Hi to everyone, since it's the first time I write in this NG

I've little experience in switching PSUs ...
I've one of them (coming from an Amiga 2000) which shuts off at the first
load.
With no load at all, it delivered 5.9V out of 5 nominal.
I reduced it (with the trimmer) to about 5 with no load.
Inserting a little dummy load (47 Ohm) now it doesn't shut anymore, but
with more serios loads (e.g. an HD), it shuts again.
I checked in-circuit capacitors for shorts and ESR increase, tested
diodes and transistors.
It seems an aging problem (the trimmers were still sealed, so no
tampering), but who's bad ?

supervinx wrote in message
...
Hi to everyone, since it's the first time I write in this NG

I've little experience in switching PSUs ...
I've one of them (coming from an Amiga 2000) which shuts off at the first
load.
With no load at all, it delivered 5.9V out of 5 nominal.
I reduced it (with the trimmer) to about 5 with no load.
Inserting a little dummy load (47 Ohm) now it doesn't shut anymore, but
with more serios loads (e.g. an HD), it shuts again.
I checked in-circuit capacitors for shorts and ESR increase, tested
diodes and transistors.
It seems an aging problem (the trimmers were still sealed, so no
tampering), but who's bad ?


As it happens I will try fathoming a dedicated SMPS today that as it stands
is in tick-tick mode, 1 ultrasonic cycle per second.
Putting a dropped LED on each output shows life on all rails.
Plan of campaign will be
Run up on variac to find lowest point and will still run of sorts, tick-tick
Put a x10 larger timing capacitor over the one on the 555 that feeds the
UC3842 PWM controller. Then make up an exerciser to feed into the ,isolated,
LED of the optoisolator, over-riding the error feed. This will be a
monostable of variable period from 1mS to a second or so with push switch,
hopefully to elicit which area is playing up by gradually ramping up .
All active, big Rs seem ok and caps ESR-check ok. I suspect false error
signal.
Anyone else any ideas/advice on a generic plan of action for these sorts of
situations.?

On 2010-03-30 10:01:16 +0200, N_Cook said:

supervinx wrote in message
...
Hi to everyone, since it's the first time I write in this NG

I've little experience in switching PSUs ...
I've one of them (coming from an Amiga 2000) which shuts off at the first
load.
With no load at all, it delivered 5.9V out of 5 nominal.
I reduced it (with the trimmer) to about 5 with no load.
Inserting a little dummy load (47 Ohm) now it doesn't shut anymore, but
with more serios loads (e.g. an HD), it shuts again.
I checked in-circuit capacitors for shorts and ESR increase, tested
diodes and transistors.
It seems an aging problem (the trimmers were still sealed, so no
tampering), but who's bad ?


As it happens I will try fathoming a dedicated SMPS today that as it stands
is in tick-tick mode, 1 ultrasonic cycle per second.
Putting a dropped LED on each output shows life on all rails.
Plan of campaign will be
Run up on variac to find lowest point and will still run of sorts, tick-tick
Put a x10 larger timing capacitor over the one on the 555 that feeds the
UC3842 PWM controller. Then make up an exerciser to feed into the ,isolated,
LED of the optoisolator, over-riding the error feed. This will be a
monostable of variable period from 1mS to a second or so with push switch,
hopefully to elicit which area is playing up by gradually ramping up .
All active, big Rs seem ok and caps ESR-check ok. I suspect false error
signal.
Anyone else any ideas/advice on a generic plan of action for these sorts of
situations.?

you checked the esr and caps but I would desoldier the main 400v cap
and test its capacity and esr out of the box. compare with a new one.
also as you have a uc3842, I would do the same with the small (10uF or
so) cap that is on the + and - pins of this uc. often this is the
problem.
regards,

--

Jean-Yves.

"supervinx"


I've little experience in switching PSUs ...
I've one of them (coming from an Amiga 2000) which shuts off at the first
load.
With no load at all, it delivered 5.9V out of 5 nominal.

** Many PC type switching PSUs need a load to be applied on ALL the outputs
on order to work properly.

Why not apply load resistances ( not incandescent lamps either) to each DC
rail and see if it holds up then ??

Thing is, when you load just the 5 volt rail, the other DC rails go UP in
voltage - and that may trigger the out of range voltage detector circuit
and stop the whole damn show.



..... Phil

Il Tue, 30 Mar 2010 21:14:04 +1100, Phil Allison ha scritto:

"supervinx"


I've little experience in switching PSUs ... I've one of them (coming
from an Amiga 2000) which shuts off at the first load.
With no load at all, it delivered 5.9V out of 5 nominal.

** Many PC type switching PSUs need a load to be applied on ALL the
outputs on order to work properly.

Why not apply load resistances ( not incandescent lamps either) to each
DC rail and see if it holds up then ??

Thing is, when you load just the 5 volt rail, the other DC rails go UP
in voltage - and that may trigger the out of range voltage detector
circuit and stop the whole damn show.



.... Phil

The PSU shuts off under normal operations while connected to his MB. I
connected it to an external HD (load on both +5 and +12): I see the
voltage raising and the voltage detector shutting the PSU down.
I have no oscilloscope handy ... with a digital multimeter I can see the
+5 rising about 4.8, the +12 about 9 and then stop.

The main capacitors are 200V rated, but I see 215 across them

"N_Cook" wrote in
:

supervinx wrote in message
...
Hi to everyone, since it's the first time I write in this NG

I've little experience in switching PSUs ...
I've one of them (coming from an Amiga 2000) which shuts off at the
first load.
With no load at all, it delivered 5.9V out of 5 nominal.
I reduced it (with the trimmer) to about 5 with no load.
Inserting a little dummy load (47 Ohm) now it doesn't shut anymore,
but with more serios loads (e.g. an HD), it shuts again.
I checked in-circuit capacitors for shorts and ESR increase, tested
diodes and transistors.
It seems an aging problem (the trimmers were still sealed, so no
tampering), but who's bad ?

is there any OVERvoltage shutdown circuit? some protection crowbar you've
missed? some SMPS have crowbars on the 5v logic rail to protect the digital
logic on the load.I've seen where a zener broke too soon and triggered the
SCR too early and caused shutdown.


As it happens I will try fathoming a dedicated SMPS today that as it
stands is in tick-tick mode, 1 ultrasonic cycle per second.
Putting a dropped LED on each output shows life on all rails.

scope each rail to see how far it climbs.
Maybe one rail is not climbing as much as it should.

Plan of campaign will be
Run up on variac to find lowest point and will still run of sorts,
tick-tick Put a x10 larger timing capacitor over the one on the 555
that feeds the UC3842 PWM controller. Then make up an exerciser to
feed into the ,isolated, LED of the optoisolator, over-riding the
error feed. This will be a monostable of variable period from 1mS to a
second or so with push switch, hopefully to elicit which area is
playing up by gradually ramping up . All active, big Rs seem ok and
caps ESR-check ok. I suspect false error signal.
Anyone else any ideas/advice on a generic plan of action for these
sorts of situations.?




use a scope with a current probe to see if you're hitting the current
limit.
Check the PWM IC's housekeeping cap.Maybe it's not holding the IC supply
enough to keep the PWM IC running.(a common failure)

--
Jim Yanik
jyanik
at
localnet
dot com

supervinx wrote in
:

Il Tue, 30 Mar 2010 21:14:04 +1100, Phil Allison ha scritto:

"supervinx"


I've little experience in switching PSUs ... I've one of them (coming
from an Amiga 2000) which shuts off at the first load.
With no load at all, it delivered 5.9V out of 5 nominal.

** Many PC type switching PSUs need a load to be applied on ALL the
outputs on order to work properly.

Why not apply load resistances ( not incandescent lamps either) to each
DC rail and see if it holds up then ??

scope each rail to see if they rise proportionally.


Thing is, when you load just the 5 volt rail, the other DC rails go UP
in voltage - and that may trigger the out of range voltage detector
circuit and stop the whole damn show.



.... Phil

The PSU shuts off under normal operations while connected to his MB. I
connected it to an external HD (load on both +5 and +12): I see the
voltage raising and the voltage detector shutting the PSU down.
I have no oscilloscope handy ... with a digital multimeter I can see the
+5 rising about 4.8, the +12 about 9 and then stop.

A DMM will not respond fast enough.

The main capacitors are 200V rated, but I see 215 across them


By "main caps" do you mean the line supply filter caps,right after the
rectifier? The raw DC supply for the switcher?
If it's a doubler,you should have around 300VDC,if a single-ended
rectifier,about 170VDC. Perhaps you don't have enough input DC.
Or maybe you're seeing some AC along with the DC.

Many SMPS use a cap-rectifier doubler for 120 VAC operation,and a switch
changes it to single-ended for 220VAC operation.

--
Jim Yanik
jyanik
at
localnet
dot com

By "main caps" do you mean the line supply filter caps,right after the
rectifier? The raw DC supply for the switcher? If it's a doubler,you
should have around 300VDC,if a single-ended rectifier,about 170VDC.
Perhaps you don't have enough input DC. Or maybe you're seeing some AC
along with the DC.

Many SMPS use a cap-rectifier doubler for 120 VAC operation,and a switch
changes it to single-ended for 220VAC operation.

The PSU is 250Vac: the two capacitors are 680uF/200V (they seem to be
series-connected) and they show, with no load, 215V across each capacitor.
This value is suspect: I would expect 170V, as you stated, so the problem
could lie in the primary side of the PSU.

The 680uF/200V is mandatory ? Can I try to substitute them with, for
example, 1000uF (or more)/400V ? I have them handy ...

--
--
Jim Yanik wrote in message
4...
"N_Cook" wrote in
:

supervinx wrote in message
...
Hi to everyone, since it's the first time I write in this NG

I've little experience in switching PSUs ...
I've one of them (coming from an Amiga 2000) which shuts off at the
first load.
With no load at all, it delivered 5.9V out of 5 nominal.
I reduced it (with the trimmer) to about 5 with no load.
Inserting a little dummy load (47 Ohm) now it doesn't shut anymore,
but with more serios loads (e.g. an HD), it shuts again.
I checked in-circuit capacitors for shorts and ESR increase, tested
diodes and transistors.
It seems an aging problem (the trimmers were still sealed, so no
tampering), but who's bad ?

is there any OVERvoltage shutdown circuit? some protection crowbar you've
missed? some SMPS have crowbars on the 5v logic rail to protect the
digital
logic on the load.I've seen where a zener broke too soon and triggered the
SCR too early and caused shutdown.


As it happens I will try fathoming a dedicated SMPS today that as it
stands is in tick-tick mode, 1 ultrasonic cycle per second.
Putting a dropped LED on each output shows life on all rails.

scope each rail to see how far it climbs.
Maybe one rail is not climbing as much as it should.

Plan of campaign will be
Run up on variac to find lowest point and will still run of sorts,
tick-tick Put a x10 larger timing capacitor over the one on the 555
that feeds the UC3842 PWM controller. Then make up an exerciser to
feed into the ,isolated, LED of the optoisolator, over-riding the
error feed. This will be a monostable of variable period from 1mS to a
second or so with push switch, hopefully to elicit which area is
playing up by gradually ramping up . All active, big Rs seem ok and
caps ESR-check ok. I suspect false error signal.
Anyone else any ideas/advice on a generic plan of action for these
sorts of situations.?




use a scope with a current probe to see if you're hitting the current
limit.
Check the PWM IC's housekeeping cap.Maybe it's not holding the IC supply
enough to keep the PWM IC running.(a common failure)

--
Jim Yanik
jyanik
at
localnet
dot com


So far confirmed the UC3842 functions properly on a bench 13.5V supply with
original 2nF and also27nF // to it. But gets overriden by a protect sub-cct
on the hot oscillator side. Also that the 2 switching powerfets are
conducting balanced. Problem seems to be a goodness of oscillation monitor
on the hot side of the transformer that rectifies some of the drive and if
not balanced or large enough starts the tick-tick mode. Rectified it must
exceed 15V set by a zener or this mode cuts in , it would seem. Only
reaching 4.9V. But in the process of checking this area a small electo now
has an ESR of 200R wheras checking before any power on it must have been sub
2R or so sitting around idle. 1988 ps with a load of early SM and this cap
of course in an awkward section to replace rather than test, a job for
tomorrow.

Thanks for your help, the fault begins to be more clear ...

Having no oscilloscope, I used spartan methods

1) connected a 12V fan pair: the voltage dropped to 3.5V, but the PSU ran
well
2) added another +12V fan pair, but connected to the +5V line: +12 line
raised to about 9, +5 to 4.80 ... ran with no troubles
3) added another small load on the +5V (47 Ohm) and everything shut
down ... The +5V line connected fans were motionless (too low voltage)
and no motion was noticed adding the 47 Ohm load.

So I must assume there's no voltage peak: the protection scheme could be
a "current sense" and gone wild ?

4) Retained the +12V line connected fans and the 47 Ohm load on the +5
Line (removed the fans on this line): starts ok
5) Tried to move the trimmer (which should adjust the +5V, very slowly)
and power up, with the aforesaid loads.
6) Until 4.59 on the +5 line ... ok
4.60-4.66 starts, lasts few seconds, then shuts off
4.66 starts-stops immediately

That trimmer is connected to two transistor and some diodes (the small
orange-glass-like ones, not the "usual" black ones.

So, I suspect the fault is in the protection circuit, generating a false
positive.

I think of removing the two transistors and identify them.
If some diode is a Zener, the only way I have to test is to desolder them
and build a small auxiliary circuit (diode-resistor), to check the
breakdown voltage.

One tool I don't have in my toolkit - a sniffometer. Nothing smelled while
in use or close handling of this power supply. But on removing the electros
, that distinctive fishy smell of leaked elecrolyte, with soldering iron
temperature on the pins.
Tiny space available to fit some capacitance so they used 2x squat format
22uF,35V in parallel. One leg pulled out of one cap on desoldering and the
other a telltale drop of brown gloop immediately under the cap but not
enough to be visible over the pcb.
Both bright green nichicon 105 deg C. Found space on the hot side for
replacement and powered up long enough at about 3KHz instead of 40KHz to
give full V on all o/p ( unloaded) with very audible 3KHz coming off the
transformer. As , presumably, the pair of switching power fets were
operating at least partially linearly at an undesigned 3 KHz they heated up
quite quickly. 15 V over that sensing zener . Removed the 27nF and now
operating correctly with substantial test loading on all rails.

One mystery remains , the original tick-tick was about 1 a second, on adding
the 27nF to the 555 timing capacitor the tick-tick rate increased to about
10 a second. I've not traced out the path from the "goodness of oscillation"
circuit to the 555 but what sort of intermediary cct would lead to an
increase in that tick-tick rate ?

"N_Cook" wrote in
:

One tool I don't have in my toolkit - a sniffometer. Nothing smelled
while in use or close handling of this power supply. But on removing
the electros , that distinctive fishy smell of leaked elecrolyte, with
soldering iron temperature on the pins.
Tiny space available to fit some capacitance so they used 2x squat
format 22uF,35V in parallel. One leg pulled out of one cap on
desoldering and the other a telltale drop of brown gloop immediately
under the cap but not enough to be visible over the pcb.
Both bright green nichicon 105 deg C. Found space on the hot side for
replacement and powered up long enough at about 3KHz instead of 40KHz
to give full V on all o/p ( unloaded) with very audible 3KHz coming
off the transformer. As , presumably, the pair of switching power fets
were operating at least partially linearly at an undesigned 3 KHz they
heated up quite quickly. 15 V over that sensing zener . Removed the
27nF and now operating correctly with substantial test loading on all
rails.

at TEK,I used to replace a lot of the green nichicon 100uF/25v
electrolytics in the 1700 series waveform monitors and vectorscopes.ESR
problems,probably aggravated by the long operating hours in hot racks at TV
stations.



One mystery remains , the original tick-tick was about 1 a second, on
adding the 27nF to the 555 timing capacitor the tick-tick rate
increased to about 10 a second. I've not traced out the path from the
"goodness of oscillation" circuit to the 555 but what sort of
intermediary cct would lead to an increase in that tick-tick rate ?




I think you answered your own question;you ADDED capacitance to the 555
timing capacitor.

what sort of SMPS uses a 555 for a controller?

--
Jim Yanik
jyanik
at
localnet
dot com

Jim Yanik wrote in message
4...
"N_Cook" wrote in
:

One tool I don't have in my toolkit - a sniffometer. Nothing smelled
while in use or close handling of this power supply. But on removing
the electros , that distinctive fishy smell of leaked elecrolyte, with
soldering iron temperature on the pins.
Tiny space available to fit some capacitance so they used 2x squat
format 22uF,35V in parallel. One leg pulled out of one cap on
desoldering and the other a telltale drop of brown gloop immediately
under the cap but not enough to be visible over the pcb.
Both bright green nichicon 105 deg C. Found space on the hot side for
replacement and powered up long enough at about 3KHz instead of 40KHz
to give full V on all o/p ( unloaded) with very audible 3KHz coming
off the transformer. As , presumably, the pair of switching power fets
were operating at least partially linearly at an undesigned 3 KHz they
heated up quite quickly. 15 V over that sensing zener . Removed the
27nF and now operating correctly with substantial test loading on all
rails.

at TEK,I used to replace a lot of the green nichicon 100uF/25v
electrolytics in the 1700 series waveform monitors and vectorscopes.ESR
problems,probably aggravated by the long operating hours in hot racks at
TV
stations.



One mystery remains , the original tick-tick was about 1 a second, on
adding the 27nF to the 555 timing capacitor the tick-tick rate
increased to about 10 a second. I've not traced out the path from the
"goodness of oscillation" circuit to the 555 but what sort of
intermediary cct would lead to an increase in that tick-tick rate ?




I think you answered your own question;you ADDED capacitance to the 555
timing capacitor.

what sort of SMPS uses a 555 for a controller?

--
Jim Yanik
jyanik
at
localnet
dot com

Adding C usually decreases the f

UC3842 PWM controller , instead of simple RtCt pin , it uses the o/p of a
555 so that there can be diode steering for different timings

"N_Cook" wrote in -
september.org:


UC3842 PWM controller , instead of simple RtCt pin , it uses the o/p of a
555 so that there can be diode steering for different timings




Ah,nothing like taking a great simple PWM IC circuit and making it more
complex....

Now I wonder about the circuit makes the ramp for the internals of the
3842,while not using the RtCt function.Seems like that could affect the
current limit function.

--
Jim Yanik
jyanik
at
localnet
dot com

Jim Yanik wrote in message
4...
"N_Cook" wrote in -
september.org:


UC3842 PWM controller , instead of simple RtCt pin , it uses the o/p of
a
555 so that there can be diode steering for different timings




Ah,nothing like taking a great simple PWM IC circuit and making it more
complex....

Now I wonder about the circuit makes the ramp for the internals of the
3842,while not using the RtCt function.Seems like that could affect the
current limit function.

--
Jim Yanik
jyanik
at
localnet
dot com


There is a lot more SM around the UC and 555 than any of the datasheets I've
seen. There are at least 1 diode for changing the duty cycle of the 555
waveform. But increasing the single timing cap from 2 to 29nF upped the
error mode frequency. Perhaps the duty cycle changed so much that it
inverted and played havoc with timings or limits in the UC. I will use x5
rather than x10 next time . Incidently slowing things down so less likely to
be destructive to drivers if the problem is shorted turns in the
transformer.

I removed an SCR, 2N5061, with this behaviour:
connecting an HD (or a load) doesn't shut off the PSU anymore, but
voltage drops from 5 to 4.2, and it's not enough.
I moved the trimmer with slight advantage (4.5) then the PSU shutdown
again.
I had to wait some minuites, discharge all caps before it started too
work again.
Under the SCR there's a fast switching diode 1N4148F, with a constant
reverse voltage of 1.40. When everything goes bad, it's the first to
receive 0 voltage.
Across A-K of the SCR I read about 4.34 V, G and K are equipotential.
May be this is the right zone to investigate ?
A friend of mine owns a similar PSU (same PC), working, but he'll be back
after easter.
Having a similar model could help me tracing down the fault...