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Oscilloscope horizontal line is curved. How fix this?
Posted by
Dhanushka
on 24 Mar, 2019 18:18
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My analog oscilloscope is a PINTEK PS-350 one. Now the horizontal line (ground line) of that is slightly curved. Due to this reason, now the edges (rising and falling) of the calibration square wave signal also have slight slopes. Every time/div have same slope. This is not a problem of probe compensation or trace rotation. I checked them. The calibration signal is correct. Also I have checked that. I expect a solution for this. Thank you.
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#1 Reply
Posted by
james_s
on 24 Mar, 2019 18:24
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It's very difficult to say for sure without doing some tests, but often this is the result of internal damage to the CRT. Can you try swapping the horizontal and vertical wires to the deflection plates? That should result in a vertical trace instead of horizontal but if the trace is still curved then the problem is probably not with the CRT.
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#2 Reply
Posted by
jdragoset
on 25 Mar, 2019 01:01
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Since it is not time/div related, almost seems like unwanted magnetization of CRT components or shield.
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#3 Reply
Posted by
Dhanushka
on 25 Mar, 2019 03:18
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Thanks jdragoset.
Can you tell me that is there a way to demagnetize that components?
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With persistence and luck you can compensate for the magnetised parts with another magnet in the appropriate position. The problem is finding said magnet and position. Watch the screen while moving magnets around and use that as guidance.
Obviously this means putting your hand (the old high voltage rule is one hand at a time) near high voltage circuits while they are operating. Be very careful.
The second problem is fixing the compensating magnet in position, and the possibility that the situation will drift over time. Sometimes it is just best to move on to another scope.
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#5 Reply
Posted by
james_s
on 25 Mar, 2019 06:36
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Wouldn't it be easier to use a degaussing coil to demagnetize rather than try to compensate for magenrised parts? Of course it it turns out to be internal damage to the CRT then compensating may be the only option.
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#6 Reply
Posted by
tautech
on 25 Mar, 2019 07:07
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My analog oscilloscope is a PINTEK PS-350 one. Now the horizontal line (ground line) of that is slightly curved. Due to this reason, now the edges (rising and falling) of the calibration square wave signal also have slight slopes. Every time/div have same slope. This is not a problem of probe compensation or trace rotation. I checked them. The calibration signal is correct. Also I have checked that. I expect a solution for this. Thank you.
Email Pintek for the service manual.
pintek@pintek.com.tw
User manual:
http://www.pintek.com.tw/pdf/Analog_Oscilloscope_0224.pdf
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#7 Reply
Posted by
jdragoset
on 26 Mar, 2019 01:24
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Many Degauss coils are on ebay.
That being said, Maybe a tool demag would be better if there are any magnetized higher carbon steel causing the problem.
Degauss coils generate low AC magnetic field that work on soft iron (shadow mask, deflection plates, grid supports, cathode assembly and maybe the mu-metal shield) at some distance.
Tool demag devices generate very strong AC magnetic field that work close-distance with a death grip until the tool is moved away and the field shut off.
For most of these, the on time is only allowed for ten seconds to avoid over heating.
We used to make a simple version with the coil from a two pole shaded pole fan motor, disconnected from the dovetail joints connecting it to the armature/shaded pole assembly.
Bear in mind this drastically decreases the AC impedance and will rapidly over heat.
Also the near-pole magnetic field will be very strong and should be isolated with 1/2' wood thickness.
Energize near the demag target, move away slowly and energize.
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#8 Reply
Posted by
jdragoset
on 26 Mar, 2019 02:11
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Last line should read:
Energize near the demag target, move away slowly and de-energize.
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#9 Reply
Posted by
Dhanushka
on 27 Mar, 2019 03:47
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Thanks for all comments.
Can you you tell me how make a DIY degassing coil using a degassing coil of a computer monitor? I have old 14" CRT PC monitor. I meant that what are the parameters such as diameter, voltage, current limiting resistor value etc. that suitable for 6" oscilloscope CRT. Also using these coils, can it be caused to make a problem?
Thank you.
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#10 Reply
Posted by
james_s
on 27 Mar, 2019 15:45
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Careful about tearing apart a 14" Pc monitor, those are becoming collectible. For degaussing the specs are not critical at all. I've used a 100W incandescent bulb as a current limiter for a degaussing coil from an old TV. I doubled the coil up a couple times so it was a loop about 4" diameter.
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#11 Reply
Posted by
GreyWoolfe
on 27 Mar, 2019 15:57
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I was shown how to use a Weller soldering gun as a degaussing tool.
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#12 Reply
Posted by
jfphp
on 27 Mar, 2019 17:26
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On some old analog scopes there was a trimmer to adjust the "cushion" : a rectangular pattern had to be adjusted without been neither crunched nor inflated. Found on some digital CRT like Spectrum analyzer or network analyzers. The rectangular pattern is internaly generated with a lot of adjustement patterns. Magetizing of the CRT is not common with a mumetal shield found in almost every scope.
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#13 Reply
Posted by
Dhanushka
on 28 Mar, 2019 03:47
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Thanks for all new comments.
I am going to degassing my oscilloscope CRT using a 4" DIY degassing coil. Could you please tell me whether there is how much probability to damage my oscilloscope. I have attached a image of the calibration signal.
Thank you.
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#14 Reply
Posted by
james_s
on 28 Mar, 2019 04:26
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It's hard to tell for sure, but that looks to me like the trace rotation just needs to be adjusted, or possibly the circuit powering the rotation coil is not functioning. Is there a setting for that? Often it's a small recessed trimmer.
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#15 Reply
Posted by
Dhanushka
on 28 Mar, 2019 04:54
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Thanks james_s.
When I turn the trace rotation for straight the vertical lines the ground horizontal line get a an angle with X axis. Also, ground horizontal line is curved little.
Thank you.
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#16 Reply
Posted by
Dhanushka
on 28 Mar, 2019 05:33
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When I contact PINTEK, They says that the attached one should be used to demagnetize. Compared with this and DIY one, may DIY one have a probability to damage the CRT?
Thank you.
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#17 Reply
Posted by
Dhanushka
on 30 Mar, 2019 06:47
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I used a DIY 4" degassing coil in series with 100W bulb but the problem is not solved yet. PINTEK sent me the attached adjustments details, troubleshooting manuals and circuit diagram. Could you please send me a solution for this using that documents.
Thanks.
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#18 Reply
Posted by
Dhanushka
on 30 Mar, 2019 06:48
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#19 Reply
Posted by
tautech
on 30 Mar, 2019 07:21
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PINTEK sent me the attached adjustments details, troubleshooting manuals and circuit diagram.
Circuit diagram, please post it.
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#20 Reply
Posted by
Dhanushka
on 30 Mar, 2019 08:05
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#21 Reply
Posted by
tautech
on 30 Mar, 2019 09:07
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Circuit diagram
Now you have enough info to start from the basics and check all voltages in the PSU are OK and ripple is reasonable.
There is no ripple spec but for all the low voltages it should be less than 20 mV. The 140V rail shouldn't be much more.
Can you safely measure the EHT ? It could be -2000V !
This might help understand all the things you need to check:
http://www.sphere.bc.ca/test/tek-parts/troubleshooting-scopes.pdf
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#22 Reply
Posted by
Dhanushka
on 30 Mar, 2019 11:06
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Thanks tautech.
I do not have voltmeter to measure the 2000 V now. But can I use a florescent (taking that close to high voltage line) bulb to check that qualitatively?
Thanks.
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#23 Reply
Posted by
ginji
on 30 Jan, 2022 02:46
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Hello.
My analog oscilloscope (IWATSU SS-6421) also has a curved horizontal line.
When the horizontal line is at the center of the top and bottom, it is not curved, but when I move it upward or downward, it is curved.
The readout display is also curved.
What is the cause? Please help!
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#24 Reply
Posted by
james_s
on 30 Jan, 2022 08:00
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Probably internal damage to the CRT. Bent deflection plates or broken support inside the neck of the tube. It's also possible I suppose that something is magnetized.