Oscilliscope advice......

[nflutter]

First time poster and a very newbie to electronics but trying to learn!

I'm learning about audio amp repair and I'm a bit puzzled with oscilliscope use. I've bought a 4ch digital scope, budget model Hanmatek DOS1104. I,ve been working on an amp which had a completely dead SMPS which I seem to have fixed with new PWM chip, a FET which was for the PFC circuit. Whilst working on it I was powering the SMPS section up with a dual power supply, 18v and 5v this got the PFC circuit running and the SMPS driver and FETs, I could verify this using my new scope. I could test the PWM chip for the PFC circuit and see the oscillator, the output to the FET and read DC voltages etc nice and cleanly on the scope, also everything on the SMPS chip driving FETs. Now the whole power supply section seems good, everything reads on scope as it should so now connected to mains through a current limiter. the amp is not working as it seems there is further issues on the output stage but all the power rails read good on my DMM +5, +12, +15, +-39 and +-75 all look good on DMM..... BUT now nothing reads good on scope, just a complete mess, no recognisble PWM or even DC volts. I'm using the same ground as before which is power ground on the low voltage side. I've checked I have the 18v and the 5v where I had it when connected to bench supply and I have all voltages inc 380v after PFC so everything seems to be working just can't use scope which I need to check output stage.

Where am I going wrong??

Hope it all makes sense, appreciate any help.

Thanks

[Grandchuck]

I am not sure what you are doing but there could be safety implications.



Since you are a beginner .....

[tggzzz]

Welcome to the forum.

It is a bit difficult to understand what exactly you are doing from your description. Perhaps you could re-word it so that each experiment/conclusion is in a separate paragraph, and preferably show a block diagram of how things were connected together.

Be aware that SMPSs are constant output power devices. Hence if the input voltage is reduced (e.g. by a current limiter) then the input current increases. Too high a current might damage the input switching transistors.

Also be aware that you shouldn't connect the scope probe shield to some parts of the SMPS, since the probe shields are connected to protective mains earth. Never ever "float the scope" by disconnecting the protective mains earth; that can be lethal to your UUT, your scope, you, or combination thereof.

To start to understand why that's the case and how to avoid the problems, have a look at the references in https://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/

[nflutter]

Yes I know, I've watched literally every YT vid on safe scope use before I even connected it first time. I'm not connecting to earth or anything on mains side, also scope is USB powered from a double insulated supply so no connection to earth, I'm only probing low voltage side I'm just confused why i get a good reading when on bench supply but not mains. I've confirmed a power ground point of the SMPS circuit which does not connect to ground in any way and the circuit probes fine on bench supply but not mains. I've even used my DMM to check if a voltage is present back at scope ground which it isn't. Maybe I'm missing something and need to go back to YT!

[nflutter]

An example of what I'm testing.....

Hook the amps PFC/SMPS circuit to dual bench supply with an 18V and a 5V supply needed, this powers up the PWM chip (UC3842) that drives a FET for th PFC circuit to give the 380v DC required. When connectied to bench supply I connect my scope ground to the power ground of the circuit, effectively pin 5 of the chip, and the probe pin 6 which is the output to the FET. I can see a nice square pulse running steady at 67.9Khz (spec is 68Khz) so PFC circuit is working fine. Now connect to mains and read the HV with DMM and I get 381v DC as expected and all voltage rails read good on DMM. Attach scope as before, press 'Autoset' button and all I get is lots of noisy peaky readout, no square pulse as before and frequency just reads a ? basically looks like scope can't read anythng.

[nflutter]

Whilst I say I'm a very newbie to electronics I'm not a newbie to electrics, I've been dealing with electrics for years, I'm just a newbie to electronics just wanting to learn more about how stuff actually works. My job is as a stage lighting and sound engineer so I deal with electrics every day but now just wanting to fix some of my dead kit and learn how it works in the process.

[nflutter]

To help clarify the process I've done is pretty much this fix on youtube, same amp, same repair, same method using scope with same grounding point, all good until I connect with mains then scope just reads garbage.

[Ice-Tea]

When connectied to bench supply I connect my scope ground to the power ground of the circuit, effectively pin 5 of the chip, and the probe pin 6 which is the output to the FET.

For your information: you're probing the mains side of your SMPS with what is effectively a floating scope. Which is, as Tgggzzzz has already commented, a bad, bad idea.

AFAIC: if you describe yourself as a newbie with electronics, don't probe mains. If you're an intermedate: don't probe mains. And if you're advanced you'll probably no longer want to probe mains unless it is unavoidable ;-)

[nflutter]

Okay, can you help me understand where I'm probing mains? The only points I probe are the oscillator of the IC and the output from IC to the gate of the FET. As I understand the HV would be present on drain and source of FET?

[nflutter]

The actual problem I have now is that now the power rails are all good and I have an issue at the output stage, I am actually trying to probe the LV side of the amp, the driver for the power Mosfets which should have a 450Khz square wave 50% duty cycle but I just get noise, I just tried probing the PWM controller in the SMPS to confirm the scope was working as before on bech supply but now on mains supply it isn't.

[ZGoode]

When connectied to bench supply I connect my scope ground to the power ground of the circuit, effectively pin 5 of the chip, and the probe pin 6 which is the output to the FET.

For your information: you're probing the mains side of your SMPS with what is effectively a floating scope. Which is, as Tgggzzzz has already commented, a bad, bad idea.

AFAIC: if you describe yourself as a newbie with electronics, don't probe mains. If you're an intermedate: don't probe mains. And if you're advanced you'll probably no longer want to probe mains unless it is unavoidable ;-)

Absolutely 100% agree here.  I hate probing anything directly referenced to mains.  It made my 4277A repair kind of suck.  On the off chance I have to do it, that is the perfect application for a battery powered isolated scope like a Fluke Scopemeter with an appropriately rated 10:1 or 100:1 probe.

[Ice-Tea]

Okay, can you help me understand where I'm probing mains? The only points I probe are the oscillator of the IC and the output from IC to the gate of the FET. As I understand the HV would be present on drain and source of FET?

It's not because it has "GROUND" in the schematic that it is fine. If you have trouble wrapping your head around that, think a bit about this:

[nflutter]

I get what you're saying about the mains side of the power supply.

As I understand, the risks of using an oscilliscope are that you do not want to create a path from your circuit back to mains ground, effectively shorting your mains to ground, even though as in my house all mains radials are protected by type A RCBOs.

I'm not sure how this could even occur if I'm using my USB powered scope which is essentially the same as using a battery powered scope, as suggested by ZGoode

And as i said all the repair work I did on the SMPS was only whilst connected to a bench supply, the only time I probed whilst connected to mains was very carefully the LV output of the FET driver to verify if I had the same trace as when connected to bench supply, and I didn't, so that's my real question.

I guess no one has any advice as to why this is so I guess I'll just go on to a gas engineer forum and ask how to connect up a gas barbecue in my lounge.

Thanks guys.

[tggzzz]

I'm not connecting to earth or anything on mains side, also scope is USB powered from a double insulated supply so no connection to earth,

So, you have floated a scope. Bad idea. Very bad practice.

The entire scope case and connectors will now be at the same voltage as the node you have connected the probes' shield to.

There will probably be a few hundred pF between the shield and ground/neutral; your UUT might break because of that.

[SteveThackery]

Now the whole power supply section seems good, everything reads on scope as it should so now connected to mains through a current limiter. the amp is not working as it seems there is further issues on the output stage but all the power rails read good on my DMM +5, +12, +15, +-39 and +-75 all look good on DMM..... BUT now nothing reads good on scope, just a complete mess, no recognisble PWM or even DC volts. I'm using the same ground as before which is power ground on the low voltage side. I've checked I have the 18v and the 5v where I had it when connected to bench supply and I have all voltages inc 380v after PFC so everything seems to be working just can't use scope which I need to check output stage.

Where am I going wrong??

Let's get back to the original question now that the electrical safety issues have been hammered home.

@nflutter: Just to be clear, you're saying all the outputs from the power supply section read correctly when you use a DMM? So in that respect the power supply seems to be working perfectly?

But when you use your oscilloscope on those same test points, it shows a "complete mess" on the screen?

Sorry to labour the point, I just want to be perfectly clear on where you are.

I'm hoping our experts will jump in at this point, but when I get a complete mess on my scope screen it is almost always a triggering problem. Have you tried sweeping the trigger level through the complete range on the screen? 

You say your oscilloscope is USB-powered, and is being powered from a USB charger? I wonder if you have one of those USB power bricks that you can try, so that you can see if running it from the power brick's internal battery makes a difference.

Finally, could you describe what you mean by the "current limiter" which you are using?

[tggzzz]

I get what you're saying about the mains side of the power supply.

As I understand, the risks of using an oscilliscope are that you do not want to create a path from your circuit back to mains ground, effectively shorting your mains to ground, even though as in my house all mains radials are protected by type A RCBOs.

No, you don't get it.

You have mentioned one problem with floating a scope. There are many others.

If you rely on yoootooob vids, then you will struggle to understand the problems.

I strongly suggest you look at the references in the link I gave. They give solid explanations and solutions.

[nflutter]

I'm not connecting to earth or anything on mains side, also scope is USB powered from a double insulated supply so no connection to earth,

So, you have floated a scope. Bad idea. Very bad practice.

The entire scope case and connectors will now be at the same voltage as the node you have connected the probes' shield to.

There will probably be a few hundred pF between the shield and ground/neutral; your UUT might break because of that.

I haven't floated the scope, by design it is USB power only, there is np ground there is no IEC just a USB cable to barrel cable. The whole case is plastic and I have the plastic caps on all probe connections. I could not actually connect it to ground if I wanted to.

[nflutter]

Never use floating scope!!!

If you need to measure the voltage between two points (neither of which is at 0V relative to ground) with an oscilloscope, you connect the ground clips of both probes together. Set the first channel’s probe to a specific voltage. Then, invert the second channel and press the “Add” button on the oscilloscope.

I haven't floated the scope, by design it is USB power only, there is np ground there is no IEC just a USB cable to barrel cable. The whole case is plastic and I have the plastic caps on all probe connections. I could not actually connect it to ground if I wanted to.
Modify message

[tggzzz]

I'm not connecting to earth or anything on mains side, also scope is USB powered from a double insulated supply so no connection to earth,

So, you have floated a scope. Bad idea. Very bad practice.

The entire scope case and connectors will now be at the same voltage as the node you have connected the probes' shield to.

There will probably be a few hundred pF between the shield and ground/neutral; your UUT might break because of that.

I haven't floated the scope, by design it is USB power only, there is np ground there is no IEC just a USB cable to barrel cable. The whole case is plastic and I have the plastic caps on all probe connections. I could not actually connect it to ground if I wanted to.

The scope is floating, whether or not you did that explicitly.

Clearly you do not understand the many consequences of the way you are using the scope. It also seems you are reluctant to take that hint and to follow suggestions as to how you could learn.

I can't overcome that, so I won't waste my time. Sorry.

[Ice-Tea]

Never use floating scope!!!

If you need to measure the voltage between two points (neither of which is at 0V relative to ground) with an oscilloscope, you connect the ground clips of both probes together. Set the first channel’s probe to a specific voltage. Then, invert the second channel and press the “Add” button on the oscilloscope.

I haven't floated the scope, by design it is USB power only, there is np ground there is no IEC just a USB cable to barrel cable. The whole case is plastic and I have the plastic caps on all probe connections. I could not actually connect it to ground if I wanted to.
Modify message

Yes, you have. The shells of those BNCs, per example, are not connected to mains earth as on other scopes. You could say they are... drum roll... floating. There is a reason why they are earthed on normal machines: for safety.

There are scopes designed for this kind of work. With probes designed for this kind of work. And this ain't it.

Now you could listen to dudes with several hundred years of experience between them telling you you should stop doing what your doing or, you know, enter your ticket in the great Darwin Award Sweepstakes.

[nflutter]

Let's get back to the original question now that the electrical safety issues have been hammered home.

@nflutter: Just to be clear, you're saying all the outputs from the power supply section read correctly when you use a DMM? So in that respect the power supply seems to be working perfectly?

But when you use your oscilloscope on those same test points, it shows a "complete mess" on the screen?

Sorry to labour the point, I just want to be perfectly clear on where you are.

I'm hoping our experts will jump in at this point, but when I get a complete mess on my scope screen it is almost always a triggering problem. Have you tried sweeping the trigger level through the complete range on the screen? 

You say your oscilloscope is USB-powered, and is being powered from a USB charger? I wonder if you have one of those USB power bricks that you can try, so that you can see if running it from the power brick's internal battery makes a difference.

Finally, could you describe what you mean by the "current limiter" which you are using?
[/quote]

Firstly thank you for adressing the actual question.

Yes all voltage rails read as they should and are stable using my DMM so I assume the power supply is working correctly.

I have 380v DC with DMM after PFC so again I assume the PFC circuit is working correctly.

Yes scope is USB powered ONLY, all plastic case with plastic cover fitted on all exposed BNCs, no IEC socket, no connection to ground.

I am using a 100w light bulb current limiter, I only used this when initially powering the amp after carrying out repairs and all was good, a brief bright bulb as caps charged then pretty much fully off so bypassed so now its on full mains no limiter.

I now seem to have issue with output stage, schematics show a test point where I should get 450Khz square wave but when I probed it just noise. I've rechecked all voltage rails to the amp and have 5v, 12v, +&- 39v and +&- 75v. ALL of this is on LV side of amp.

I will check the triggering but everything is packed away at the mo.

One thought is that if there was a smoothing problem in the power supply would that possibly give me the correct voltage but with lots of noise?

Thanks again

[nflutter]

Never use floating scope!!!

If you need to measure the voltage between two points (neither of which is at 0V relative to ground) with an oscilloscope, you connect the ground clips of both probes together. Set the first channel’s probe to a specific voltage. Then, invert the second channel and press the “Add” button on the oscilloscope.

I haven't floated the scope, by design it is USB power only, there is np ground there is no IEC just a USB cable to barrel cable. The whole case is plastic and I have the plastic caps on all probe connections. I could not actually connect it to ground if I wanted to.
Modify message

Yes, you have. The shells of those BNCs, per example, are not connected to mains earth as on other scopes. You could say they are... drum roll... floating. There is a reason why they are earthed on normal machines: for safety.

There are scopes designed for this kind of work. With probes designed for this kind of work. And this ain't it.

Now you could listen to dudes with several hundred years of experience between them telling you you should stop doing what your doing or, you know, enter your ticket in the great Darwin Award Sweepstakes.

Again, the scope is designed this way to be USB powered only, there is no IEC connector, there is no earth connection and I have the plastic covers fitted on all exposed BNCs, I have not altered it in any way, it may be floating but I did not do this, and again all repair work was carried out whilst connected to BENCH SUPPLY with 18v and 5 v DC ONLY.

If all you want to do is get on your high horse about what could theoretically happen then please don't get on this thread, if you want to be of some help to the actual problem of probing the LV side of the amp then all help welcome.

Tomorrow at work I will be dealing with 400 amp 3 phase Powerlock systems so please don't come on to me about earthing, i deal with it every day in every secenario.

[tggzzz]

One thought is that if there was a smoothing problem in the power supply would that possibly give me the correct voltage but with lots of noise?

In my first job I designed a test set to verify that a mechanical microwave switch was switching correctly. It was protected against reversed input voltages, short circuit outputs, and everything else we could think of.

It worked successfully. Until, that is, a technician who was universally known as "bigfoot" managed to set the PCB on fire.

It transpired that a switch he was testing was giving intermittent results. Bigfoot, however, thought (and I use that term loosely) "problems are often caused by PSU ripple, PSU ripple is improved by a large capacitor, therefore I'll connect a large capacitor across the output". Did he think that maybe the tester was working and doing its job correctly? Oh no. He decided to be an exemplar of "you can't make something foolproof, because fools are so damn ingenious".

As a last attempt at helping you understand how mistaken you are, I do have a scope that is designed for the kind of things you are trying to do. It is a Tektronix THS720. Here's a picture of the BNC inputs, one of the probes that comes with the scope (right) and a traditional scope probe (left). Note two things:

• which bits of the probes are metal and plastic
• the material the BNC input is made from. Hint: not metal

That should be sufficient for you to pause and think "maybe I am missing something and have something to learn".

[ZGoode]

I'm not sure how this could even occur if I'm using my USB powered scope which is essentially the same as using a battery powered scope, as suggested by ZGoode

The scopemeter I mentioned and the Tek THS720 that tggzzz are very different from you DOS USB scope.  The fluke and the Tek are specifically designed with isolated outputs, not just from earth/ground, but from each other as well.  They are designed for precisely this application in mind.
Your scope, even though not directly mains powered still shares grounds between the different inputs it has.

[ZGoode]

Without knowing more about the device you're trying to measure it's hard to say that the probes/scope you are using are right for this application.  Not saying your scope is bad, its not, its just we really don't have a lot to go off of here