Author Topic: How on earth do I get the PSU out of this old scope??? (retitled)  (Read 10979 times)

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Offline Atheus

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Hi all,

Thank you all for all your help so far on my venture into electronics. So I just acquired a bit of kit from Ebay - a "Flue and Phillips" PM3055 60Mhz analog oscilloscope for next to nothing. It was sold as for parts, but one pic showed a trace on the screen, and it didn't look too bad apart from one crack in the plastic and one non-working button (the auto set). I spoke to the guy and he said he didn't really know if it worked, he didn't know how to test it, and it was old so he was just getting rid of it.

I lucked out. The crack in the plastic was purely cosmetic damage - no components directly under it - and it was just a bit of old plastic from the crack preventing the auto set button working. Clear the switch and a bit of duck tape later I have an (I think?) fully functional 60Mhz scope. It even came with probes. Herein lies the problem...

The probes have no ground clip attached! I don't really understand much about the operation of a scope yet, but I know it needs two points per probe, the trace for that probe being the difference between those points over time. They look like the attached photo.



Why would someone remove the clip? Are these of any use without it? Can I just wire-wrap one in or is there some kind of standard connector for that end which I am missing?

Confused,  :-// ~Atheus

/edit: Okay it doesn't like dropbox shared images. I'll give it a real URL...
« Last Edit: April 01, 2017, 04:33:36 pm by Atheus »
 

Online Brumby

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Re: Scope probes have no ground connector!
« Reply #1 on: March 26, 2017, 05:17:19 pm »
Google search "oscilloscope probe" and look at some images.

The ground clip is usually a short alligator clip with an end that screws into or clips onto a point along the probe.  There's nothing exceptionally special about them - other than they are kept short as possible.  (Big loops are great antennas and will pick up any EMF and add it to the signal you want to measure.)

If you want to get into RF work, then you will need to be especially careful with probes and technique.

Starting out in the audio spectrum is where I would point you.
« Last Edit: March 26, 2017, 05:25:42 pm by Brumby »
 

Online Monkeh

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Re: Scope probes have no ground connector!
« Reply #2 on: March 26, 2017, 05:20:15 pm »
The ground leads for those probes would just have a little sprung clip on them. Honestly though, it's easier to just buy some probes than get new ground leads.

A pair of perfectly functional probes will cost as little as £6 on eBay, £10 if you want them this week.
 

Offline amspire

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Re: Scope probes have no ground connector!
« Reply #3 on: March 26, 2017, 05:23:35 pm »
Probe accessories often get lost.

Anyway, you can get excellent probes very cheaply now.

https://www.aliexpress.com/item/SDFC-1Set-High-Quality-P6100-Oscilloscope-Probe-DC-100MHz-Scope-Clip-Probe-100MHz/32650681057.html

If you can fabricate your own spring clip for the probe, you can make your own ground cables. But you will find it good to have two identical probes.

 

Offline Atheus

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Re: Scope probes have no ground connector!
« Reply #4 on: March 26, 2017, 05:29:26 pm »
Oh and I forgot to mention - there's another weirdness - the probe on the left measures 1M \$\Omega\$ end-to-end on the centre pin of the co-ax while the other only 0.5k \$\Omega\$... I thought all probes were high impedance? Is at least one of these safe to use to test the device fully? Is it something to do with the 10x on the probe? I haven't got that far (10x/100x probes) yet.

Re: the existing replies already - thank you! What a great forum. So I'll probably buy two new identical probes, but in the meantime, if I make a solid connection between the alligator clip and the outer co-ax can I use this to test? Or is it too sensitive for simple wire-wrapping or some kind of drilled tab connectors?

/edit: Also is there any specific spec I'm looking for in new probes to match the scope? Or will any generic probe be safe to use?

Pics of scope in action coming ASAP...
« Last Edit: March 26, 2017, 05:39:04 pm by Atheus »
 
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Online Monkeh

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Re: Scope probes have no ground connector!
« Reply #5 on: March 26, 2017, 05:31:20 pm »
Sure, you can just hook an alligator lead up to it. Tin the end of a bit of wire and wrap it tight if you need.

I spend more time using a steel spring as a ground contact than a lead!

If you flip the switch on the right-hand probe to x10 it should read much higher. This is quite normal. (x10 is usually what you'll use anyway)
« Last Edit: March 26, 2017, 05:33:48 pm by Monkeh »
 

Online Brumby

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Re: Scope probes have no ground connector!
« Reply #6 on: March 26, 2017, 05:33:56 pm »
Why would someone remove the clip? Are these of any use without it?

All you need is a reference point - and if the item you are checking is already connected to earth then you can often get away without a ground lead for non critical measurements.  Just remember you might be including a very big earth loop in your signal path.
 

Online Brumby

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Re: Scope probes have no ground connector!
« Reply #7 on: March 26, 2017, 05:42:37 pm »
Mind you, at RF, distance matters and you will see approaches like this:

That coiled section near the fingertip is the earth connection.
 

Offline Atheus

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Re: Scope probes have no ground connector!
« Reply #8 on: March 26, 2017, 06:08:20 pm »
How about these probes?

https://www.amazon.co.uk/Oscilloscope-Scope-Probes-Probe-100MHz/dp/B00GHIHW0K/

They don't specify compatibility with my scope but they fit the spec unless there's something I'm missing. BNC connectors and up to 100Mhz. Is there a reason to pay more for better scopes at this stage?

BTW what is P6100?

And (from specs) "Attenuation ratio: 1:10" - these are 'x10' probes then?
« Last Edit: March 26, 2017, 06:16:18 pm by Atheus »
 

Offline helius

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Re: Scope probes have no ground connector!
« Reply #9 on: March 26, 2017, 06:10:00 pm »
X1 scope probes are a fool's paradise: their performance is unbelievably poor. There are a few situations where they will work, but not many.

In OP's picture, both probes have ground ferrules around their tips. This is where the ground lead attaches, but they accept other connections, like spring clips.
« Last Edit: March 26, 2017, 06:23:42 pm by helius »
 

Online T3sl4co1l

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Re: Scope probes have no ground connector!
« Reply #10 on: March 26, 2017, 06:27:00 pm »
Oh and I forgot to mention - there's another weirdness - the probe on the left measures 1M \$\Omega\$ end-to-end on the centre pin of the co-ax while the other only 0.5k \$\Omega\$... I thought all probes were high impedance?

Click the "X1/X10" switch. ;)

You need the ground clip to reference the probe to your circuit.  It clips into the shrouded slot near the handle.

You can also (or should be able to) slide off the shroud, exposing a coaxial tip, which can be used with high bandwidth / high precision test points as pictured above.

The scope should have a "PROBE COMP" or squarewave output somewhere on the front panel.  This is used to compensate your probe (search on that for info), and also serves as a reference to verify it's working.  (You don't need a ground clip to measure this signal, because it's already grounded to the scope.  Remember that the scope grounds are all common, and you cannot clip ground onto anything that's not also grounded!)

Tim
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Online tggzzz

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Re: Scope probes have no ground connector!
« Reply #11 on: March 26, 2017, 07:15:05 pm »
If you are new to probes, then the references on
https://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/
will help.

Start by paying attention to the basic stuff, including types of probes and safety.
There are lies, damned lies, statistics - and ADC/DAC specs.
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Offline alsetalokin4017

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Re: Scope probes have no ground connector!
« Reply #12 on: March 26, 2017, 10:23:44 pm »
This is what's missing.

But you may as well order a matched set of 1x-10x probes 100MHz from Ebay, they are inexpensive and will work perfectly with your scope, and will include everything you need.
The easiest person to fool is yourself. -- Richard Feynman
 

Offline tooki

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Re: Scope probes have no ground connector!
« Reply #13 on: March 26, 2017, 11:18:57 pm »
I wouldn't replace that nice Hameg probe with a cheap Chinese one. Yes, the Chinese one will work fine. But the cable on better probes is so much softer and nicer to use!

So I'd just buy some ground clips.
 

Offline alsetalokin4017

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Re: Scope probes have no ground connector!
« Reply #14 on: March 26, 2017, 11:27:05 pm »
And these are available separately where?

He wants to _use_ the 60MHz scope, not sleep with it.
The easiest person to fool is yourself. -- Richard Feynman
 

Offline amspire

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Re: Scope probes have no ground connector!
« Reply #15 on: March 26, 2017, 11:34:51 pm »
I wouldn't replace that nice Hameg probe with a cheap Chinese one. Yes, the Chinese one will work fine. But the cable on better probes is so much softer and nicer to use!

So I'd just buy some ground clips.
Sure, keep the probes, but if a pair of identical 100MHz probes can be bought for $10 in total (see my link), doesn't it make sense to get them? Differential mode works much better on an analog scope to a digital one, but you do want two identical probes. Never hurts having spare probes for the times when you need to use the EXT input.

If the probe hook clip accessories are missing, then definitely get a new pair of probes.
 

Online Ian.M

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Re: Scope probes have no ground connector!
« Reply #16 on: March 26, 2017, 11:43:34 pm »
You can probably form a ground clip to fit from some plated steel wire, then its just a 3" piece of flexible black wire, a croc-clip and a bit of heatshrink to make a nice job of it. That will make the Hameg probe usable.

Otherwise, as has just been pointed out, its got two input channels + Ext for external trigger (including trigger view) and X-Y mode, so to make full use of it you really need three probes.  I'd pick up a budget set of 100MHz x1/x10 switchable probes and keep the Hameg one as well.   If you get lucky, you may be able to find a spare sprung hook tip for the Hameg probe - its nice to be able to clip a probe on and leave it, especially when you have multiple waveforms to monitor.

N.B.  hook clips often aren't interchangable between different brands of probe.
 

Online 2N3055

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Re: Scope probes have no ground connector!
« Reply #17 on: March 27, 2017, 01:31:47 am »
 

Offline Atheus

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Re: Scope probes have no ground connector!
« Reply #18 on: March 27, 2017, 01:41:56 pm »
Click the "X1/X10" switch. ;)

Ah! Thank you! 10 meg. Looks like I have a lot to learn...

You need the ground clip to reference the probe to your circuit.  It clips into the shrouded slot near the handle.

You can also (or should be able to) slide off the shroud, exposing a coaxial tip, which can be used with high bandwidth / high precision test points as pictured above.

... that much I do understand though :) - I was just wondering if there's any reason I can't wrap a wire round this outside co-ax connector (indeed revealed by removing the sheath) just for a once off test of the thing, or if the ground wires have component(s) in them like the co-ax side clearly does. Seems there shouldn't be a problem doing this.

I haven't yet plugged a probe in as I've not been confident enough. I think I will now though, and try something VERY simple.

The scope should have a "PROBE COMP" or squarewave output somewhere on the front panel.  This is used to compensate your probe (search on that for info), and also serves as a reference to verify it's working.  (You don't need a ground clip to measure this signal, because it's already grounded to the scope.  Remember that the scope grounds are all common, and you cannot clip ground onto anything that's not also grounded!)

Thank you again! I was not aware of this tiny tab in a sea of buttons. My primary objective right now is to see if it works properly - because I have nothing plugged in I have not yet seen the trace deflected in the Y direction apart from with the manual adjustment buttons (which only mean the CRT works).

Anyone who wants to know what the faceplate looks like just Google image search PM3055. Or for the extra lazy...

https://www.google.co.uk/search?q=pm3055&safe=off&source=lnms&tbm=isch&sa=X&ved=0ahUKEwis55u1zPXSAhVlDMAKHU3vB-UQ_AUICSgC&biw=1440&bih=776

... that little one on the left says "Cal 1.2V [squarewave sign*]" which must be the thing of which you speak. Just to be clear, I can plug a ground-less probe into the scope, and safely prod this tab to see a square wave on the CRT?

I'm trying to unravel that last sentence about grounding... does something special happen when I use this calibration tab? Or is it simply due to the scope itself being grounded?

The scope ground is obviously the wall/house ground, and lets just imagine I am testing another non-isolated device, also grounded - lets say to the very same power socket extension. The non-isolated component is common ground to the tab on the scope. Can I use a ground-less probe in this situation? If not why not? We would just be measuring relative to wall/house ground, rather than relative to whatever the ground probe would be connected to.

I am aware most of the devices I test WILL be isolated and I will need ground probes, however, just for a test, I'll try the calibration and once I'm sure it works I will continue my reading/watching about scopes and probes. Unfortunately the video stickied (while great and I will watch the whole thing) concentrates on older scopes which do not feature a digital "main time base" button etc.

I assume with your last sentence you're just telling me not to put current up the ground wire. Sounds sensible. But... 'you cannot clip ground onto anything that's not also grounded!'... really? Can't I just measure the difference between any two points on an isolated device like with a multimeter?

Cheers!

~Atheus

/edit: *why doesn't this forum have a square-wave sign? Is that TinyMCE you use for a text editor? You can definitely put extra buttons on TinyMCE. It'd have to be custom because there's no Unicode square wave. Two types of sine and a sawtooth but no square. Who knew.
« Last Edit: March 27, 2017, 01:45:49 pm by Atheus »
 

Offline Atheus

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Re: Scope probes have no ground connector!
« Reply #19 on: March 27, 2017, 01:55:57 pm »
Re: alsetalokin4017: thanks for the pic! now I know what the other end looks like I have a chance of finding or making one.

Re: Ian.M: "You can probably form a ground clip to fit from some plated steel wire, then its just a 3" piece of flexible black wire, a croc-clip and a bit of heatshrink to make a nice job of it. That will make the Hameg probe usable."

Excellent. Thank you.

It looks like I have one "nice" probe and one average one. I did wonder because one is significantly sharper than the other. You could put a hole in someone with the Hameg (or measure a very small pin... whatever you're into). I won't buy more until I know a lot more. If it's once-in-a-decade buy then I'll go for quality. Not even sure about different types of triggering (yet!) but I'll probably follow your advice and get two cheap ones and keep the Hameg.

Thanks again,

~Atheus
« Last Edit: March 27, 2017, 02:00:02 pm by Atheus »
 

Offline Atheus

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Re: Scope probes have no ground connector!
« Reply #20 on: March 27, 2017, 03:22:56 pm »
It works!!! :P



It's npt much, it's not even square, but it's a signal! My settings will just be way off. What a score for £50 shipped!
 
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Online Brumby

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Re: Scope probes have no ground connector!
« Reply #21 on: March 27, 2017, 04:27:28 pm »
That's a good start!

I'd try a faster timebase, just one or two clicks, to stretch the X axis and increase the vertical sensitivity - again just one or two clicks - to stretch the Y axis.  This should give you a nicer display.

As for the signal not looking square - that might just be a matter of adjusting the compensation trimmer on the probe.  If that sorts out this little problem, you are indeed looking very good.
 

Offline alsetalokin4017

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Re: Scope probes have no ground connector!
« Reply #22 on: March 27, 2017, 05:30:08 pm »
1. The trace should go all the way across the screen horizontally no matter what the timebase (x axis) setting is. Try using the Horizontal Position control to move the trace rightwards and get the trace to fill the screen horizontally.

2. At this stage please _do not even consider_ probing the Mains with your scope. This can be dangerous to you and to the scope if you don't do it right, and even pros sometimes don't do this correctly. So ... don't.   Yes, you can probe "ungrounded" stuff like battery-powered DMMs or whatever as long as you provide a "ground" connection to the scope probe, using a clip or wire or whatever. Remember though that "all" the scope's BNC connector outer shields are grounded together and to the scope chassis and thus back to the mains cord ground pin... and thus back to everything else that is connected to your mains wiring ground circuit.

3. A complete probe set will include spring clips that slip over the probe sharp pointy tip and provide a means for grabbing onto small wires so you can do your probing "hands free". So please go ahead and order a pair of the inexpensive Chinese 100MHz probes.

For example: (not necessarily a recommendation just an example)

http://www.ebay.com/itm/2x-100MHz-Oscilloscope-Scope-analyzer-Clip-Probe-test-leads-kit-for-HP-Tektronix-/162220256702?hash=item25c514adbe:g:LgsAAOSwmLlX64uT

Note the accessories in the photos. Spring grabber clips, anti-shorting covers for probing IC pins, color-coded rings to identify probes, compensation adjustment screwdriver tool. Some probe kits will also include short spring groundpins and sometimes even BNC adapter sleeves for the probe tips.

Don't worry about the "for Tektronix" or other mfg. names used by the sellers. As long as they have a BNC connector on the scope end and are switchable 1x-10x and 100MHz rated they will be usable with any modern scope of low bandwidth like yours.
The easiest person to fool is yourself. -- Richard Feynman
 

Offline alsetalokin4017

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Re: Scope probes have no ground connector!
« Reply #23 on: March 27, 2017, 06:07:30 pm »
Quote
How about these probes?

https://www.amazon.co.uk/Oscilloscope-Scope-Probes-Probe-100MHz/dp/B00GHIHW0K/

They don't specify compatibility with my scope but they fit the spec unless there's something I'm missing. BNC connectors and up to 100Mhz. Is there a reason to pay more for better scopes at this stage?

BTW what is P6100?

And (from specs) "Attenuation ratio: 1:10" - these are 'x10' probes then?

Yes, those would be fine. Yes, they are compatible with your scope. No, there is no reason to pay more for better probes at this stage. P6100 is sort of a manufacturer's designation (following the Tektronix part number style) for this class of probes. Yes, these are switchable 1x-10x probes.

By shopping around you can find many vendors selling these or similar probe kits for this price or slightly higher. Some of the accessory kits may vary, some may have the compensation capacitor in the BNC connector and some may have it in the probe handle itself, but I think all of them probably come from the same factory or two in China.
The easiest person to fool is yourself. -- Richard Feynman
 

Online Ian.M

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Re: Scope probes have no ground connector!
« Reply #24 on: March 27, 2017, 06:34:52 pm »
*MANDATORY* viewing before you attempt to probe anything that isn't either battery powered or fully floating.

The original discussion topic:
https://www.eevblog.com/forum/blog/eevblog-279-how-not-to-blow-up-your-oscilloscope!/
« Last Edit: March 27, 2017, 06:37:34 pm by Ian.M »
 
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Online Brumby

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Re: Scope probes have no ground connector!
« Reply #25 on: March 27, 2017, 07:42:25 pm »
1. The trace should go all the way across the screen horizontally no matter what the timebase (x axis) setting is. Try using the Horizontal Position control to move the trace rightwards and get the trace to fill the screen horizontally.

I wondered if the half width trace was the result of photographing the screen.  It has some curious artefacts at the end.

However, upon reflection I have doubts, so the best thing is to ask Atheus...  Do you have a full width trace?
 

Offline macboy

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Re: Scope probes have no ground connector!
« Reply #26 on: March 28, 2017, 12:13:04 am »
Usually the cal signal is a 1 kHz signal, so that if you select a timebase (horizontal) of 500 us/div, then you can get up/down transitions of the square wave lined up with the graticule (division markers) on the screen. it also has a specific voltage (1.2 V in your case) so that with the right vertical settings you get a specific height waveform. For you, setting to 200 mV/div should give you a 6 division tall waveform (20 mV/div with 10x probes). Knowing this can help you do a basic sanity test on the scope.

The cal signal is a square wave, but it doesn't look square because your probe isn't compensated to the scope. There is a trimmer on the probe to adjust the compensation so that the top of the square wave is flat. This trimmer is clearly visible as the orange thing on the grey Hammeg probe. Look for something similar on the other. (On probes that are not 1x/10x switchable, and on most higher frequency ones, this adjustment is in the "compensation box" which is attached to the BNC connector, rather than on the probe itself).  Adjust the trimmer so that the majority of each square wave top is as horizontal as possible. There may still be a little over- or under-shoot at the left most edge that can't be trimmed away, especially when using probes that aren't specifically made for the scope. The key is getting the bulk of it to be horizontal.

Note that the scope will recognize Philips brand 10x probes and automagically adjust the displayed vertical scale when one is connected, to compensate for the 10x (really 0.1x) attenuation of the probe. This will likely not happen with the probes you have or cheap Chinese ones, so you may need to mentally do the math.

Buy those cheap Chinese P6100 probes. You can very likely use the ground clips they come with on the probes you have. You probably won't find just the ground clips any cheaper. You may find you like the new probes too.
 

Offline SingedFingers

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Re: Scope probes have no ground connector!
« Reply #27 on: March 28, 2017, 02:00:57 am »
Just a heads up on that scope. Those PM3055's have three little hand grenades in the power supply which I've marked up in the diagram at the bottom of this post. Make it a project to replace them soon. When they explode they do a lot of damage, make one of the worst smells you have ever had to experience and will fill the room with smoke in 10 seconds flat. They are about the right age to go bang. I've had one go off in my PM3217:



And the ones to replace:



This is what you can expect from an actual PM3055 if they explode - throw the whole unit away:



Anything marked RIFA X2 has to be replaced.
« Last Edit: March 28, 2017, 02:07:31 am by SingedFingers »
 

Online T3sl4co1l

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Re: Scope probes have no ground connector!
« Reply #28 on: March 28, 2017, 07:50:16 am »
I'm trying to unravel that last sentence about grounding... does something special happen when I use this calibration tab? Or is it simply due to the scope itself being grounded?

The scope ground is obviously the wall/house ground, and lets just imagine I am testing another non-isolated device, also grounded - lets say to the very same power socket extension. The non-isolated component is common ground to the tab on the scope. Can I use a ground-less probe in this situation? If not why not? We would just be measuring relative to wall/house ground, rather than relative to whatever the ground probe would be connected to.

I am aware most of the devices I test WILL be isolated and I will need ground probes, however, just for a test, I'll try the calibration and once I'm sure it works I will continue my reading/watching about scopes and probes. Unfortunately the video stickied (while great and I will watch the whole thing) concentrates on older scopes which do not feature a digital "main time base" button etc.

I assume with your last sentence you're just telling me not to put current up the ground wire. Sounds sensible. But... 'you cannot clip ground onto anything that's not also grounded!'... really? Can't I just measure the difference between any two points on an isolated device like with a multimeter?

Grounded in the sense of delivering current, yes.  So isolated isn't grounded galvanically, but won't deliver current, so it's good.

Basically you want to clip onto the common terminal of the circuit, making sure there isn't a voltage there, that would cause a ground loop current.

A low impedance ground return (the case of EUT being safety-grounded) can be worse, because of ground loops, as I think you kind of hinted about, earlier, so you understand that.

Remember that there's no such thing as isolation at high frequencies: you can't simply stick in an isolation transformer (or lifted ground, on EUT or scope), and expect the signals to measure correctly between any other points.  This only works at DC, and works less and less well at AC.

An extreme case is measuring a gate driver in a switching circuit, where the driver's "ground" is floating on top of the main switching voltage.  It may be tempting, with isolation, to simply clip the probe on, but you get huge errors due to the switching edges, so bad you might not even be able to see the flat-top level of the square wave you're supposed to be reading.

For those cases, you need extreme measures: a differential probe with high common mode rejection, ideally one with isolation too.

Tim
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Bringing a project to life?  Send me a message!
 

Offline Atheus

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Re: Scope probes have no ground connector!
« Reply #29 on: March 28, 2017, 09:41:23 am »
@T3sl4co1l and @macboy I'm gonna have to read your responses again, maybe do some research, and get back to you. You're much further into this stuff than me but I pick stuff up quickly. I'll probably be testing it again tonight.

1. The trace should go all the way across the screen horizontally no matter what the timebase (x axis) setting is. Try using the Horizontal Position control to move the trace rightwards and get the trace to fill the screen horizontally.

I wondered if the half width trace was the result of photographing the screen.  It has some curious artefacts at the end.

However, upon reflection I have doubts, so the best thing is to ask Atheus...  Do you have a full width trace?

Yes I do :) - it's the digital camera. When I was looking at that I had it set so I could not see the dot move left-right (but the camera could) nor did I have the incorrect X/Y pos settings. That much I figured out.

*MANDATORY* viewing before you attempt to probe anything that isn't either battery powered or fully floating.

Seen it :) - thank you!

Yes, those would be fine. Yes, they are compatible with your scope. No, there is no reason to pay more for better probes at this stage. P6100 is sort of a manufacturer's designation (following the Tektronix part number style) for this class of probes. Yes, these are switchable 1x-10x probes.

Excellent.

[pics of FRIED kit!!!]

Wow! Thank for the warning! What year is yours? Does it have Fluke/Phillips written on it or just Phillips? I hear they are different. I'm gonna check if mine match your components...

~Atheus
 

Offline SingedFingers

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Re: Scope probes have no ground connector!
« Reply #30 on: March 28, 2017, 05:48:23 pm »
Mine was 1983 but it was in storage for around 20 years. There are a number of ones into the white Fluke era that have exploded as well. They both use RIFA X2 capacitors. I've found them in HP stuff from the late 1990s as well. In fact there's was one in my HP E3630A :(
 

Online Brumby

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Re: Scope probes have no ground connector!
« Reply #31 on: March 28, 2017, 06:46:27 pm »
Yeowch on those caps.

Yeah, I'd be replacing them - very soon - especially now we can see life in the scope!
 

Offline SingedFingers

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Re: Scope probes have no ground connector!
« Reply #32 on: March 28, 2017, 07:00:01 pm »
Yeah. There's a whole thread here about those: https://www.vintage-radio.net/forum/showthread.php?t=69128

First thing I do now is replace them on sight.

Have some more motivation from the above thread:

« Last Edit: March 28, 2017, 07:01:34 pm by SingedFingers »
 

Offline John at the Falls

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Re: Scope probes have no ground connector!
« Reply #33 on: March 28, 2017, 07:28:20 pm »
I have 4 sets of the P6100 Probes for an old 5 channel USB scope. They are very nice and well made.


You have a good start in that you understand about ground paths and Mains ground.
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Online tggzzz

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Re: Scope probes have no ground connector!
« Reply #34 on: March 28, 2017, 08:07:23 pm »
You have a good start in that you understand about ground paths and Mains ground.

And in the fullness of time you will begin to understand that "ground" is a convenient fiction that is useful in a limited range of circumstances.

Start by considering how a low frequency monopole antenna is "turned into" a dipole when mounted vertically above the earth. https://en.wikipedia.org/wiki/Monopole_antenna#Radiation_pattern

For a different example consider why, if you are caught outside in a thunderstorm, you should keep your feet together and not apart.
There are lies, damned lies, statistics - and ADC/DAC specs.
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Offline Atheus

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Re: Scope probes have no ground connector!
« Reply #35 on: March 28, 2017, 08:21:34 pm »
Wow - warning heeded! I'll avoid even turning it on again until I replace those caps. A foul 20-year-old PSU frying is NOT a thing I want in my house. I had a modded ATX PSU blow up on my once, it was only about 2 years old let alone 20, but it went bang audibly and if I hadn't been in the room to switch it off at the wall who knows it might have caused a fire. That smelled bad enough!

An ESR meter is on my list to build/buy so I might go ahead and get that to check other caps. I am also building a new ATX modded PSU so it'll be useful there too.

I don't suppose anyone knows where I can pick up the Dave Parker ESR tester kit in the UK for a reasonable price? I was surprised I only saw it on Ebay under different names (dodgy) and a Portuguese store (expensive shipping plus weak £ vs Euro)...

You have a good start in that you understand about ground paths and Mains ground.

Heh thanks :) - I _think_ I understand it anyway, I've only been at this 2/3 weeks, but I'm already a software dev and general purpose nerd so it comes naturally :)

On that note I'm still a little unclear about what happens if you have two different voltages in your device (such as a microcontroller switching high power via MOSFETs), obviously separate from each other in main circuit from PSU onwards, but which will have to meet at a common ground to work... is that right? They must meet, and only once at the end?

Do I have to make sure both end up at 0V at the ground plane? That might sound dumb... most components will "use up" all voltage provided obviously and simply explode if the power is too much, but what about non-resistive components such as an LED, or an accidental short of high volts into ground? I just have to be careful not to do that I suppose?

But what if the micro is battery driven (therefore has a different ground) - do the grounds still have to meet for it to work? Or can a battery powered micro trigger the switch with one wire only while the transistor is referenced to a different ground? As long as the battery V goes far enough above the required to switch on the transistor would it work?

What about ground bounce? I assume with only one source and one ground (but different voltages used at different parts of the circuit - high side and low side I think is the term) you will still get everything 0V at the ground plane naturally when they meet right? Unless you do something stupid like hook up only an LED without a resistor? You still have to worry about noise when thinking about ground bounce I read - usually high side noise (perhaps out of a DC motor) hitting the low side with the micro and cooking it. Will simply fitting diodes prevent this? If so... where does the energy go? Or do I need to use the right smoothing caps without diodes?

Would two separate circuits (one battery powered as above, and one mains) send a voltage into the micro frying it? Is that just a random thing about whether your house ground is a different potential vs the battery negative?

And in the fullness of time you will begin to understand that "ground" is a convenient fiction that is useful in a limited range of circumstances. [snip] For a different example consider why, if you are caught outside in a thunderstorm, you should keep your feet together and not apart.

I shall consider that at great length lol :)

Seriously though I'll look closer at what you said tomorrow. I hope what I said above lines up, or just about, at least.

... this post has become much longer than I wanted... sorry TLDR folks but I'm just musing about grounding. Probably most of what I typed was utter crap :)

Thanks!

~Atheus
 

Offline SingedFingers

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Re: Scope probes have no ground connector!
« Reply #36 on: March 28, 2017, 08:30:48 pm »
Might be a good project to build your own ESR meter. There's a project here: https://www.eevblog.com/forum/projects/5-transistor-esr-meter-design/
 

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Re: Scope probes have no ground connector!
« Reply #37 on: March 28, 2017, 09:10:02 pm »
Wow - warning heeded! I'll avoid even turning it on again until I replace those caps.

If there is cracking in the external plastic, moisture will have got in. I've seen it in a 2465 (30s after power up a resistor in series with one ceased to exist) and a 2465B where there was no problem (but I still replaced them). Make sure you get the right X/Y rating; that's more important than the exact capacitance.


Quote
Probably most of what I typed was utter crap :)

The unasked questions are the ones that get you in the end. As I taught my daughter, "let's make new mistakes".
There are lies, damned lies, statistics - and ADC/DAC specs.
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Online T3sl4co1l

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Re: Scope probes have no ground connector!
« Reply #38 on: March 29, 2017, 01:13:08 am »
On that note I'm still a little unclear about what happens if you have two different voltages in your device (such as a microcontroller switching high power via MOSFETs), obviously separate from each other in main circuit from PSU onwards, but which will have to meet at a common ground to work... is that right? They must meet, and only once at the end?

Yes, they must meet.  Though preferably, they should meet many times, a continuous number of times -- like on a ground plane. :)

Quote
Do I have to make sure both end up at 0V at the ground plane? That might sound dumb... most components will "use up" all voltage provided obviously and simply explode if the power is too much, but what about non-resistive components such as an LED, or an accidental short of high volts into ground? I just have to be careful not to do that I suppose?

They will.  Consider a constrained geometry problem:

Suppose you have a slot that's just wide enough to hold a rectangular bar.  The bar has two holes, one in each end.

Suppose there are two bars, in facing slots, so they can both slide independently, like sashes in a double-hung window frame.

A pin can be placed through the holes, when the holes in the bars line up with each other.  When this is done, they will not move independently.  The pair will move independently, as a set, but will not move relative to each other.

Finally, suppose you have a pair of dividers (no relation to the "voltage divider", this is a geometric tool that looks like a compass), marked with a scale.  One leg is labeled "+" and the other "-".  You can set the legs of the dividers to any distance, and measure that distance.  The tips fit particularly well into the holes in the bars...

Now.  If we pin the bars together (pick any pair of holes), the pair will slide independently.  Suppose we grab onto the "-" leg of the divider, to fix it in place, and measure distance to one of the holes.  This distance is meaningless, because -- what are we grabbing?  It has no relation to the position of the bars: they can slide anywhere we want, so there is no correct answer.  It's an arbitrary floating measurement!

Okay, so let's place the "-" leg in one of the holes.  Now we can, say, measure the length of one bar, from hole to hole.  No matter where each bar is positioned, we measure this consistently.

This is identical to measuring a supply's voltage, with a meter, and not caring what common mode (or absolute) voltage it has relative to its surroundings.  A 9V battery is still a 9V battery, whether it's sitting on the table, or one leg is grounded to Earth, or it's sat on top of a Tesla coil!

When we pin the bars together, we have three options:
- Bar 1 'finish' to bar 2 'start'.  The bars are in series, and the total length adds.
- Bar 1 'start' to bar 2 'start'.  The bars are in series, and the total length subtracts.  (Or finish to finish, which flips the sign.  If the total is zero, then it doesn't matter which hole is pinned: a degenerate case.  You can intuit that, if they are slightly mismatched, then any "slop" to the pin in the hole allows them to be connected in parallel (two pins, one each end) without too much trouble...)
- Bar 1 'start' to bar 2 'finish'.  They're backwards; we've flipped the polarity from the first case.  The total length adds, but it's going the opposite direction (if we label the holes, and keep the measurement +/- matched up accordingly, we have to flip the dividers so they read negative).

When the bars are in series (adding), we have three choices where to place the dividers: start, middle or finish.  If we put "-" on the start, then we measure two different positive distances at the remaining hole positions.  This is using the 'start' as common ground reference.

If we put "-" in the middle, we measure one positive and one negative distance.  This is using the 'middle' as common ground.  This is a bipolar supply.  (These are handy for circuits that need to drive symmetrical signals, like audio amplifiers.  If the distances aren't equal, we will probably call this an "unsymmetrical bipolar supply"; these are handy for some op-amp and logic circuits.)

If we put "-" on the finish, both are negative.  Same thing as before, just backwards.

About connecting things in parallel.  If the bars are very stiff, and the holes are very tight, it takes very little mismatch to make it impossible to pin them together.  The mechanical reason is that it will take a lot of force to pin them together.  Whereas if the setup is much more flexible, they can be connected with little force.

This is analogous to connecting power supplies in parallel, given differences in output voltage, and equivalent output resistance.  Two sources of different voltage will draw a current between them, and if they are very low resistance, the current can be very high indeed.

Now, speaking of current, let's also consider a load.  In this mechanical analogy, if we stretch a spring across one bar (hooking the ends into the holes), it carries some tension (current).  If we stretch it across the other bar, same again (proportional to length, that is).  Or if the bars are in series, it stretches even further, and carries even more tension.

That's all your circuit will do: you don't have to worry about it stacking up correctly, it will find its own balance.

Bad circuit designs can suffer from this, though, so be careful.  Example: using zener diodes in series to create a midpoint voltage.  Zener diodes exhibit a ~constant voltage drop, so if the total voltage imposed on a chain of zeners is more than the total, current skyrockets.  (The usual design approach is to use a series resistor, limiting current to a reasonable value.  Inefficient, but it works well.)

Quote
What about ground bounce? I assume with only one source and one ground (but different voltages used at different parts of the circuit - high side and low side I think is the term) you will still get everything 0V at the ground plane naturally when they meet right?

Bounce!  Ah, you're ahead of yourself!i

Now let's go through all that again, but instead of leaving the bars sitting there, we're tapping them with hammers, and instead of one hole at one end, there's a bunch of holes in a row, across the bar.  Give or take angle, the holes are all the same distance to the far end; but if we start tapping on the bar with a hammer, it vibrates, and each hole has a slightly different position during that vibration!

Clearly, if we built the bar out of a wire frame, the vibrations could be very large; and if we use solid metal, it's much reduced.  Alternately, we could use large chunks of metal, separated by wire frames, so that local areas are stiff, but allowed to flex with respect to other local areas.

In this way, we can see that ground plane under a circuit provides a stable (but not perfect) reference for all the signals in the circuit; that signals are only referenced to their local environments; and that, if we should connect local areas together, we must be mindful of the voltage drop that may exist between their grounds (which is called a common mode voltage).

Tim
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Offline SingedFingers

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Re: Scope probes have no ground connector!
« Reply #39 on: March 29, 2017, 08:49:02 am »
Great explanation. I could have done with that years ago.

My first high current accident, back in 1996 was due to a bad grounding issue. I built an audio amplifier. Not a small one. It was supposed to deliver 100W into 8 ohms from a 500va transformer based power supply (the power supply was a bad mistake as well but that's another story). The feedback loop in it was 100% uncompensated and ground referenced and it self destructed due to oscillation as I had wired the whole damn thing with bits of mains flex I had lying around. No ground plane. No PCB. It was an instant disaster. An attempt to rebuild the smouldering remains over a ground plane (FR4 sheet), add compensation, fuses and clue was successful and it was my amp for about 2 years until I got fed up of the persistent burning smell :D

I still stare at those Gainclone amps you see and cry slightly when I see the ground toplogies used.
« Last Edit: March 29, 2017, 08:52:02 am by SingedFingers »
 

Offline tautech

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Re: Scope probes have no ground connector!
« Reply #40 on: March 29, 2017, 09:24:25 am »
Back to the OP for a bit if we may.

I have seen repair techs immediately remove the reference leads from probes, instead they use a long mains Gnd lead with a croc clip that in some way connects directly to the scope chassis to connect to the gnd on the DUT.
Some scopes have a GND terminal that can be utilised for this direct probe reference connection.

Why do they do this ? Well this style of use has a few advantages for those that know scopes and testing procedures well.
1. No flailing reference lead to short anything on the DUT while accessing various test points.
2. Much faster diagnostic procedures without the need to continually shift the reference lead due to it not being long enough.
3. Once a safe signal reference point is found, no need to find another.
4. No chance of the reference lead becoming unattached during measurements.

Now all this implies the tech has good experience, knows that his probe reference connection is sub-optimal and waveforms are likely to be affected as a result BUT this is where experience counts as one mostly searches for indications of signal of approximately the correct amplitude and frequency.

Designers and developers use a much more precise approach.  ;)
Avid Rabid Hobbyist
 

Offline Atheus

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Re: Scope probes have no ground connector!
« Reply #41 on: March 29, 2017, 02:38:27 pm »
I can't quite see past the blue caps but there doesn't seem to be any damage.

How'd you get at the board like that? Did you have to unhook those little grey tabs? It sill looks like it would be blocked though.

Also it was turned on until recently and I don't want to touch the PSU. I'll leave the caps to discharge. Unless I can short them safely - how do I do this if it's possible?

Thanks.

~Atheus
 

Online Brumby

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Re: Scope probes have no ground connector!
« Reply #42 on: March 29, 2017, 04:18:11 pm »
Just drag out your meter and check each of the caps you are concerned about.  They may not have any voltage left on them worth worrying about.  Note: when checking caps in an unpowered circuit, always use DC voltage scales.  Without a source driving AC into the circuit, capacitors can, at best, maintain their last instantaneous voltage.

In circuit capacitor discharging rates vary from circuit to circuit.  Some will be drained within a second.  Others can take many hours.  The big ones you can find in microwave ovens can take days - and with the high voltages they carry at a respectable capacitance, represent a real and present danger... dramatic and lethal.  If you ever come across them, treat them with the greatest respect.
« Last Edit: March 29, 2017, 04:25:07 pm by Brumby »
 

Online denverpilot

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Re: Scope probes have no ground connector!
« Reply #43 on: March 29, 2017, 04:41:01 pm »
And these are available separately where?

He wants to _use_ the 60MHz scope, not sleep with it.
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Online Brumby

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Re: Scope probes have no ground connector!
« Reply #44 on: March 29, 2017, 04:49:09 pm »
To discharge a capacitor, you just bridge the terminals with a resistor.  Some people will just stick a screwdriver across the terminals.  For high value caps, this is not a great idea.  The current surge will be extremely high and has the potential to cause damage to the cap.  You would NOT do this with one of the aforementioned microwave oven caps.

If you are doing it a bit, you could make up a resistor with fly leads attached with, maybe alligator clips at each end.  If you want to get fancy, you could make provision for meter connection so you can verify the residual voltage.  Just make sure there is no exposed conductor as these things often end up just laying across circuit and chassis - and you don't want any unexpected shorts.

What sort of resistor? Well, it depends on the situation in which you are going to be using it.  For general discharge, I would go for something like a 100 ohm 5W.  The 5W rating is for situations where you might be discharging some high value caps in a power amplifier.  For example, I have an amp with two 8000uF 75V caps in the power supply.
 

Offline SingedFingers

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Re: Scope probes have no ground connector!
« Reply #45 on: March 29, 2017, 05:45:58 pm »
Yeah don't use a screwdriver. A friend of mine did this with a Sprague 36D in a printer that had keep skip dived for parts immediately after power up. The manufacturer had not bothered with a bleeder resistor.  The last 3mm of the screwdriver evaporated.

I'd go for a higher resistance myself. 1k, 2W. You can afford to wait a few seconds :)

Also beware. Just because you discharged them, don't think they won't have another go. They may hold a respectable voltage again a minute or so after you stop discharging them due to dielectric absorption.

Philips tend to be good citizens though. I don't think I've seen a design of theirs that doesn't power off to a safe state after a few seconds. Also they are usually friendly to bringing the entire unit up on a bench supply if you apply DC across the filter caps which is nice for debugging. No nasty mains hanging around. PM3217 and PM2524 at least.


EDIT: WARNING. I just looked at the SMM for the PM3055 and the primary caps have mains potential across them on the hot side. Can't see an obvious discharge path either. Be very careful.



EDIT 2: Also looking at some other explosions on the internet it is probably a good idea to replace C6007 and C6008 as well. Also need to be careful with C6002, 6003, 6004, 6006 as they are used as the line trigger pick off.
« Last Edit: March 29, 2017, 06:20:50 pm by SingedFingers »
 

Offline Atheus

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Re: Scope probes have no ground connector!
« Reply #46 on: March 30, 2017, 12:33:17 pm »
I've never shorted a cap of this value before... I do have two big fat 4.7Ohm 50W heat-sunk resistors which I was going to use as a load in a modded ATX PSU... That should be safe to discharge most things right? With some equally fat wires/probes soldered on?

I've already left it 24H unplugged and pushed the power button a few times during that period... would that do it?

I Think I am able to remove the bottom shell of the scope if I remove the handle, but that would involve shoving a pair of insulated long-nose pliers under the power board, so I wouldn't want to accidentally short two components with the pliers... I think I'll have a go tonight so I'll be back shortly!

~Atheus
 

Online Ian.M

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Re: Scope probes have no ground connector!
« Reply #47 on: March 30, 2017, 12:55:43 pm »
A 60W or 100W incandescent bulb makes a fairly good discharger.  Its a reasonable compromise between peak current and time constant, though you do need to check the filament is good before and after use.
 

Offline Atheus

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Re: Scope probes have no ground connector!
« Reply #48 on: March 30, 2017, 01:30:52 pm »
Okay removing the bottom cover does not provide useful access to anything. It's all blocked off with molded plastic. Really interested in how you took it apart SingedFingers!

Cheers,

~Atheus

/edit: Okay, with much lying at funny angles with a powerful LED lamp (I've not removed the board), I can see a cap marked 'X2' and it even has the little 'a' in a circle to the right of it... looks the same as yours :/

Just to check though - my scope is marked Fluke/Philips rather than just Philips, and I found a code 9444 (44th week of 1994?) on a sticker on the back, making me think this is a later model scope. Fluke bought Philips measuring equipment division in 1993 according to a bit of research. Is this a later year than yours? Is yours marked Fluke?

Also I see no damage AT ALL in that area (or anywhere in fact) and usually caps show some signs before blowing... right? Or is this not a type which shows it physically? It is clear the scope has not been exposed to moisture (mentioned as the main problem in other thread) or dust.

I am very reluctant to take it apart without knowing exactly what I'm doing. I'll probably break the little tabs regardless if I try - the old plastic is so brittle. Also knowing if/when these were made by Fluke would be nice too... If they had Fluke involvement they must have tested and modified it... unless this is a problem which only manifests after 20 years?

I need more information...

I am very tempted to just continue using the scope as it shows no damage and I've not even established it's fully working condition. I am yet to see the square wave produced by the cal tab clearly...

I think I will reassemble it and continue testing unless someone can provide some info on how to get that board out, or knows the years these were manufactured, by whom, and if/when it was revised.

/edit 2: I see one side and the top (side says rated 250V... is that the right one? It's a different angle for me to see the 'X2' on top and the the '250'...) and I don't see 'RIFA' anywhere on it... but that doesn't mean it's not there...

Also my caps are gold colored - notably the 1000pF right next to the X2 - the blown up one looks white. Also the same 1000pF is not as wide and is marked ONLY with the rating not the rest of the stuff on the burnt-out photo. It's much more like the one in the not-burned out photo. I dunno what this means in terms of manufacturing years or versions.

At the end of the day, I bought this for £50 as a punt, and I'm lucky it works at all! I want to use it. I'm going to reassemble and try to calibrate. I will not probe any high (or even high-ish) voltage points until I am more sure about the thing, though I'm not sure if this will help as the problem is on the AC side of the PSU.
« Last Edit: March 30, 2017, 03:18:05 pm by Atheus »
 

Offline SingedFingers

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Re: Scope probes have no ground connector!
« Reply #49 on: March 30, 2017, 05:50:08 pm »
The gold ones are the ones you want to replace. They will be marked with RIFA on the side.

Check the SMM on how to remove the power supply: http://www.qsl.net/vk5bar/AHARS-Resources/Philips%20PM3055/PM3055%20Service%20Manual.pdf

Section 12-1.
 

Offline Atheus

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Re: Scope probes have no ground connector!
« Reply #50 on: March 30, 2017, 09:37:11 pm »
Are they metal poly or some other kind of film (which I'd have to buy)? I suppose I should replace like-for-like rather than using electrolytics and/or ceramics (which I have in large quantities already)?
 

Offline timb

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Re: Scope probes have no ground connector!
« Reply #51 on: March 30, 2017, 10:11:38 pm »
Are they metal poly or some other kind of film (which I'd have to buy)? I suppose I should replace like-for-like rather than using electrolytics and/or ceramics (which I have in large quantities already)?

Replace it *only* with an X2 rated plastic film (metalized and non-metalized polyester/PPS are common).

Digi-Key, Mouser, e14, etc. will have what you need.

It absolutely cannot be an electrolytic. These caps go across the mains and are non-polarized, whereas most AE caps aren't rated for that sort of voltage and *are* polarized.

Ceramic caps *can* be rated for mains voltage and are non-polarized, however they're generally special purpose, so I doubt you'd have one laying around. (Your garden variety ceramic is rated for 50V.) Even if you had one with the appropriate voltage rating, it still wouldn't be appropriate to replace a film cap with.
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Online Brumby

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Re: Scope probes have no ground connector!
« Reply #52 on: March 30, 2017, 10:16:45 pm »
Are they metal poly or some other kind of film (which I'd have to buy)? I suppose I should replace like-for-like rather than using electrolytics and/or ceramics (which I have in large quantities already)?

Replace it *only* with an X2 rated plastic film (metalized and non-metalized polyester/PPS are common).


Do not even consider going any other way than this.  There are a couple of reasons - which we can go into - but for now, just trust us.
« Last Edit: March 30, 2017, 10:18:34 pm by Brumby »
 

Offline SingedFingers

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Re: Scope probes have no ground connector!
« Reply #53 on: March 30, 2017, 11:47:01 pm »
+1 for the above. In fact, more than +1!

For ref, I usually buy the X2 replacements here in the UK:

https://www.bitsbox.co.uk/index.php?main_page=index&cPath=65_81&zenid=jdn7p07fkqqkhaf5aqm4kngki0

They're made by Tai Yao. All approvals. Been using them for about 5 years with no problems and there isn't a silly minimum order quantity. RS for example make you buy 5 at once which is no good for one repair job.

If you have to replace the electrolytics, don't buy them from the above site. Get some decent ones from RS made by Rubycon or Vishay or some non shoddy crap pusher. I like the Vishay Sprague or Vishay BC units - the latter are descended from the electrolytic caps already in it :)
« Last Edit: March 30, 2017, 11:48:33 pm by SingedFingers »
 

Offline Atheus

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Re: Scope probes have no ground connector!
« Reply #54 on: March 31, 2017, 01:46:38 pm »
Thanks all! Point taken! Buying parts... will probably buy something else expensive while I'm at it, 'to save on shipping' or some other made up reason :)

It does seem to have been fine for 20+ years though. I'll probably use it before I replace these. I'm not familiar with the type of solder, the specific heat capacity of surrounding board/components etc, and the plastic I have to bend is old... I'm afraid I might do more harm than good :/

+1 for the above. In fact, more than +1!

For ref, I usually buy the X2 replacements here in the UK:

https://www.bitsbox.co.uk/index.php?main_page=index&cPath=65_81&zenid=jdn7p07fkqqkhaf5aqm4kngki0

They're made by Tai Yao. All approvals. Been using them for about 5 years with no problems and there isn't a silly minimum order quantity. RS for example make you buy 5 at once which is no good for one repair job.

If you have to replace the electrolytics, don't buy them from the above site. Get some decent ones from RS made by Rubycon or Vishay or some non shoddy crap pusher. I like the Vishay Sprague or Vishay BC units - the latter are descended from the electrolytic caps already in it :)

These are the exact parts I think I need (from bitsbox; they are already my go-to for small quantities):

https://www.bitsbox.co.uk/index.php?main_page=product_info&cPath=65_81&products_id=2312&zenid=jdn7p07fkqqkhaf5aqm4kngki0
https://www.bitsbox.co.uk/index.php?main_page=product_info&cPath=65_81&products_id=3179&zenid=jdn7p07fkqqkhaf5aqm4kngki0 (x2)

Those repair kits do not contain the 1000pF/0.1nF values required. Am I correct in my order? There's no problem with those 1000pF units being 'y2' class caps? I understand this is just a safety rating and does not make them unsuitable (more so if anything)? I'll double/triple buy of course for spares.

I'll order soon so someone please shout if I've got it wrong!!! Thanks again.

~Atheus

Oh BTW, Re: "If you have to replace the electrolytics, don't buy them from the above site. Get some decent ones from RS made by Rubycon or Vishay or some non shoddy crap pusher."

Bitsbox do stock Nichion and Panasonic. I buy these when available, but they're only in a few common values on that particular site. I'll go elsewhere if needed. Anyway caps for the scope are ordered. Time for a new thread for the repair I think.

~A
« Last Edit: March 31, 2017, 04:31:32 pm by Atheus »
 

Offline SingedFingers

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Re: Scope probes have no ground connector!
« Reply #55 on: March 31, 2017, 05:20:44 pm »
Class X capacitors go across the line. Class Y capacitors go from the line to ground.

Don't substitute. The failure modes are different. Y2 has higher insulation breakdown voltages as well I understand to reduce risk of exposure.

The thing is you want the class X capacitors to pop the dielectric if there is a line transient. They will absorb the transient and heal; that's the point. You don't want the Y class ones doing that.
« Last Edit: March 31, 2017, 05:23:56 pm by SingedFingers »
 

Online Brumby

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Re: Scope probes have no ground connector!
« Reply #56 on: March 31, 2017, 05:48:13 pm »
Yes.  Looking at each as a basic capacitor, there's not much difference between X rated, Y rated and regular capacitors.

Where there is a HUGE difference is how they behave under failure conditions.


Imagine what a capacitor across the live and neutral would do if it were to fail as a short.  X2 rated caps are designed not to do that.
 

Offline Atheus

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Re: Scope probes have no ground connector!
« Reply #57 on: March 31, 2017, 06:07:25 pm »
Class X capacitors go across the line. Class Y capacitors go from the line to ground.

You mean X in parallel and Y in series? I've not looked at the traces on the power board closely enough to know...

Don't substitute. The failure modes are different. Y2 has higher insulation breakdown voltages as well I understand to reduce risk of exposure.

The thing is you want the class X capacitors to pop the dielectric if there is a line transient. They will absorb the transient and heal; that's the point. You don't want the Y class ones doing that.

Sounds like it's most important to get the X cap right then? Since it's behaviour is so different than standard? Is 'Y' essentially a 'normal' cap?

I just ordered 3 * 0.22uF X2 metal poly and 5 * 1nf/1000pF Y2 metal poly. Yes I only need 1 and 2 respectively but I always overbuy...

So I will replace the big X2 with an X2, but on my scope (and in the photo you posted), the 1000pf is not marked X2. Nor are they marked RIFA as far as I can see. Just the Farad rating. I would be replacing these with the 1000pF 'Y' caps. At Bitsbox the 1000pf value was only available in 'Y' or non-specific ceramic of various kinds. See the links in previous post for what exactly I bought.

Did I screw up? Do I need to find X2 caps at 1000pF? :/ - note again I cannot see any 'X2' on the 1000pF(s) - only the big 0.22uF.

Parts arrive tomorrow. Someone please tell me if I got the wrong bits so I can kick myself. Cheers!

~Atheus

P.S. I am still reluctant to do this due to my soldering experience... I may sound like I know what I'm doing (the best way to succeed as an engineer I find!) but I've been in software for 10+ years and have hardly touched a soldering iron since university (apart from the last few weeks of course). I'll be putting a kit together this weekend though (one of those cheap Chinese ESR meters which are apparently not too bad) so I think if that works first time (well... say third time lol...) I'll have a go at the scope. Might practice some desoldering too... I really do not want to f**k this up and break an instrument I'm damn lucky worked in the first place!

/edit: looking again, I don't think my 0.22uF X2 is mains voltage rated. The 1000pF Y2 is (250V). This might be okay since Y2 goes power -> ground and X2 goes across in parallel... or I might have completely misread the caps and ordered the wrong parts. Yay.

One thing is for sure - there is no 1000pF/1nF X2 for sale on the Bitsbox site (though I see it on others so know it exists) so it can't be too common. I hope I did the right thing!
« Last Edit: March 31, 2017, 06:51:56 pm by Atheus »
 

Offline SingedFingers

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Re: Scope probes have no ground connector!
« Reply #58 on: March 31, 2017, 08:23:26 pm »
This is the best drawing I can find that explains X vs Y class capacitors:



If you have only one RIFA capacitor, you just need to replace that single one, none of the others. They are probably 100% fine. It's only the RIFA ones I've ever heard of exploding.

Y2 caps aren't normal caps. They have a massive insulation breakdown resistance. Their objective is NOT to conduct if there is a mains transient so you'll see them good for 5kV impulse. We want an X2 to break down and short the line out before those conduct as ideally we want the current to stay on the line and not hit the equipment.

The capacitor is C6002 and is marked as a 275v 20% 1NF capacitor, X2 rated according to the manual. That's the one to replace. Don't stick those Y2 ones in there. Keep them around though - what you have bought will be useful in the future. I'm always replacing these damn things.

That 220nF will probably be 100% fine in there assuming it is rated at 275v+. It has two purposes, firstly to shoot down any transients which is independent of capacitance value. Secondly to eat any high frequency noise, which is dependent on capacitance value. The power supply switches at 43KHz according to the service manual so this will eat up more of that than the 1nF one would as the reactance at that frequency is lower. In the MHz+ reason where scopes get tetchy, it'll kill more of that.

The low value ones aren't really sold any more in large quantities because in the 1980s a 200nF one was quite large and expensive. Dielectrics have improved as have fabrication methods so it's cheaper to knock out larger ones now.

Wait for someone else to confirm this as well. Always get things reviewed.

As for soldering, don't worry about it. These ones are usually pretty easy to get out. If you can, snip the leads off a part you are replacing first from the component side, then use braid or a vacuum pump to get the gunk out of the hole. Then it's like building a kit when you put the new part in :)
« Last Edit: March 31, 2017, 08:28:09 pm by SingedFingers »
 

Offline Atheus

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Re: Scope probes have no ground connector!
« Reply #59 on: April 01, 2017, 03:19:32 pm »
I'm not seeing how I can remove the PSU from the scope... see below image:



See that overcomplicated tab thingy holding it in? There's one at each end. The manual simply says 'push [those] "sidewards" and gently lift the unit out... well which bloody way is sidewards??? I've tried the obvious - bending the little plastic clip things (you know what I mean) to free the board - but a) there doesn't seem to be enough room to clear the board properly and b) even if I got it that far there is a little plastic bar (the thin one directly above the clip) on each side preventing it's removal.

This thread https://www.eevblog.com/forum/repair/philips-pm3055-repair-and-teardown/ makes it sound much more complicated. The OP says he had to access clips on the underside of it but I can't get to them under the bottom cover - there is a non removable plastic cover under it - and certainly not from the top. One of his photos seems to show it's possible to remove the clip things completely! I have no idea how.

Only once I get to board out can I be 100% certain about caps.

J.R.
 

Offline tautech

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Re: Scope probes have no ground connector!
« Reply #60 on: April 01, 2017, 03:46:54 pm »
Use a screwdriver to bend the tab towards the top side of the PCB. Once the PCB has slid out just a little the tabs will slide on the top face while the PCB is removed.
That's how it looks to me.
Avid Rabid Hobbyist
 

Offline Atheus

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Re: Scope probes have no ground connector!
« Reply #61 on: April 01, 2017, 04:31:22 pm »
Use a screwdriver to bend the tab towards the top side of the PCB. Once the PCB has slid out just a little the tabs will slide on the top face while the PCB is removed.
That's how it looks to me.

I'll try those tabs again but it'll be the tenth time :/

Those things go all the way down to the bottom, where there s a small access port:



Yes I've obviously had a go at it ineptly with a screwdriver... although there is enough cosmetic damage on this thing for me not to care. You see the thing at the top of the circled area? The only thing that isn't grey plastic? That's the board. It prevents me pushing the tab 'down' (into the pic z axis) which would free the whole clip. The service manual insists however that removing the bottom cover is not required for servicing...

... One last go at the top tabs and I'll call it a day.

Thanks all,

~Atheus

/edit: Aha! Now I see why the other guy needed cable ties! There are two holes in the board where you can put something to give some purchase - without crushing the PCB with pliers or grabbing by a component and ripping it off. This is really the only effective tip I've found so far.

Without touching the bottom tabs at all (as the service manual says) pull the tabs back and yank the board up with the cable tie. You will think this is not going to work, but it will, it just requires a lot of force to pull the tabs back while bending the board with pliers to get the clearance, and then a huge amount of force to pull the board out. Not the kind of force I would ever think of putting on a bit of electronics kit. More like what is requires to remove a 6" nail from a roof timber with a claw hammer. Seriously. I have now got one side loose and have broken two cable ties (!!!) (without a hint of success I might add) on the other side... there MUST be some other fixing! If I stand up and REALLY pull it I'll rip the PCB in half or something.

:/ ~A
« Last Edit: April 01, 2017, 05:09:15 pm by Atheus »
 

Offline danadak

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Re: How on earth do I get the PSU out of this old scope??? (retitled)
« Reply #62 on: April 01, 2017, 10:14:42 pm »
500 ohm scope probes do have utility -


https://www.eevblog.com/forum/testgear/fifty-ohm-probes/


Selection guide showing 500 ohm probe, Cin < .15 pF.


http://www.tek.com/document/selection-guide/probe-specifications



Regards, Dana.

Love Cypress PSOC, ATTiny, Bit Slice, OpAmps, Oscilloscopes, and Analog Gurus like Pease, Miller, Widlar, Dobkin, obsessed with being an engineer
 

Online rstofer

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Re: How on earth do I get the PSU out of this old scope??? (retitled)
« Reply #63 on: April 02, 2017, 06:38:55 am »
One thing I always think about:  Somebody put <this> together and they didn't destroy the <whatever> doing it.  I'm smarter than they are so there's no excuse for my not being able to disassemble it.  Arrogant, for sure, but the thought has served me well over the years.

But I might look for a screw down low.  Maybe more than one...

 

Offline SingedFingers

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Re: How on earth do I get the PSU out of this old scope??? (retitled)
« Reply #64 on: April 02, 2017, 06:54:13 am »
Indeed.

It's like a Rubik's cube, or Pandora's box.

Try disassembling and reassembling a PM2524; that'll have you in tears :D
 


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