem is that Variable knob(sometimes called CAL on Tektronix) affects the vertical position of the signal beam.
I demonstrate it in the video

Any ideas where to start looking ?

Here is the schematic for this oscilloscope : https://drive.google.com/file/d/1V1R-zMm9nyLlfCIqLlVuUQoSgcbpBajZ/view?usp=share_link
And this folder contains lots more, also pictures : https://drive.google.com/drive/u/2/folders/1-KAHHziQhGj40ROpt0AJ1LJmDQhn_cz9
So far I tested the power supply and its pretty much spot on.

 

[danadak]

I believe its the adjustment of the balance control in the vertical amplifier.

What you can do is mark the position its in, and then do small adjustments, turning the
cal adjustment between each adjustment until cal no longer affects position. Usually this
adjustment procedure is in the service manual.

This is a different scope but general principles apply -

balance control vertical amplifier oscilloscope - Google Search

www.google.com

Regards, Dana.

 

[Lightium]

I hope you have plastic screw drivers. Don't adjust with the power on.

 

[danadak]

If you dont have a plastic tool using a sharp knife and a bamboo skewer stick for barbeque
you can make one.


Regards, Dana.

 

[Lightium]

If it's intermittent, then clean the potentiometers on the panel..

 

[komrad2236]

Hey folks
First of all thanks for phenomenal advice.
I do have plastic screwdriver so I used it while having oscilloscope turned on.(but I am also going to make a wooden one, I actually do need it, never thought of using those barbecue sticks)

I was able to adjust channel B by moving R16 pot and doing what danadak suggested, and I was able to successfully fix the problem with channel B.

However, channel A still has the same problem no matter how much I adjust the pot, in fact I don't think the pot had any effect at all.

As far as is it intermittent, seems its not and I did clean out potentiometers few days ago when I noticed there were some problems while moving the knobs slowly like signal skipping or jittering, those problems disappeared after cleaning.

edit : I went back to check channel A little bit more in detail, and I noticed that I can not raise the signal beam above certain point almost like there is a ceiling the signal is hitting and can not go above, and if I move the R16 pot on channel A board, it seems it just changes where this imaginary ceiling is.
Even using vertical adjustment knob won't allow me to go above this ceiling.
Here is the video to demonstrate : (uploading)

 

[komrad2236]

Here is the video demonstration

 

[rjenkinsgb]

I'd go through the amp and check the collector voltages of pair (upper/lower).

Or compare voltages between the two channels, with the controls in identical positions. It's possibly a failed transistor, or just a bad connection somewhere.


I got a Solatron dual beam scope in the 70s which had a similar problem, & back then I had next to no test gear & it was long pre-internet...
I could not find anything with a multimeter but the guy I got it from kept telling me to just change all the transistors in that channel board. I eventually did, for ones salvaged from scrap - and it then worked perfectly!

Transistor failure is a definite possibility, though I'm not suggesting the change-everything approach!

 

[komrad2236]

Sounds good, I will go ahead and do that.
The voltage compare thing, as I do not have transistors used in this board.
I assume there is a possibility of resistor or something else malfunctioning and messing up the transistors bias and operation mode.
It might be good just in off state, remains to be seen.

 

[komrad2236]

I did some measurements, can you spot which component is bad based on these voltage readings ?

Bad board A​

Good board B​

 

[rjenkinsgb]

It looks like somewhere around R24 to R27, something is high resistance, shorted or a bad joint?

IC1/T5, the one below the 1 / 2 / 5 attenuator switch, has a different base-emitter voltage, compared to all the others.

Note that R25/26/26 have no voltage across them on the good channel, but a large voltage drop on the bad one, especially an odd voltage on R29.

It could be something in the switches even, corrosion or dry joint?

I suspect something in that area is messing up the voltages at the transistor.