Vintage oscilloscope, channel A beam not good, channel B is good

[komrad2236]

Intro :
Hi guys, how you all doing ?

I have been gifted my first ever oscilloscope, 2 channel  - 20 MHz.
I attached some photos of how it looks like when its not disassembled, as I have already taken it apart for cleaning and repair.


I know very little about this tech.
Only from what I was able to learn online in few days I have it but I am excited and willing to learn much more.

I noticed it has issues, hoping I can get help here and I also have a question or two.
Question #1:
Do oscilloscopes have channel A on by default ?
On this oscilloscope, if both channel A and B are off, channel A beam is shown.
Manual however states following(used Google translate from Slovenian to English) :

Code: [Select]

A,B - button for choosing the mode of operation of the vertical amplifier. 
If button A, or B is pushed in, channel A, or B is on. 
If both buttons are pushed in, both channels work alternatively or via "chopper", depending on the continued speed of the time base. 
Triggering in that case is only through the A channel. 
This means that in the case when the oscilloscope works as a two-channel, it is not possible to trigger the time base via channel B. 
In that case, we have to replace the inputs.
[b]If both buttons are pushed out, the operation is not regular[/b].

Video that showcases this :




Now onto the problem with channel A
-Channel A , I can't get the beam to appear on the screen, its all the way at the bottom, off screen, unless I switch to 5V, .5V, 50mV and 5mV(yes, all 5s,weird but specific? ) (talking about volts/div dual knob)
In this case I can only get the beam slightly above middle of the screen.
Also, specifically on 5V position, if I wiggle the big knob, the one that clicks, beam will twitch or jump up and down the screen(fixed by cleaning volts/div potentiometer and its contacts, image below in reply)

Video of that here :


But for some reason both knob that adjusts the vertical height, and small knob on Volts/div(I assume it means Volts per division) , they both affect vertical position of the beam(video below that shows this).
This oscilloscope has sort of a dual knob, it consists of big knob and small knob on top of big one and it looks like this

Big knob clicks in place, and can be moved to 20v to 10v to 5v,2V,1V, .5V, .2V, .1V, 50mV, 20mV, 10mV and 5mV.
It can not be moved in between, only to set positions I just mentioned.
Small knob doesn't click, instead it just rotates smoothly like regular knob does.
I assume this small knob is for fine adjustment like getting to 13v/division or something like that ?
Same dual knobs can be seen here, it says "CAL" on it
https://youtu.be/5F92iSgEQ-M?t=56

-Pressing beam finder button does show that beam is there, just off screen

-There is a button that makes the beam move from left to right if switched on or make it stationary if off, and indeed it works.
Both channels beams do move or stay in place depending on this button,if switched on or off.

Video that showcases channel A problems, I go over what happens on every position :

[komrad2236]

If I am allowed one more reply, here are schematics and layout of components on channel A's PCB :

[sarahMCML]

Somewhere internally you should be able to find a trimmer potentiometer marked R15, and if you adjust this carefully whilst switching the volts/div switch over 3 ranges, you should be able to adjust out the movement of the beam. Just be extremely careful of high voltages in the later sections of the circuit. Can't see it on the board you showed us!

[komrad2236]

R15 is actually just a regular resistor of 47 ohms, I found it and indeed its 47 ohms spot on, tested with DMM.
If you take a look at schematic there is R16 little bit underneath it, that is a symbol for trimmer potentiometer in this case.

[Chris56000]

Is there any Member that can read Slovakian or is registered on "elektronik.si" that can download the complete 12.47 MB Iskra MA4049 manual for me please? I can translate it into English and upload it to k04BB if someone can get it from "elektronik.si" for me!

I tried registering on there so I could get it but I kept getting a red error message and I'm not sure what that forum is wanting as I filled in everything it appeared to require!

Chris Williams

[komrad2236]

@sarahMCML , I still want to thank you !
Thanks to you as I was looking for R15 resistor, I looked into the knob for volts/div and it was DIRTY ! 

Cleaned it and now twitching is gone. 
100% gone, it doesn't twitch a bit.

Every other issue is still present.

@Chris56000
I have them here,
https://drive.google.com/drive/folders/1-KAHHziQhGj40ROpt0AJ1LJmDQhn_cz9

[2N3055]

Is there any Member that can read Slovakian or is registered on "elektronik.si" that can download the complete 12.47 MB Iskra MA4049 manual for me please? I can translate it into English and upload it to k04BB if someone can get it from "elektronik.si" for me!

I tried registering on there so I could get it but I kept getting a red error message and I'm not sure what that forum is wanting as I filled in everything it appeared to require!

Chris Williams

It is Slovenian not Slovakian... 

That forum is complicated.
I have it from different source.

Here is schematic.

User manual is 13 M. It is in next 4 posts as multifile zip.

[2N3055]

remove .txt from end of filenames and combine them together. Pdf is inside.

[TimFox]

R15 is actually just a regular resistor of 47 ohms, I found it and indeed its 47 ohms spot on, tested with DMM.
If you take a look at schematic there is R16 little bit underneath it, that is a symbol for trimmer potentiometer in this case.

R16 = 25k trimpot is the "balance" pot, which should do what sarahMCML suggested:  the print in your schematic is a bit fuzzy.
Very vintage CROs put that pot on the front panel, or reachable through the front panel, since it was important.
Vacuum-tube amplifiers were less long-term stable, and the balance pot was almost always on the front panel:  ignorant students treated it as a vertical position pot and messed it up for us knowing students in the lab course.

[2N3055]

p3

[2N3055]

p2

[2N3055]

p1

[2N3055]

https://drive.google.com/drive...Opt0AJ1LJmDQhn_cz9?usp=sharing
https://cloud.mail.ru/public/8SJf/hFd3Ky2DL

[komrad2236]

R16 = 25k trimpot is the "balance" pot, which should do what sarahMCML suggested:  the print in your schematic is a bit fuzzy.
Very vintage CROs put that pot on the front panel, or reachable through the front panel, since it was important.
Vacuum-tube amplifiers were less long-term stable, and the balance pot was almost always on the front panel:  ignorant students treated it as a vertical position pot and messed it up for us knowing students in the lab course.

You were right, it was accessible trough a hole.
I turned R16 all the way to the right as far as it goes, which was about half the turn, and the beam moved up a little bit, that's all.
Attached the image so you can see before and after

[sarahMCML]

Did you try changing to the 1, 2 and 5 positions of the volts/div switch as you moved the pot slightly? It interacts with all three positions and is very sensitive. Try it again whilst doing the above, with the vertical centering control set to its mid position.

[komrad2236]

Yes, I tried also some combinations to see if I get anything.
First I tried like you said, having the vertical centering knob to its mid position, moved it slowly as I switched between 1,2 and 5V positions on Volts/div switch.
Beam still remains off screen, on bottom, until I move the pot all the way to the right to its maximum right rotation.

I then also tried moving the small knob on top of volts/div switch, the one I mentioned in my first post and posted a photo of, which I believe is called "CAL" knob, I tried having it all the way to the left, and then repeated what you said, then I also tried having the CAL knob all the way to right, same thing happens.

I also tried having vertical centering knob all the way to the left, mid and right positions, to try different situations.
I tried moving the pot slowly, and little faster, while switching positions as well, no change.

In all situations, same exact outcome occurs, beam is off screen,bottom, unless I move the pot all the way to the right.

What bothers me is that this "CAL" knob, I do not think it is supposed to move the beam up and down like mine does.
I can confirm this because channel B does not do this.
And here on this post
https://www.eevblog.com/forum/beginners/v-cal-knob-on-a-scope-what-is-it-used-for/

Seems like "CAL" knob is what I suspected it to be, sort of a fine adjustment, when you need 7 Volts/div for example
As I found online, sometimes its called "VAR".
Explained here : http://www.tpub.com/neets/book16/71e.htm

In the schematic, 5mV , 50mV, .5V and 5V are in fact grouped together, have a look at attachment in this reply
That might give some clue as to what component might be faulty ?

[2N3055]

CAL is for calibrating gain of amp. VAR is manual control of gain for user.

I CAL is changing vertical postion (and you are right it should not) that means that top and bottom transistor are have different static work point, i.e. something is wrong with them.

measure voltage over E-C on TR5 and TR6  and voltage drops over respective  collector and emitter  resistors. It all should be symetrical. Also measure previous stages in a same way.
If you look at schematics, amplifier is differential and symmetrical and DC coupled from beginning to the end.....
Follow the signal path and find a point where things are not symmetrical anymore.

[komrad2236]

I appreciate the help but you will have to be little bit more specific on exactly what do I do as I am a newbie.

Few things confused me so let me ask on specifics.

measure voltage over E-C on TR5 and TR6

Do you want me to turn on this non working oscilloscope, then connect one multimeter lead to ground and other one to emitter and see its voltage then same with collector
Or do you want me to turn on the non working oscilloscope and then connect one multimeter lead to emitter and other lead to collector ?

voltage drops over respective  collector and emitter  resistors. It all should be symetrical.

I assume turn off the non working oscilloscope, put multimeter to diode mode and then test the transistor ?
I don't understand which resistors you mean exactly as Tr5 has R53 and R51 above it, there is R49 and R50 below it which are also above Tr6, and R52 and R54 are under Tr6, so how do I exactly test ?

amplifier is differential and symmetrical and DC coupled from beginning to the end.....

No idea what this means. 

Follow the signal path and find a point where things are not symmetrical anymore.

This sounds like you want me to use another oscilloscope(or no?) which I do not own.

[2N3055]

I appreciate the help but you will have to be little bit more specific on exactly what do I do as I am a newbie.

Few things confused me so let me ask on specifics.

measure voltage over E-C on TR5 and TR6

Do you want me to turn on this non working oscilloscope, then connect one multimeter lead to ground and other one to emitter and see its voltage then same with collector
Or do you want me to turn on the non working oscilloscope and then connect one multimeter lead to emitter and other lead to collector ?

voltage drops over respective  collector and emitter  resistors. It all should be symetrical.

I assume turn off the non working oscilloscope, put multimeter to diode mode and then test the transistor ?
I don't understand which resistors you mean exactly as Tr5 has R53 and R51 above it, there is R49 and R50 below it which are also above Tr6, and R52 and R54 are under Tr6, so how do I exactly test ?

amplifier is differential and symmetrical and DC coupled from beginning to the end.....

No idea what this means. 

Follow the signal path and find a point where things are not symmetrical anymore.

This sounds like you want me to use another oscilloscope(or no?) which I do not own.

That is OK.

For start you need only multimeter.  You can, at first, do that: black multimeter probe to ground, and then measure in pairs voltage on emiters  and collectors of IC1(T3/T4),  collectors of Tr3/Tr4, collectors and emitters of Tr5/6. These will be some voltage (not important what voltage at this moment) and two pairs should be same value but opposite polarity.  So if collector of Tr3 is 6V , Tr4 will have to be -6V. Most of schematic should have like nodes positive and negative that should be mirrors of each other. Look at schematic and you will see what I mean. So you probe one point and then find it's mirror and then probe there.  If you find a node where there is not so, you are getting close.

If you measure very similar voltage on E and C of any transistor, that means transistor is either fully conducting because previous stage is feeding too much of excitation (voltage/current) or some of resistors are open or transistor shorted and is dead.
It is detective work, where you slowly and methodically go from input to output to find a point where you start to see something wrong and then snoop around that point to figure out what..

I would suggest to print out schematic and write down voltages as you measure them.

[komrad2236]

Thanks for patience, really means a lot !
Yes I can see it now, how board is essentially mirrored.

So here's what I got.
Black probe on ground, volts/dev switch to 20V position
(reason I am writing this is because I noticed voltage measurements are different, earlier I wrote :

-Channel A , I can't get the beam to appear on the screen, its all the way at the bottom, off screen, unless I switch to 5V, .5V, 50mV and 5mV(yes, all 5s,weird but specific? )

and in those positions, voltage measurements are different as opposed to all other positions, so I had the switch put to one of these other positions, 20V in this case as the beam is off screen)

IC1/T3
pin 7(emitter)    =  -1.18 V
pin 8(collector)  =    5.55 V

IC1/T4
pin 10(emitter)    =  -0.41 V
pin 11(collector)  =   1.12 V
Tr3
Emitter                =   3.48 V
Collector             =  -1.54 V

Tr4
Emitter                =   3.48 V
Collector             =   1.63 V
Tr5
Emitter                =  -2.13 V
Collector             =   6.10 V

Tr6
Emitter                =  -1.28V
Collector             =   4.51V



Edit :
Ok I see what you mean how its smart to slowly track the signal and find whats going on, I am going to do that, if you have any requests, or suggestions, please post them.
I attached the photo, right at the start I can see the difference at input voltages to the board, I wrote them down, should I have any concerns ?

[sarahMCML]

I think I may have confused you in my instructions, and not given you enough information.
What I meant to say was:

Set the Vertical Adjust knob to its Mid point.
Set the Small Centre knob on the Volts/Div control to its fully anti-clockwise position (there looks to be a switch there, so turn it right off!).
Then, trying 1V, 2V and 5V positions on the Main Volts/Div switch as you do so, adjust R16 slowly, all the while checking to see if the beam moves back into the centre. That should balance up the input stage.

[komrad2236]

Just to be sure I understand you, I am attaching the photo.
I am trying this atm, I will give it a shot for for some time to see if anything happens.




Update :

So I have been trying this for some time now, no change.
Beam is off screen.

[komrad2236]

I've been testing few points on both boards, here are results, for comparison.
As they are very similar and I assume I could use board B to fix board A as in use working board B as a reference to what voltages I should be getting at certain points

[sarahMCML]

Forgetting my previous question for a moment, if you go to your diagram in post #13, can you connect your meter between points F4 and F5, and adjust R16 until the voltage becomes 0V. Then see if it changes if you change the Volts/Div switch to another setting, as before.

[Chris56000]

My thanks for the Iskra MA4049 manual – I'll have a go at translating this, and the information on the circuit and layout diagrams into English over the next few weeks!

I'll also study the circuits and add some fault–finding suggestions as well!

Chris Williams