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  #31  
Old 09-28-2013, 09:23 PM
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Quote:
Originally Posted by N2IXK View Post
What is the amplitude of that ringing on the 240V line? What is the DC voltage?

Can you try temporarily bridging another cap between 240V line and ground to see if it improves the bars or cuts the amplitude of that ringing?

Post the section of the schematic that shows C270 and this supply for those of without a Sams handy...
You'll have to forgive my lack of knowledge. I'm learning how to use the scope as I go along. All I can tell you is it was 5 volts/div with 10x probe and 20us time/div. How can I get the amplitude of the ringing? Do I increase decrease the volts/div and use the vertical scale just on the ringing portion?

As for the cap, I know I need one over 240V, but is there a particular value I should use? Also, should I parallel it with the current one or can I just find a 240V source and put the cap on it to ground?

I've attached the section of the schematics with the C270 a 10uf 300V. The T207 is the Flyback.
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  #32  
Old 09-28-2013, 09:46 PM
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The waveform looks to be about 1.5 divisions peak to peak, so that could be either 7.5V or 75V, depending on whether or not the "5V/div" is taking the 10:1 probe into consideration.

75 vpp ripple on a 240V supply is a problem. 7.5V, not so much.

The doubled waveforms might be caused by improper triggering. Make sure you are triggering on whatever channel the probe is connected to, and if your scope has a TVH trigger mode, use it. Try rocking the trigger level back and forth around zero, and see if that makes the trace clearer.

I would start with the same value cap as you already have installed, and putting it right in parallel should be fine if it actually goes to ground. The attachment isn't showing up for me...
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  #33  
Old 09-28-2013, 10:49 PM
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Quote:
Originally Posted by N2IXK View Post
The waveform looks to be about 1.5 divisions peak to peak, so that could be either 7.5V or 75V, depending on whether or not the "5V/div" is taking the 10:1 probe into consideration.

75 vpp ripple on a 240V supply is a problem. 7.5V, not so much.

The doubled waveforms might be caused by improper triggering. Make sure you are triggering on whatever channel the probe is connected to, and if your scope has a TVH trigger mode, use it. Try rocking the trigger level back and forth around zero, and see if that makes the trace clearer.

I would start with the same value cap as you already have installed, and putting it right in parallel should be fine if it actually goes to ground. The attachment isn't showing up for me...
The probe was in the 10x mode. Also, the triggering is from a Leader LCG-396 Pattern Generator.

The cap is connected the following way. There's a lead from the flyback to a leg of a diode, the other leg of the diode connects to a leg of a 150 ohm 2W, the other leg of the resistor goes to the positive side of the cap and a lead that connects the 240V around the chassis and finally the cap goes to ground.

I'll have to find a local source for a 10uf 300V cap. The local Radio Shack doesn't have them in-store. Must be a place around here somewhere. Not looking forward to paying 7 bucks for shipping of a single cap.
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Last edited by TinCanAlley; 09-28-2013 at 10:52 PM.
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  #34  
Old 09-29-2013, 01:14 AM
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Got any dud CFL bulbs? They usually have a 10uF 250-450V cap in the base...Though not exactly as likely to be good as a new cap.
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  #35  
Old 09-29-2013, 09:23 AM
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Quote:
Originally Posted by TinCanAlley View Post
The probe was in the 10x mode.
But some scopes can automatically detect the probe setting, and adjust the vertical scale range to reflect the probe attenuation and prevent confusion. This requires both a scope and a probe that support this feature. Do yours?

If your scope is doing this, then the scale really is 5 volts/division. If your scope or your probe doesn't do this for you, then the scale is actually 50 volts/division. This makes all the difference in the world when trying to interpret the waveforms as ripple on a power supply rail.
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  #36  
Old 09-29-2013, 11:08 AM
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Quote:
Originally Posted by N2IXK View Post
But some scopes can automatically detect the probe setting, and adjust the vertical scale range to reflect the probe attenuation and prevent confusion. This requires both a scope and a probe that support this feature. Do yours?

If your scope is doing this, then the scale really is 5 volts/division. If your scope or your probe doesn't do this for you, then the scale is actually 50 volts/division. This makes all the difference in the world when trying to interpret the waveforms as ripple on a power supply rail.
I have a Tektronix 465M. No auto settings for the probe.

I've been thinking about the waveform. It's a 240V source, so the 5V/Div would be too low to show it and the 10x probe would make it 50V/Div and it would cover almost 5 divisions. What I got was way under that. I'm going to use the DMM to go all the way back to the boost diode and when I find the point it's 240V, I'm going to scope it. With a setting of 5V/Div and the probe in 10x, I should get a form that covers over 4 divisions, right?
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  #37  
Old 09-29-2013, 11:10 AM
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Quote:
Originally Posted by Electronic M View Post
Got any dud CFL bulbs? They usually have a 10uF 250-450V cap in the base...Though not exactly as likely to be good as a new cap.
Thanks for the heads up. I have a crap load of new 26 and 13 watt CFLs from China and I don't mind gutting one or two. I'll let you know what I find.
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  #38  
Old 09-29-2013, 11:48 AM
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Quote:
Originally Posted by TinCanAlley View Post
I have a Tektronix 465M. No auto settings for the probe.

I've been thinking about the waveform. It's a 240V source, so the 5V/Div would be too low to show it and the 10x probe would make it 50V/Div and it would cover almost 5 divisions. What I got was way under that. I'm going to use the DMM to go all the way back to the boost diode and when I find the point it's 240V, I'm going to scope it. With a setting of 5V/Div and the probe in 10x, I should get a form that covers over 4 divisions, right?
Where is your baseline set? Are you operating the scope AC or DC coupled?

Typically, for checking ripple on a power supply, you would operate AC coupled, so you only see the AC component of the signal, and ignore the DC. If you are running DC coupled, the baseline should rise a bit less than 5 divisions (250V) from zero, with ripple superimposed on it. You can do a quick sanity check with a DMM, by measuring the 240V supply twice, both in DCV range (should show ~240V), and in ACV (should show close to zero if no significant ripple is present).

From what you are telling us (5 V/div with a 10:1 probe and no auto priobe ranging), the scope is actually showing you 50V per division, or a waveform of ~75 Vpp. This is WAY too much noise on a supply rail, and would easily explain the jailbars.

You really need to learn how to use a scope effectively in order to get useful information from it. Take a couple hours and watch this GREAT YouTube training session:

http://www.youtube.com/watch?v=8ZKMrzTGxLQ
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  #39  
Old 09-29-2013, 12:28 PM
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Quote:
Originally Posted by N2IXK View Post
Where is your baseline set? Are you operating the scope AC or DC coupled?

Typically, for checking ripple on a power supply, you would operate AC coupled, so you only see the AC component of the signal, and ignore the DC. If you are running DC coupled, the baseline should rise a bit less than 5 divisions (250V) from zero, with ripple superimposed on it. You can do a quick sanity check with a DMM, by measuring the 240V supply twice, both in DCV range (should show ~240V), and in ACV (should show close to zero if no significant ripple is present).

From what you are telling us (5 V/div with a 10:1 probe and no auto priobe ranging), the scope is actually showing you 50V per division, or a waveform of ~75 Vpp. This is WAY too much noise on a supply rail, and would easily explain the jailbars.

You really need to learn how to use a scope effectively in order to get useful information from it. Take a couple hours and watch this GREAT YouTube training session:

http://www.youtube.com/watch?v=8ZKMrzTGxLQ
Gonna get some popcorn and watch that video.

Okay, just did a scope of the 240V line just after it comes off of the resistor into the main feed (where the capacitor is and the where the 240V gets distributed). I had the scope set to AC coupling, V/D set at .5, the T/D set to 20us and probe at 1x. The rippled waveform covered 1.75 divisions peak to peak. So that would make the AC less than a volt, correct?
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Last edited by TinCanAlley; 09-29-2013 at 12:33 PM.
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  #40  
Old 09-29-2013, 12:33 PM
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.5 V/div and a 10:1 probe is 5 V/div. A signal 1.75 divisions high would be 1.75 x 5, or 8.75 Vpp.
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  #41  
Old 09-29-2013, 12:43 PM
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Originally Posted by N2IXK View Post
.5 V/div and a 10:1 probe is 5 V/div. A signal 1.75 divisions high would be 1.75 x 5, or 8.75 Vpp.
Sorry, forgot to mention it was in 1x setting on the probe.
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  #42  
Old 09-29-2013, 07:14 PM
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Last edited by andy; 12-05-2021 at 07:52 PM.
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  #43  
Old 09-29-2013, 09:02 PM
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It's best to keep the probe in X10 mode unless you're looking at very low level signals. A x10 probe provides a higher input impedance 10M, instead of 1M), so it won't load down the signal as much. It also provides some over voltage protection to the scope since there is a 9 meg resistor in series with the signal. If you forget it's in X1 mode and accidentally touch something with a high voltage on it, you could easily damage the scope. In 10X mode, the worst that's likely to happen is damaging the probe.
Anything 40V and over I use the 10x.

I just about drove myself crazy wondering why I could see the ripple of AC on the DC lines, but couldn't see the DC. I thought I had all the settings correct, but still couldn't see a waveform. After realizing I wasn't going to figure it out on my own, I search the net. I figured out how to measure the voltage of DC, but still can't get a waveform. So does the DC on the B+ have a waveform? If so, how do I see it? What would be the settings other than DC coupling?

Thanks
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  #44  
Old 09-29-2013, 09:14 PM
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Ideally, the B+ has no waveform at all, just a flat line of constant DC voltage. Practically, there will always be a small amount of time variation due to incomplete filtering out of the AC source (unless the device is battery powered). Even if the device is battery powered, there can be small variations due to varying load current drawn by the circuits that use the B+ supply.
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  #45  
Old 09-29-2013, 09:23 PM
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Originally Posted by old_tv_nut View Post
Ideally, the B+ has no waveform at all, just a flat line of constant DC voltage. Practically, there will always be a small amount of time variation due to incomplete filtering out of the AC source (unless the device is battery powered). Even if the device is battery powered, there can be small variations due to varying load current drawn by the circuits that use the B+ supply.
So the flat line I see is correct. About the only thing I can do is move the coupler to GND, move the trace line to the bottom of screen, set the voltage/div, move the coupler to DC and then measure voltage after the trace line moves up. That's okay with me.

If I see ringing on the base of a transistor, do I look at the collector to see if the ringing has been amplified? Also, I seem to recall someone suggesting I ground the base of the ABL transistor to see if the jail bars go away. The voltage on the base is under 1V, so is it okay to ground it? Doing so won't short anything out?
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