Electronics > Repair
MA 4049 Oscilloscope repair
RiverTown:
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 nothing :(
Blank 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.
RiverTown:
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 |O.
echen1024:
--- Quote from: RiverTown on April 15, 2014, 12:23:45 pm ---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 |O.
--- End quote ---
The problem with scopes is you need another scope to fix them.
tautech:
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.
RiverTown:
--- Quote from: echen1024 on April 15, 2014, 01:38:33 pm ---The problem with scopes is you need another scope to fix them.
--- End quote ---
Yes, 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 pm ---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.
--- End quote ---
I 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.
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