Leaky Electolytics in Philips 'scope

[Cursitor Doom]

On Mon, 23 Mar 2015 13:39:08 -0400, JC wrote:

The power board is very easy to remove, a couple of screws and plugs and
it pulls out the rear of the scope. I suggest you get the service
manual. Philips power supplies all have dried out axial caps and won't
start up, you wont get anywhere without replacing them. That's usually
all you need to do, they are well built apart from that.

Thanks for that. I've now got it out and it's evident it's been repaired
before. I'll try to post some pictures later on. Actually all the axial
electrolytics have tested fine, surprisingly. I have spotted one small,
old-style polyester cap that's cracked but aside from that, nothing
obvious apart from 'historical charring' which someone has cleaned up and
replaced a couple of diodes.
At least now I can get at it, power it up and make some voltage checks!
Does anyone know if it needs to see a load to produce an output? Being of
a certain age, I'm more of a linear psu person...

 

[Mike Tomlinson]

En el artículo <memlgt$tl2$3@dont-email.me>, Cursitor Doom
<curd@notformail.com> escribió:

I've got one of the PSU covers off now and have identified a suspect
component. It's some sort of capacitor with the following markings:

250V, 0.22uF@X fo=3,00Mhz

and it has a fine web of cracks over its encapsulation. I had one of
these go short-circuit once before in a microscope power supply and that
was the cause of it blowing fuses too.
I don't understand the markings on the component, though. IIRC, "fo"
refers to the self-resonant frequency and I don't see what purpose there
would be in that at 50hz mains where it's located in that part of the
device pre-PSU!

It's a class X mains filter capacitor. The appearance you describe
(mesh of cracks) is common when they fail. Usually they go with a bang
and a lot of smoke.

It will be ok to test the oscilloscope with out it in circuit to see if
it is the cause of the blowing fuses, but you should replace it
eventually. Note again this is class X, and should only be replaced
with another class X.

--
:: je suis Charlie :: yo soy Charlie :: ik ben Charlie ::

 

[Cursitor Doom]

On Mon, 23 Mar 2015 19:46:56 +0000, Arfa Daily wrote:

However, if the line input filter has definitely failed - which it
sounds like is the case - then there is unlikely to be any further
problems downstream of that filter, and I would just go ahead, bite the
bullet, and whack the volts up.

Thanks, Arfa. Well I snipped the filter completely out of circuit and
checked it and it clocked at 0.9ohms so for sure something fused inside.
It's rated at 250V/1A and I just happen to have a new spare one which is
unusually fortunate. Now the psu board is out of circuit I feel a bit
happier about zapping it with full volts. I'll give it a whirl in the
morning as it's getting late here now.
cheers, cd.

 

[JC]

On 3/23/2015 9:47 AM, Cursitor Doom wrote:

Have now done this and it isn't blowing fuses any more. However, I'm up
to 100V on the variac and no sign of life from the scope yet (230V main
here). Unfortunately the PSU section is sandwiched in the middle of other
boards, so probing for voltages within it is not possible without removal
(which looks a right PITA). I'll have to have a good think about how to
proceed further.

The power board is very easy to remove, a couple of screws and plugs and
it pulls out the rear of the scope. I suggest you get the service
manual. Philips power supplies all have dried out axial caps and won't
start up, you wont get anywhere without replacing them. That's usually
all you need to do, they are well built apart from that.

 

[DaveC]

Have now done this and it isn't blowing fuses any more. However, I'm up
to 100V on the variac and no sign of life from the scope yet (230V main
here). Unfortunately the PSU section is sandwiched in the middle of other
boards, so probing for voltages within it is not possible without removal
(which looks a right PITA). I'll have to have a good think about how to
proceed further.

How is 115/230 volts mains selected? Is it in the line filter/fuse/IEC
connector housing? Or...?

Of course when bypassing the line filter you're inserting your mains voltage
to the proper transformer taps...?

 

[DaveC]

How is 115/230 volts mains selected? Is it in the line filter/fuse/IEC
connector housing? Or...?

Switch on the rear panel (yes, it's in the right pos.)

Of course when bypassing the line filter you're inserting your mains
voltage to the proper transformer taps...?

It's a SMPS!

If it's a SMPS (with presumably universal input voltage), what's the switch
for?

 

[Cursitor Doom]

On Mon, 23 Mar 2015 13:40:47 -0700, DaveC wrote:

And so I repeat (to Cursitor): when you are bypassing the input filter
be sure your mains voltage is being applied to the proper input to the
SMPS.

Dave

That makes a lot more sense now, Dave, thanks.

 

[Arfa Daily]

Have now done this and it isn't blowing fuses any more. However, I'm up
to 100V on the variac and no sign of life from the scope yet (230V main
here). Unfortunately the PSU section is sandwiched in the middle of other
boards, so probing for voltages within it is not possible without removal
(which looks a right PITA). I'll have to have a good think about how to
proceed further.

If it hasn't gone bang with 100 volts on it, then it's not going to. SMPSs
are very ill behaved in that respect. If it's got a front end problem, then
it's got it at 20 volts, let alone 100. If it goes beyond 20 volts, then
it's going to do one of three things. It's either going to burst into life
when the input volts are closer to normal, or do nothing at all no matter
what you do, or not start up until it has full input volts applied straight
off. It depends on the configuration of the control IC. Usually, they have
what's called an under volts lockout (UVL) feature on them. This prevents
the control IC from trying to start the PSU if the input volts are below a
certain threshold. Sometimes, if the volts are pushed progressively beyond
the threshold value by continuing to turn up the variac, the IC will
'recover' and start. However, some ICs never recover in this scenario, and
the only way out is to take all the volts back off, and then just go for it
with full line voltage. If it were not to start by either of those methods,
then there is a fault in the control circuit.

That is not uncommon with switchers, and is usually due to either a faulty
startup resistor, or a high ESR buffer cap on the supply volts pin to the
chip.

However, if the line input filter has definitely failed - which it sounds
like is the case - then there is unlikely to be any further problems
downstream of that filter, and I would just go ahead, bite the bullet, and
whack the volts up.

I didn't have any trouble finding the schematic online, but if you are
having trouble, mail me off-group at the address used to post this, and I'll
send it on to you.

Arfa

 

[Arfa Daily]

"DaveC" <invalid@invalid.net> wrote in message
news:0001HW.D135A3180B4A7865B04179BF@news.eternal-september.org...

How is 115/230 volts mains selected? Is it in the line filter/fuse/IEC
connector housing? Or...?

Switch on the rear panel (yes, it's in the right pos.)

Of course when bypassing the line filter you're inserting your mains
voltage to the proper transformer taps...?

It's a SMPS!

If it's a SMPS (with presumably universal input voltage), what's the
switch
for?

It's not 'universal' as many older switchers weren't.

Arfa

 

[DaveC]

If it's a SMPS (with presumably universal input voltage), what's the
switch
for?

It's not 'universal' as many older switchers weren't.

Arfa

And so I repeat (to Cursitor): when you are bypassing the input filter be
sure your mains voltage is being applied to the proper input to the SMPS.

Dave

 

[DaveC]

> If you look at the schematic,

Are these available on-line?

it only has one two-wire input for the mains
to be applied to. Whether that input is expecting 110 v or 230 v is a
function of which way the voltage select switch in the bridge circuit is set
Arfa

If the PS can accept 115 or 230 and if there's only 2 wires for mains input
to the PS, then it *is* universal input.

If there's a switch, then I can't imagine only 2 wires input to the PS (the
filter/switch can't drop half the mains without a transformer)...?

Admittedly confused,
Dave

 

[Arfa Daily]

"DaveC" <invalid@invalid.net> wrote in message
news:0001HW.D135C85F0B533505B01029BF@news.eternal-september.org...

If it's a SMPS (with presumably universal input voltage), what's the
switch
for?

It's not 'universal' as many older switchers weren't.

Arfa

And so I repeat (to Cursitor): when you are bypassing the input filter be
sure your mains voltage is being applied to the proper input to the SMPS.

Dave

If you look at the schematic, it only has one two-wire input for the mains
to be applied to. Whether that input is expecting 110 v or 230 v is a
function of which way the voltage select switch in the bridge circuit is set

Arfa

 

[Cursitor Doom]

On Tue, 24 Mar 2015 08:11:30 +0000, Gareth Magennis wrote:

"DaveC" wrote in message
news:0001HW.D1362C590B6AA366B01029BF@news.eternal-september.org...

If you look at the schematic,

Are these available on-line?

it only has one two-wire input for the mains to be applied to. Whether
that input is expecting 110 v or 230 v is a function of which way the
voltage select switch in the bridge circuit is set Arfa

If the PS can accept 115 or 230 and if there's only 2 wires for mains
input to the PS, then it *is* universal input.

If there's a switch, then I can't imagine only 2 wires input to the PS
(the filter/switch can't drop half the mains without a transformer)...?

Admittedly confused,
Dave

There are *two* mains input sockets on the board - in addition to the
rear panel switch. I plan to post a pic of the board shortly, then all
will become clear hopefully.

 

[Phil Allison]

Cursitor Doom wrote:
Phil Allison wrote:

Interesting, but it still doesn't answer what the relevance of the
"fo=3Mhz" parameter is.


** fo is the *series* resonant frequency of the cap.

Such caps across the AC supply act as an EMI filter so their high
frequency characteristics are important.

Above 3MHz, the impedance of that cap will begin rising and its
effectiveness lost. A lower value cap will have a higher resonant
frequency.

Allow about 15nH inductance and you will easily see how he game works.



Thanks for that. I'll try simulating it later. And this is for what
purpose exactly? To prevent noise on the mains from other appliances
getting into the scope, or to prevent noise from the scope getting into
the mains?

** Seeing as the unit has a SMPS - it's the latter.




.... Phil

 

[Cursitor Doom]

Changed the subject to reflect the progress made over the last day or
two. I've now uploaded pics of the PSU board. You can see where it's been
repaired before and evidence of a previous burn-out. It looks a little
tired in places overall. Check it out:

https://www.flickr.com/photos/128859641@N02/

 

[Gareth Magennis]

"DaveC" wrote in message
news:0001HW.D1362C590B6AA366B01029BF@news.eternal-september.org...

> If you look at the schematic,

Are these available on-line?

it only has one two-wire input for the mains
to be applied to. Whether that input is expecting 110 v or 230 v is a
function of which way the voltage select switch in the bridge circuit is
set
Arfa

If the PS can accept 115 or 230 and if there's only 2 wires for mains input
to the PS, then it *is* universal input.

If there's a switch, then I can't imagine only 2 wires input to the PS (the
filter/switch can't drop half the mains without a transformer)...?

Admittedly confused,
Dave





I believe some are converted by how the initial diodes and capacitors are
switched/connected to rectify the mains.
For 110v operation, they incorporate a voltage doubler, 240v not so.



Gareth.

 

[Arfa Daily]

"DaveC" <invalid@invalid.net> wrote in message
news:0001HW.D1362C590B6AA366B01029BF@news.eternal-september.org...

If you look at the schematic,

Are these available on-line?

it only has one two-wire input for the mains
to be applied to. Whether that input is expecting 110 v or 230 v is a
function of which way the voltage select switch in the bridge circuit is
set
Arfa

If the PS can accept 115 or 230 and if there's only 2 wires for mains
input
to the PS, then it *is* universal input.

No it isn't. The term "universal" when applied to a SMPS is very specific in
that it applies to designs that by whatever method - usually by having a PFC
front end in better designs - can work with any input voltage from anywhere
in the world without requiring any kind of user intervention.

If there's a switch, then I can't imagine only 2 wires input to the PS
(the
filter/switch can't drop half the mains without a transformer)...?

Admittedly confused,

You are misunderstanding how this front end topology works. The high mains
voltage in Europe compared to the low voltage in the U.S. - or even lower in
Japan - is not reduced by the input circuitry. Rather, those low voltages
are doubled to bring the resulting output from the bridge up to the same
ballpark as is achieved when European mains is applied. I would suggest that
you have a read of the second heading down - "Voltage Doubler and Tripler"
in this Wiki

http://en.wikipedia.org/wiki/Voltage_multiplier

which details exactly how the front end of SMPSs like the one in the Philips
scope, achieve this end with the aid of a single pole voltage select switch

Arfa

Dave

 

[DaveC]

which details exactly how the front end of SMPSs like the one in the Philips
scope, achieve this end with the aid of a single pole voltage select switch

Arfa

Ah, so the Philips' ps has a voltage doubler at its front end controlled by
the "mains switch". The input to the ps *proper* is the same--240--fed by
either the 240 mains directly or the 120 mains, doubled.

Thanks,
Dave

 

[Ian Field]

"Ralph Mowery" <rmowery28146@earthlink.net> wrote in message
news:nNydndhe9vZroJLInZ2dnUU7-XudnZ2d@earthlink.com...

"mike" <ham789@netzero.net> wrote in message
news:men52i$925$1@dont-email.me...
T-rated fuses. In fact I was running off a Variac when the replacement
fuse blew at only about 80V!

Simple experiment.
80V x 2A = 160W.
run it at 75V and see what melts.

When fuses are blowing, that is not a good idea. You often melt the
transformer or something else in adition to what blows.

Fuses are often rated at somewhat above the normal current draw. With a
very bad overload , they will blow quickly enough, but at just a slight
overload, they will not blow and often the transformer or other components
will also overheat and go bad.

Its also a pretty good way to fry a variac - they're not as robust as
regular fixed mains transformers.

 

[Ian Field]

"Cursitor Doom" <curd@notformail.com> wrote in message
news:menm3p$4rf$6@dont-email.me...

On Sun, 22 Mar 2015 16:43:12 -0700, mike wrote:

Had a laptop I couldn't fix because the power supply wouldn't stay up.
had to put a DC supply downstream of the inductors and stuffed an amp
into the board. Took a millivoltmeter to trace the short to a SMT cap
under a heat sink. Was really shorted, didn't get warm at all.

Interesting point there I'd not considered before: if something's
*completely* shorted-out, then it won't get warm!

An old service engineer's trick is to bridge the fuse with a 60W light bulb,
then you can check voltages into the mains front end and get some idea
what's holding things up.

Its worth remembering the lightbulb filament has a PTC characteristic, so
there will be some inrush surge - calculating the value for an equivalent
power resistor would avoid that.