The horizontal line on oscilloscope display - Page 1
Forum section: electronics / beginners

author: gkmaia
author_rank: Frequent Contributor
timestamp: April 20, 2019, 09:10:11 am
content: While the scope is not receiving any signal it displays a straight horizontal line. Is this line what is referred as blanking?If it is not it.What does the blanking circuit of an ocilloscope outputs to the crt?And what are simptoms of a faulty blanking circuit?What block of the scope circuit has failed when there is no horizontal straight line while it is not receiving signals?

author: tggzzz
author_rank: Super Contributor
timestamp: April 20, 2019, 09:13:58 am
content: Quote from: gkmaia on April 20, 2019, 09:10:11 amThe scope displays the voltage at its input. If the input is constant, then the trace is a horizontal line.Blanking refers to turning off the beam when it is "flying back" from the right hand side of the screen to the left hand side, and optionally to turning the beam off before the a sweep is triggered.Experiment with the "norm" triggering mode and with slow sweep speeds.

author: gkmaia
author_rank: Frequent Contributor
timestamp: April 20, 2019, 09:34:05 am
content: Thanks for your answer. Blanking makes sense now.I understand if I am scoping a 5v dc signal it will display a straight line. My scope does that fine.But I do not see a straight flat line when not scoping any signal or when on GROUND coupling and it is not because the vertical position is off the screen or vertical V is out of range.What block of the scope is possibly faulty in a case like this?Horizontal amp? Sweep? Blanking? Trigger?

author: StillTrying
author_rank: Super Contributor
timestamp: April 20, 2019, 09:42:57 am
content: "But i do not see a straight flat line when not scoping any signal or when on gnd coupling."Is trigger set to AUTO.

author: tggzzz
author_rank: Super Contributor
timestamp: April 20, 2019, 10:13:31 am
content: Quote from: gkmaia on April 20, 2019, 09:34:05 amBefore thinking that the scope may be faulty, assume that you haven't set the controls appropriately!You need to do some research into a scope's basic controls. There are many such tutorials available, one isBe very aware of the safety hazards when using scopes, and use the correct type of probe. For pointers, seehttps://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/

author: gkmaia
author_rank: Frequent Contributor
timestamp: April 20, 2019, 06:21:02 pm
content: I appreciate the tip but how it works is not really my issue at this point. I have a few scopes, digital, transistor analog and vacuum analog. None of them behave like this one in regard the flat line. And the service manual does not mention what circuit is in fault when the absence of a flat line. So for example see this analog scope without signal. Flat line is present[url=https://www.youtube.com/watch?v=SX4HGNWBe5M]https://www.youtube.com/watch?v=SX4HGNWBe5M] https://www.youtube.com/watch?v=SX4HGNWBe5M]https://www.youtube.com/watch?v=SX4HGNWBe5M]https://www.youtube.com/watch?v=SX4HGNWBe5M] https://www.youtube.com/watch?v=SX4HGNWBe5MFor a while it had the flatline on ground or off probing. Now it does not. My question remains as per followingI do not see a straight flat line when not scoping any signal or when on GROUND coupling and it is not because the vertical position is off the screen or vertical V is out of range.What block of the scope is possibly faulty in a case like this?Horizontal amp? Sweep? Blanking? Trigger?

author: StillTrying
author_rank: Super Contributor
timestamp: April 20, 2019, 10:50:15 pm
content: Quote from: gkmaia on April 20, 2019, 06:21:02 pmAUTO trig.Does it trigger OK when there is something to trigger on, such as its own 1kHz.

author: Nerull
author_rank: Frequent Contributor
timestamp: April 20, 2019, 10:53:57 pm
content: Sounds like a fault in the user module.How it works is precisely important, because in certain modes the scope doesn't sweep until triggered.It's like complaining that your car doesn't start and then getting pissy with the mechanic when they tell you that you need to turn the key.

author: tggzzz
author_rank: Super Contributor
timestamp: April 20, 2019, 11:22:28 pm
content: Quote from: gkmaia on April 20, 2019, 06:21:02 pmIf you don't understand how your tool works, you will not learn how to use it effectively.By "effectively" I mean using the right tool to unambiguously give you the correct result.And that's true for all tools, not just scopes.

author: gkmaia
author_rank: Frequent Contributor
timestamp: April 20, 2019, 11:46:40 pm
content: Quote from: StillTrying on April 20, 2019, 10:50:15 pmIt does trigger fine either manual or auto any wave at all its horz or vert range.What circuit block is usually responsible to provide a flat line to the crt when there is no signal input or coupling is on ground?

author: james_s
author_rank: Super Contributor
timestamp: April 21, 2019, 12:35:57 am
content: There is no particular circuit block that delivers a flat line, the flat line is caused by the absence of vertical deflection, the horizontal deflection sweeps the beam across the screen, voltage at the probe causes the beam to move up for positive voltage and down for negative voltage, if the voltage is zero then the beam will sweep across the middle of the display.Note that this describes how an analog scope works, if you have a digital scope then it will tend to approximate this behavior but they don't work quite the same way. Either way I'm not at all convinced that there is anything wrong with your scope, can you post a video of what it is doing?

author: StillTrying
author_rank: Super Contributor
timestamp: April 21, 2019, 12:42:09 am
content: "What circuit block is usually responsible to provide a flat line to the crt when there is no signal input or coupling is on ground?"AUTO trig!If there's been no real triggers for around 0.8 sec AUTO trig should start unsynchronized triggering.It might be the mechanical switch.Then it's time to study the schematic, trigger -> timebase.

author: gkmaia
author_rank: Frequent Contributor
timestamp: April 21, 2019, 02:27:32 am
content: Quote from: StillTrying on April 21, 2019, 12:42:09 amThanks. That makes sense. I have an issue with intensity circuit that is flat at a node that should be oscilating. Also the blanking is not a 100% as i see a faint line conecting the ends of the signal as it sweep back. They all share the same board as the trigger. But the trigger is fine so far. I will investigate more.

author: Tom45
author_rank: Frequent Contributor
timestamp: April 21, 2019, 04:00:10 am
content: As many have said, what you are reporting is to be expected if the trigger mode is in normal mode. If trigger mode is set to auto, you should see a line.However, you haven't said if you have tried setting the trigger mode to auto. Also, what scope do you have?