🚀 go-pugleaf

The UC3842 is Sooooooooo common in monitors, and the fault you describe is
also very common, as is total failure with a shorted FET.

Generally, the UC3842's surrounding circuitry is at fault. 80% of the time
the filter electrolytic on the +12v pin of the UC3842 is faulty (typical
value ~220uf), 15% of the time the other parts that generate the +12v rail
are faulty (very often this is two series wired 270k-330k 1/2 watt jobs from
the +330v rail). The other 5% of the time, the UC chip iself is faulty.

Typically, if you're repairing a fault where the switching FET blew, change
the FET, UC, the FET drive resistor/components (there's often a small signal
diode and a zener as well), the electro on the +12v rail to the UC and the
low ohm current sense resistor as these will almost always be shot. Many
times the +12v rail generation resistors will be high and need changing too
(very often this is two series wired 270k-330k 1/2 watt jobs from the +330v
rail).

If it's hiccuping but not blown up completely, the filter electro is
normally the cause. A key testpoint is the +5v reference from the UC chip -
if this isn't present, and the +12v rail is OK, suspect the UC.

Generally we change all these parts as standard, since a UC and a handfull
of passives is cheap enough, and another blown FET + labour to do it twice
is expensive.

    Craig


"Alan Liefting" <aliefting@ihug.co.nz> wrote in message
news:3C967D91.B796C3A8@ihug.co.nz...
>
>
> Rudolf Ladyzhenskii wrote:
> >
> > In this case, get datasheet for 3842 (from Motorola -- UC3842).
>
> It is burnt into my retinas!
>
> > Make sure that its VCC is 12V. Pin7 from memory.
>
> Vcc is not critical (within limits).  It is clamped to 30V by the chip.
> Usually does not get that high.
>
> > Also check/replace capacitors in primary.
>
> Yep.  Done!
>
> >
> > If you find that VCC to 3842 pulses, I would suspect fault outside of PS
> > primary.
> >
>
> Vcc is varying due to change in loading but not pulsing.  I am sure it
> is not dropping below a threshold wich will cause the chip to shut down.
>
>
> > Rudolf
> >
> > "Alan Liefting" <aliefting@ihug.co.nz> wrote in message
> > news:3C96692D.587C9095@ihug.co.nz...
> > > I am almost 100% sure that it is a PSU fault.  The monitor will work
> > > occasionally.  The EHT is coming up.  The PSU control ic is not
shutting
> > > down due to overcurrent.  There is an additional bit of circuitry that
> > > controls the 3842 chip.  I think it is for power save.  I am sure that
> > > this is the area where the fault is.
> > >
> > > Alan Liefting
> > >
> > > Rudolf Ladyzhenskii wrote:
> > > >
> > > > Check horizontal circuitry.
> > > >
> > > > Rudolf
> > > >
> > > > "Alan Liefting" <aliefting@ihug.co.nz> wrote in message
> > > > news:3C965925.4A3EA9E4@ihug.co.nz...
> > > > > Hello All,
> > > > >
> > > > > I have the above monitor which is pulsing.  The power supply is
coming
> > > > > up and shutting down a couple of times per second.  It is
definitely a
> > > > > power supply fault.  Voltage control is done on primary.
Disconnected
> > > > > optocoupler which is for power supply standby.  Still no
difference.
> > > > > Replaced control capacitor.  No difference.  Replaced opto and
UC3842
> > > > > control ic.  Still no difference.  Replaced a capacitor in an
obscure
> > > > > bit of circuitry and came right.  Monitor then failed on test!
> > > > >
> > > > > FCC ID DK4M1566
> > > > >
> > > > > HELP??!!
> > > > >
> > > > > Alan Liefting