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From: N_Cook on 31 Mar 2010 05:04
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 ?


From: Jim Yanik on 31 Mar 2010 09:45
"N_Cook" <diverse(a)tcp.co.uk> wrote in
news:hov3ae$hvb$1(a)news.eternal-september.org:

> 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
From: N_Cook on 31 Mar 2010 09:58
Jim Yanik <jyanik(a)abuse.gov> wrote in message
news:Xns9D4C635052804jyaniklocalnetcom(a)216.168.3.44...
> "N_Cook" <diverse(a)tcp.co.uk> wrote in
> news:hov3ae$hvb$1(a)news.eternal-september.org:
>
> > 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


From: Jim Yanik on 31 Mar 2010 13:03
"N_Cook" <diverse(a)tcp.co.uk> wrote in news:hovkhd$due$1(a)news.eternal-
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
From: N_Cook on 1 Apr 2010 03:26
Jim Yanik <jyanik(a)abuse.gov> wrote in message
news:Xns9D4C84D588168jyaniklocalnetcom(a)216.168.3.44...
> "N_Cook" <diverse(a)tcp.co.uk> wrote in news:hovkhd$due$1(a)news.eternal-
> 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.


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