Vintage oscilloscope, channel A beam not good, channel B is good - Page 1
Forum section: electronics / repair

author: komrad2236
author_rank: Contributor
timestamp: March 02, 2023, 03:34:15 pm
content: 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: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.If both buttons are pushed out, the operation is not regular.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 thisBig 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 ithttps://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 :

author: komrad2236
author_rank: Contributor
timestamp: March 02, 2023, 03:51:11 pm
content: If I am allowed one more reply, here are schematics and layout of components on channel A's PCB :

author: sarahMCML
author_rank: Regular Contributor
timestamp: March 02, 2023, 05:25:00 pm
content: 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!

author: komrad2236
author_rank: Contributor
timestamp: March 02, 2023, 07:33:08 pm
content: 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.

author: Chris56000
author_rank: Frequent Contributor
timestamp: March 02, 2023, 07:38:50 pm
content: 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

author: komrad2236
author_rank: Contributor
timestamp: March 02, 2023, 08:00:18 pm
content: @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.@Chris56000I have them here,https://drive.google.com/drive/folders/1-KAHHziQhGj40ROpt0AJ1LJmDQhn_cz9

author: 2N3055
author_rank: Super Contributor
timestamp: March 02, 2023, 09:05:56 pm
content: Quote from: Chris56000 on March 02, 2023, 07:38:50 pmIt 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.

author: 2N3055
author_rank: Super Contributor
timestamp: March 02, 2023, 09:09:49 pm
content: remove .txt from end of filenames and combine them together. Pdf is inside.

author: TimFox
author_rank: Super Contributor
timestamp: March 02, 2023, 09:10:05 pm
content: Quote from: komrad2236 on March 02, 2023, 07:33:08 pmR16 = 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.

author: 2N3055
author_rank: Super Contributor
timestamp: March 02, 2023, 09:14:06 pm
content: p3

author: 2N3055
author_rank: Super Contributor
timestamp: March 02, 2023, 09:15:38 pm
content: p2

author: 2N3055
author_rank: Super Contributor
timestamp: March 02, 2023, 09:16:17 pm
content: p1

author: 2N3055
author_rank: Super Contributor
timestamp: March 02, 2023, 09:18:00 pm
content: https://drive.google.com/drive...Opt0AJ1LJmDQhn_cz9?usp=sharinghttps://cloud.mail.ru/public/8SJf/hFd3Ky2DL

author: komrad2236
author_rank: Contributor
timestamp: March 02, 2023, 09:42:16 pm
content: Quote from: TimFox on March 02, 2023, 09:10:05 pmYou 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.Attachedtheimageso you can seebeforeandafter

author: sarahMCML
author_rank: Regular Contributor
timestamp: March 03, 2023, 12:45:52 am
content: 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.

author: komrad2236
author_rank: Contributor
timestamp: March 03, 2023, 09:02:49 am
content: 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 posthttps://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 exampleAs I found online, sometimes its called "VAR".Explained here :http://www.tpub.com/neets/book16/71e.htmIn the schematic,5mV , 50mV, .5V and 5V are in fact grouped together,have a look at attachment in this replyThat might give some clue as to what component might be faulty ?

author: 2N3055
author_rank: Super Contributor
timestamp: March 03, 2023, 11:18:42 am
content: 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.

author: komrad2236
author_rank: Contributor
timestamp: March 03, 2023, 02:14:05 pm
content: I appreciate the help but you will have to be little bit more specific on exactly what do I do asI am a newbie.Few things confused me so let me ask on specifics.QuoteDo 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 collectorOr do you want me to turn on the non working oscilloscope and then connect one multimeter lead to emitter and other lead to collector ?QuoteI 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 ?QuoteNo idea what this means.QuoteThis sounds like you want me to use another oscilloscope(or no?) which I do not own.

author: 2N3055
author_rank: Super Contributor
timestamp: March 03, 2023, 03:09:58 pm
content: Quote from: komrad2236 on March 03, 2023, 02:14:05 pmThat 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.

author: komrad2236
author_rank: Contributor
timestamp: March 03, 2023, 06:10:11 pm
content: 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 :Quoteand 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/T3pin 7(emitter) = -1.18 Vpin 8(collector) = 5.55 VIC1/T4pin 10(emitter) = -0.41 Vpin 11(collector) = 1.12 VTr3Emitter = 3.48 VCollector = -1.54 VTr4Emitter = 3.48 VCollector = 1.63 VTr5Emitter = -2.13 VCollector = 6.10 VTr6Emitter = -1.28VCollector = 4.51VEdit :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 ?

author: sarahMCML
author_rank: Regular Contributor
timestamp: March 03, 2023, 06:54:02 pm
content: 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.

author: komrad2236
author_rank: Contributor
timestamp: March 03, 2023, 07:44:45 pm
content: 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.

author: komrad2236
author_rank: Contributor
timestamp: March 03, 2023, 11:03:10 pm
content: I've been testing few points on both boards, here are results, for comparison.As they are very similar and I assumeI could use board B to fix board Aas in use working board B as a reference to what voltages I should be getting at certain points

author: sarahMCML
author_rank: Regular Contributor
timestamp: March 04, 2023, 12:54:23 am
content: 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.

author: Chris56000
author_rank: Frequent Contributor
timestamp: March 04, 2023, 04:24:40 am
content: 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

author: komrad2236
author_rank: Contributor
timestamp: March 04, 2023, 12:04:13 pm
content: Quote from: sarahMCML on March 04, 2023, 12:54:23 amI don't see diagram in #13 post, but I see it in #15 so I will assume that's what you meant.So I put one probe on F4 and other on F5, I adjusted R16 to 0.000V to be precise, that's the reading I see on multimeter.Results are the same.No noticeable change.Its still the same, every position has the beam off the screen,under, except 5V, .5V, 50mV and 5mV which barely get to middle but can not go above that point.Just for good measure, I wanted to see how big/small changes in voltage between F4 and F5 are,If I rotate the pot all the way to its left, lowest I get is-0.042VIf I rotate the pot all the way to its right, I get0.020VI also decided to see what's going on with working, board B, where its E5 and E4(so letter E instead of F that's all)If I rotate the pot all the way to its right, lowest I get is-0.031VIf I rotate the pot all the way to its left, I get0.011VNote: I assume this is because boards are mirrored so that's why values for R16 pot rotated fully left on board, and R16 pot rotated fully right on board B, both have negative voltage values.I feel we have some clues here, the fact that channel A, CAL knob(small knob on top of big volts/div switch, I circled it with green in earlier photo) is affecting the vertical position of beam.The fact that every other position except 5V, .5V, 50mV and 5mV is off screen and still these 4 positions barely get to middle.There has to be a point or area where these are connected, if only I knew where that is I think best idea would be to look there. No ?Sorry I just feel little frustrated, I feel like I'm on a good path just lack knowledge to read schematics well or understand this board.

author: sarahMCML
author_rank: Regular Contributor
timestamp: March 04, 2023, 03:48:40 pm
content: Yes, sorry, my mistake, it was #15.What you are aiming for is to set the voltage between the two points F4 and F5 to 0V with respect to each other, so that if you then measure them with respect to OV (circuit ground), they measure the same voltage and are in balance.I hope you reset channel B back to where it was originally.

author: komrad2236
author_rank: Contributor
timestamp: March 04, 2023, 09:50:55 pm
content: You're right.I just tested the voltage in respect to ground, and both points had 0.82 V.(Regardless of where the volts/div switch was)QuoteI did, I actually marked and measured both channels before turning any pots.Now I am kind of hoping to get an answer to what these tests amounted to, any clues to what could be faulty ?

author: komrad2236
author_rank: Contributor
timestamp: March 07, 2023, 02:15:01 pm
content: Seeing as I do not have enough knowledge on how oscilloscopes even work, and I need more knowledge on reading circuit diagrams and schematics, I am going back to drawing board and learning all those so I can repair this device.One thing confuses me, inside oscilloscope, on both boards there is a TCA 617 which is transistor array of 5 NPN transistors, meaning 5 transistors in one IC.One thing confusing is, I tested this IC and both boards give me almost identical values.What confused me the most is that T3 in this chip, pins 6-7-8, when I tested pins 6 and 7, I got voltage drop of 112, which is weird as I should be getting a voltage drop of 600-700 which is usual voltage drop for a diode.Also if I test in reverse, I get 1100 voltage dropShouldn't it be no reading as in, a diode is blocking the reverse current ?So why am I getting a drop ?Confusing and goes against what I have learned about testing transistors.Granted I did test this in circuit, maybe I should desolder both chips but afaik, testing transistors in circuit should not pose a problem.For example testing T4 of this chip, pins 9-10-11, when I test 9 and 10 pins, I do get around 600 voltage drop.I get nothing in reverse.Which is how it should be, but with T3...weird.I got super excited when I got this oscilloscope, and thought of fixing it so I will let that drive me to learn more and ultimately fix it and use it for my future endeavors.I will update on how my progress goes in this post.If anyone has any ideas or wants me to do some tests meanwhile to help me find the fault, please shootAlso I want to thank all who chimed in, requested measurements and gave pointers, even though it did not amount to finding the fault and fixing the scope, it helped a lot to teach me about certain parts of oscilloscope and electronics in general.

author: 2N3055
author_rank: Super Contributor
timestamp: March 07, 2023, 02:54:52 pm
content: Quote from: komrad2236 on March 07, 2023, 02:15:01 pmOk this is a possible clue... First, it should be same for both channels, and yes, it is diode, so 500-600mV in one direction, and no current in other.. Of course that is outside the circuit. But it should be same for both channels.If you look at schematic impedance converter block ("impedančni prilagojevalnik") seems to be connected to IC1 T 1/5/4/3 with a connector E1-E8. Maybe it is a plugin card or cable. If you disconnect that you might not need to desolder IC because it will leave some transistor pins floating. Also check Tr2 Tr3 and Tr4 (diode check like you did) comparing good and bad channel. Something is either wrong with IC1/T3 or something connected to pins 6/7 of IC1/T3.Make sure that input attenuator is on same settings on both channels, because base of T3 is connected to switches..

author: komrad2236
author_rank: Contributor
timestamp: March 18, 2023, 11:04:00 am
content: Little update.I disassembled entire oscilloscope and cleaned it as best as I could.I would say now its 95% clean.(contacts, boards, metal case)I made sure Volts/div metal contacts were 100% clean.This was done to eliminate the possibility of dirt blocking the contacts from touching as I found out that could be the cause of problem.I put the oscilloscope together and again, it behaves the same.However, one thing I noticed that I did not before is that actually both channels vertical beam position changes if I move Volts/div switch.Channel A behaves like I mentioned earlier, beam goes off screen for most positions expect ones that have number 5 in it.But now I noticed that even Channel B, vertical beam position is affected a little bit if i switch Volts/Div.If its positioned on any position with number 2, its position is normal,Switching to any position with number 1 moves the beam down by maybe 0.5 cm.And switching it to any position with number 5 moves it down by another 0.5 cm.So there is a pattern going on here.I think using critical thinking( which I think is crucial for problem solving and fault finding, I can not go around randomly testing everything and touching random things hoping it would solve the problem ), its safe to assume both boards are actually working.Rather, something else is at fault here.It can not be power board as I am getting correct voltages.Its not the board for horizontal position either as it is functional 100%.That either leaves the board connected to CRT which is there to adjust the intensity, focus and such,it might be faulty somehow but it would make no sense as it is only there for adjustment of the image itself, or most likely, it will be the board to which all boards are connected to, which is the one I am suspecting my fault will be at.Simply process of elimination and localizing the fault.I do not understand how oscilloscope works yet fully, but I hope its safe to say that the board connected to CRT will not be where fault lies.I am sharing this phenomenal video that helped me understand stuff betterhttps://youtu.be/m0BIyurgBrwLike I mentioned, I do not understand CROs well, yet, but I do not see, guided by common sense, how a board that is supposed to handle the CRT itself could cause this issue, because its showing different stuff for channel A and channel B.So by common sense, it has to be, without a doubt, the main board, possibly something wrong with a certain transistor not amplifying the signal enough to raise the beam to center of the screen.I will start there.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: March 18, 2023, 12:50:32 pm
content: The CRT "Switches": back and forth from channel A to channel B. Either in a chop or alt pattern, so if one channel looks OK on the screen, the CRT is basically OKThe power board is OK also, as you have said.The same beam and pins on the CRT use the same inputs from each channel.The small change in the zero line is common in old scopes, there might be an adjust for the different Volts/Div ranges, but that is not your big problemThe switch itself could be faulty, or the circuits it controls. Identical circuits for each channel. As you said "mirrored"You are correct to look there. and compare the two circuits. I think you are doing this.Be sure the wires on both switches are identical first. Maybe even someone had worked on them and not made a good connection.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: March 18, 2023, 01:10:15 pm
content: On the schematic notice that the switch for the 1:1 1;2 1;5 attenuator has one switch that is active in any position that is a "5"The scope works in these positions if I recall correctly what you have written. Maybe the problem is with the other switches on this "gang" of switches. Maybe bad connection or no connection.Check the wires and make sure they are not shorted and they are correct in this switch. Compare with other channel switch.