Author Topic: How on earth do I get the PSU out of this old scope??? (retitled)  (Read 10998 times)

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Offline 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 »
 

Offline 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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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 :(
 

Offline 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/
 

Online tggzzz

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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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Offline 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 »
 

Online 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.  ;)
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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
 

Offline 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 »
 

Offline 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.
Funniest thing I've read all day. Thanks for the hearty laugh, sir. Kudos.
 

Offline 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
 

Offline 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.
 


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