How on earth do I get the PSU out of this old scope??? (retitled)

[Atheus]

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)?

[timb]

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.

[Brumby]

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.

[SingedFingers]

+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

[Atheus]

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

[SingedFingers]

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.

[Brumby]

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.

[Atheus]

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!

[SingedFingers]

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

[Atheus]

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.

[tautech]

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.

[Atheus]

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

[danadak]

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.

[rstofer]

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...

[SingedFingers]

Indeed.

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

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