I have a problem with my oscilloscope.help me

[engineser]

oscilloscope problems

when my hung chang OS-620 Oscilloscope is working in full screen, it goes to half screen. and doesnt go to full screen again.Can anyone help me?

ps:I have the schematic of the oscilloscope

 

[E-design]

oscilloscope focus repair problem

If you can be more specific, like include a photo to explain what is the problem. Also if you can post the schematic in PDF we may be able to offer some advice.

 

[albert22]

oscilloscope without high voltage problem

If the image gets smaller and blurred it might be caused by a reduced high voltage.
A reduction in the vertical size could be a problem with the vertical amp(same as for the horiontal). If both vertical and horizontal size are affected check the power supply which feeds the deflection amplifiers.

 

[engineser]

repairing my oscilloscope

Jpeg1 is the picture of the screen,when the oscilloscope works full screen.When the full screen goes to half screen, it becomes as it isshown in jpeg2 .
Thanks for your help I really appreciate it.

 

[E-design]

oscilloscope half screen

This problem should be quite easy to fix. If you look at the circuit diagram, you will notice that both circuits are the same (CH1 & CH2) pre-amplifiers up to the red lines. From there the signal gets switched (CHOP mode for slow sweeps & ALT mode for fast horizontal sweeps) if both channels are in use. The block marked in blue provides the switching signals. So your fault is most likely in the final vertical amp stage where you are losing amplification on one side of the differential stage. The final transistors often run quite hot and may have heatsinks on them. Your fault sounds to me to be heat related, where the transistor fails as it starts to heat up.

There are two ways to go about thisafter making sure all the supply voltages are correct)
1/ Using a can of circuit freeze, squirt each of the transistors in turn as soon as the fault appears. Wait a second or two between transistors to see if the fault clears. The fault may clear because freezing the transistor will tend to reduce the leakage current in the faulty transistor until it heats up again. If normal operation returns when freezing a transistor, you should remove, check and replace it if necessary. Be aware it may test quite normal on a Ohm meter when cooled off and out of circuit. I often apply some heat from a soldering iron (someone may help you) while checking the transistor. Check for excessive leakage between collector & emitter which is the most likely to show up. If in doubt, change the device.

2/ Ground CH1/2 inputs and place the vert controls of both channels in the center position and XY mode. Next by taking voltage measurement in the final stage, you should be able to see which half is faulty. The DC voltages for the top transistors should be very close to the ones on the bottom. Voltages going to the CRT should be very close to zero with no fault. Having the fault condition, you should see a DC offset on one of the CRT plate connections.

Having a second scope to check signals using the cal-out through this stage is nice.

Doing the it freeze method will be faster. Also inspect the board with a magnifier to make sure you have no dry solder joints in this section.

 

[samibel]

do oscilloscopes have fuses

I have the same problem with my os 620 the high voltage is too low and the focus is wrong, the horizontal amplitude is to short, i thing that is a to low voltage of the power supply... i hope an electric shema help me please thaks in advance.

Sorry for my poor english language i an belgian people

 

[engineser]

power profile diagram oscilloscope

I am sending. If you will have problems about opening the file, give me your e-mail adress.so that i will send the schema via e-mail.
My e-mail adress: engineser@msn.com

Added after 42 minutes:

I have the same problem with my os 620 the high voltage is too low and the focus is wrong, the horizontal amplitude is to short, i thing that is a to low voltage of the power supply... i hope an electric shema help me please thaks in advance.

Sorry for my poor english language i an belgian people

 

[crx220e1]

no trace on my oscilloscope?

i hope someone could help me... i have the same oscilloscope and it suddenly don't give any trace... when i turn on the oscilloscope there is no display though the LED indicator is on... could someone help me?

 

[E-design]

is my oscilloscope tube blown

You need to get the manual and check all the voltages given out by the power supply. One of these will be used to power the high voltage section which provides the needed voltages for the CRT to work.

 

[crx220e1]

oscilloscope crt problem

i've downloaded the schematic and studied it... there seems to be a 1.9KV supply in the CRT... I only have a DMM with a max reading of 1000V... is there any way to skip adjusting the 1.9KV supply... also could it be possible that since my oscilloscope doesn't have an output at the CRT the INTENSITY adjustment resistor is busted/uncalibrated?

 

[Borber]

oscilloscope freezing

To measure 2kV make resistor divider with 10Mohm (better is 3x3.3Mohm) and 100kohm resistors. Use 20V range.
Before any adjustments measure all supply voltages. They should be within 10% of rated values. Measure circuits voltages and find deviations from expected values.

 

[E-design]

oscilloscope focus repair

Here is a quick test to check if you have HT and heater voltages present on the tube.

Turn up the intensity on the scope and turn down any graticule lights on the display.
Darken the room.
Switch on the scope.
After about 10sec switch of the scope while watching the display.
If you see any indication of a faint flash on the CRT then you have a very good chance that the HT (-1.9kV in your case) and heater voltages are present.
I have repaired many scopes in the past with a previous company and this trick saved me a lot of time with dead CRT problems.

You may be stuck with a blanking problem. Often the blanking control get's a HV capacitor or diode failing in the DC restoration circuit thereby cutting off the beam.

You may need another scope to monitor the blanking control pulses.

 

[E-design]

scop blanking problem

I have marked the things I would check first on the diagram.

Check all parts maked with red blocks for short or open circuit faults.

 

[crx220e1]

m.descriptiva hung chang os 620

thanks for the help... i found out that the fuse was busted... i replaced it and the oscilloscope worked again... though i still have a question what does that -1.9KV do? ... i can't reach the pin to measure it... though I adjusted the potentiometer that control the -1.9KV until I have a "finer" output... is this ok even though I did not measure if its -1.9KV?

 

[E-design]

oscilloscope display blurred

The -1.9kV is used to accelerate the electrons to the front of the screen where it strikes the phospor and glows. The cathode of the CRT must be more negative wrt grid.

Added after 4 hours 49 minutes:

The blown fuse may be an indication that something is not well and that the circuit draws more current than it should. The fuse may fail again after a few on/off cycles.

 

[crx220e1]

oscilloscope and no display and crt

ok now what do i need to do to determine the main problem?

 

[E-design]

oscilloscope no trace problem

You will need to wait until it fails again and then start looking for the failed or leaky components.

You could use a meter and verify that the current drawn by the circuit is well within the rating of the fuse. If that is good then it may be just fuse fatigue causing its failure.