MA 4049 Oscilloscope repair - Page 1
Forum section: electronics / repair

author: RiverTown
author_rank: Regular Contributor
timestamp: April 12, 2014, 02:43:03 pm
content: I received an old Iskra 2 channel 20MHz analog oscilloscope from a friend. We have tested it and everything was working correctly. Scope has been working for few hours without any problems.I didn't have my set of probes so I ordered them of a flebay, and stored scope under the bench.When probes finally arrived, I've connected them to the scope, pressed the ON button and nothingBlank screen.I've tried normal time mode, XY mode, but screen is blank even if I press beam finder.At first glance I've thought that fault must be somewhere on the HV board. Luckily I have full schematics of the scope.Oscilloscope is builded as a six board construction. It has two vertical boards, power supply, horizontal and trigger, and HV board. Sixth board is used for interconnecting, and has just a few components.I almost forgot, there is one more thing that I noticed. When the scope is on for minute or so, and then when turned off and rapidly on again the beam is visible on the screen for about a second.Power supply seems fine. All rails are bang on except 21V rail which is few volts high. But that rail is unregulated, there is just a rectifier and filter cap, so that seems ok because scope has worked fine before.I've checked almost all semiconductors on HV board except HV diodes and an opamp in HV stabilizer circuit. They all seems ok.Even the voltages that go to the cathode tube seems more or less fine.Cathode voltage is little low (-459V and should be -470V), and focus voltage goes from -460- -389V but should go from -470- -430V. Every other voltage looks fine.I'm not sure if heater is working properly because glow in the tube is barely visible.Voltage between one side of a heater and ground is -415V, and between other side and ground is -421V. So voltage between two ends of a heater is about 6, 3V. Are these voltages ok?I also noticed silent high pitch buzzing. I'm not sure is it coming from the tube or HV board. Probably it is coming from HV transformer.I've almost reached dead end so I really need your help with this repair.Any help is greatly appreciated.P.S. Sorry if my english isn't the best.

author: RiverTown
author_rank: Regular Contributor
timestamp: April 15, 2014, 12:23:45 pm
content: Nobody has advice what to look next, or where fault might be?I don't have another oscilloscope so I can't measure oscillator on HV board or any others signals.Also my multimeter can't measure more then 1000V, so I can't check is HV out ok.I'm quite desperate now, because I need scope for my next project.

author: echen1024
author_rank: Super Contributor
timestamp: April 15, 2014, 01:38:33 pm
content: Quote from: RiverTown on April 15, 2014, 12:23:45 pmThe problem with scopes is you need another scope to fix them.

author: tautech
author_rank: Super Contributor
timestamp: April 15, 2014, 07:18:36 pm
content: Just had a look at 1 Schematic. Lucky you have DC voltages at various points on it.The manual should describe a setup state the for scope to be in for those DC voltages to be valid. It is a special DC test state. Work from the output end of each schematic to check voltages first until you find which board has the problem.

author: RiverTown
author_rank: Regular Contributor
timestamp: April 17, 2014, 02:58:03 pm
content: Quote from: echen1024 on April 15, 2014, 01:38:33 pmYes, I know.Unfortunately scopes in my country are very expensive, and I don't have another scope. Price for used 20 MHz, 2 channel noname scope can be high as 600 $.First schematic is power supply board. As I already say, I've checked all the voltages.Quote from: RiverTown on April 12, 2014, 02:43:03 pmI doubt that fault is on vertical, or horizontal/trigger boards because trace isn't visible even if I press beam finder.So only board that has left is HV board. I've checked all the voltages that are going to the cathode tube. Results are listed in my first post.HV board schematic has few more marked voltages around transformer and HV stabilizer circuit. I haven't measure them yet.Tomorrow I will solder few wires on the traces, which cannot be reached with multimeter probe, so that I can measure and check voltages.Is there any trick for measuring of high voltage with normal multimeter?Once again, sorry for my bad english.

author: tautech
author_rank: Super Contributor
timestamp: April 17, 2014, 04:18:22 pm
content: Quote from: tautech on April 15, 2014, 07:18:36 pmLike I said there are DC voltages marked on the schematics. Re-seat all connectors. Re-seat all socketed devices. Check any high value resistors on the HV board.

author: soldo63
author_rank: Contributor
timestamp: April 18, 2014, 12:01:28 am
content: Her is a link to a pdf file on troubleshooting oscilloscopeshttp://www.sphere.bc.ca/test/tek-parts/troubleshooting-scopes.pdfIt's for Tektronix but there might be something helpful in there for you.

author: RiverTown
author_rank: Regular Contributor
timestamp: April 20, 2014, 01:48:09 pm
content: First of all I want to thank everyone for help. This oscilloscope troubleshooting pdf is great.I've re-seated and cleaned all connectors with contact cleaner. There isn't any socketed devices on the boards.I applied negative ten volts to the Z axis input on the back of the scope, and nothing was visible on the screen. According to the above listed pdf that means that fault is probably on the HV board, or that CRT is faulty as I expected.All voltages listed on the HV board schematic have been measured. On 21.5V mark I've measured 22.3V, and 460V where 470V is marked. Other voltages are bang on (+70V, +33V, +12V).There are few more marked voltages on the PCB (-12V, 70V, 18V). -12V, and 70V were ok. 18V is marked on the weird spot, and it isn't obvious where 18V rail is so I measured few nearby traces. Closest voltage value was around 21V.There are some minor differences between schematic and PCB, probably different REV, but nothing crucial.I've become quite desperate, and decided to check almost all components on the HV board.I checked all resistors (without pots and trimmers) everything was fine. Capacitors were also fine, but I only measured capacitance. There are only two electrolytes on the board.I've checked HV diodes, they are also fine.Measured almost all semiconductors (just a diode test), nothing looks faulty.I think than next step will be to check few components on the interconnecting board.What is your sugestion? Can you comment heater voltages from the first post, are they ok?Also that high pitch buzzing sound, I am quite sure that it is coming from the transformer. I don't know is it normal. Is it possible that frequency of the oscillator has drifted?Is it worth to try and replace an op amp? It is in the TO99 case and it will be pain in the butt to replace it with DIP8 package.Do you know any tricks to measure HV with normal DMM?Should I start troubleshooting on the horizontal board?Thanks in advance..P.S.Happy Easter!!

author: tautech
author_rank: Super Contributor
timestamp: April 20, 2014, 08:01:16 pm
content: Check the DC voltages for the plate output amps on the IC board.Heater voltages seem normal.

author: RiverTown
author_rank: Regular Contributor
timestamp: April 21, 2014, 01:14:17 pm
content: These are plates voltages. Voltage on the plate D1 and D2 isn't stable.D1 -MAX: 47,4V-MIN: 21,65V-AVG: 34,7VD1 -MAX: 51,5V-MIN: 28,13V-AVG: 39,8VD3: 35VD4: 31V

author: tautech
author_rank: Super Contributor
timestamp: April 21, 2014, 01:30:46 pm
content: The output voltages should be stable if scope is set up for that measurement.Have you the calibration procedure? That will describe it in detail...output balance.Had a look at the troubleshooting pdf and it is good. Go back to the HV board and take note of the pdf chapter on HV. Try disconnecting the HV multiplier to see if the buzzing goes away.Study the pdf, use the flow charts.

author: RiverTown
author_rank: Regular Contributor
timestamp: April 27, 2014, 05:05:38 pm
content: I was quite busy last few days so didn't have time to reply earlier.Voltage on plates D1 and D2 (horizontal plates) wasn't stable because scope was in normal time mode. When I set it in the XY mode voltage is stable, 35V.There are some short calibration procedures in the scopes manual. Sadly they are very basic and the manual is written in the language that is similar to my native language, but I can't understand everything that says.Buzzing sound is still here even if multiplier is disconnected.I've re-soldered entire HV board and managed to get trace on the screen, but only when INTENSITY knob is set very high.Acording to the troubleshooting PDF main reasons for that are weak cathode voltages or weak CRT. So my next step is to check cathode voltage generation circuit (voltage is still 10V lower than it should be).If think that I won't search any further if fault isn't there, because everything else is functional and scope is functional.Once again thank you for your help.

author: tautech
author_rank: Super Contributor
timestamp: April 27, 2014, 08:01:52 pm
content: IF the scope was quiet and now buzzing, I would suspect the HV transformer is under too much load. Check or substitute componentry for cathode and intensity. It is possible a component is faulty but not dead, check all carefully.

author: RiverTown
author_rank: Regular Contributor
timestamp: April 28, 2014, 06:00:00 pm
content: Hi.I'm not sure is buzzing sound was present before, maybe it was.Ok, I'll check that.

author: robimarko
author_rank: Regular Contributor
timestamp: May 11, 2014, 10:55:35 pm
content: Its nice to see another live Iskra MA4049 oscilloscope,its a great one.I personaly use MA4048 on weekly bases in school,and they work live on day they were made.

author: RiverTown
author_rank: Regular Contributor
timestamp: May 18, 2014, 02:58:42 pm
content: Hi,I haven't seen this post before.You know me, I'm "elektor" from the elektronika.ba forum. I didn't have time to reply on your post there.I hope that you have already found/ read Tektronix troubleshooting guide from this topic.I was planing to link it to you on the elektronika.ba topic.Pozdrav!

author: robimarko
author_rank: Regular Contributor
timestamp: May 18, 2014, 03:28:08 pm
content: Unfortunately I havent seen it before.Offcourse that i know you from elektronika.baI will surely read it cause i still havent fixed my scope but i am working on it.Most likely one or more 500K resistor are faulty(One is surely cause i measured resistance of over 32 Mohm on it).Now i am trying to remove CRT to get acces to the whole HV board