300W PC power supply, no POST beep, smell, smoke, bang,dead

[N Cook]

Jou Jye 300AP
Blown 5 amp mains fuse
There is a 1/2W resistor between the common point of the 2 x 200V mains side
electrolytics
and one of the 2 leads , high V side of the large isolation transformer,
marked on pcb as R5 now reading 98 ohms
Overheated and probably had a brown and a red band somewhere in the value.
Covered with some 125 degree shrink-sleeve that has charred. This was to
protect against touching other parts but this R was on end and lead doubled
back and presumably the bang was the long lead shorted to the other barrel
end of the R as severe tell-tale smoke 'jet-trails' at that point, no
ceramic spacer or anything .

Anyone hazard a guess what the original value might be. ?
The 2 main 2SC2625 and 200V electrolytics seem OK.

Parallel to the 4 wires of the mains side bridge rectifier is a small
sub-circuit
with (2S?) K2645 and a L8561 8 pin IC, is this all a crow-bar circuit to
knock out the mains fuse?

 

[N Cook]

No this 1/2 Watt R is just in series with a small ceramic 2.2nF capacitor
as a snubber / filter across the HV primary of the main switch mode
transformer

 

[Franc Zabkar]

Jou Jye 300AP
Blown 5 amp mains fuse
There is a 1/2W resistor between the common point of the 2 x 200V mains side
electrolytics
and one of the 2 leads , high V side of the large isolation transformer,
marked on pcb as R5 now reading 98 ohms
Overheated and probably had a brown and a red band somewhere in the value.
Covered with some 125 degree shrink-sleeve that has charred. This was to
protect against touching other parts but this R was on end and lead doubled
back and presumably the bang was the long lead shorted to the other barrel
end of the R as severe tell-tale smoke 'jet-trails' at that point, no
ceramic spacer or anything .

Anyone hazard a guess what the original value might be. ?
The 2 main 2SC2625 and 200V electrolytics seem OK.

Parallel to the 4 wires of the mains side bridge rectifier is a small
sub-circuit
with (2S?) K2645 and a L8561 8 pin IC, is this all a crow-bar circuit to
knock out the mains fuse?

This smaller circuit is probably the +5VSB standby supply.


- Franc Zabkar

 

[N Cook]

This smaller circuit is probably the +5VSB standby supply.


- Franc Zabkar

It is a power factor corrector sub-circuit, would seem.
Does that mean all those millions of PC SMPS without this corrector are
getting free electicity off the generating company?
Until I got oblique lighting just right very difficult to read IC number
and it is L6561
http://www.st.com/stonline/products/literature/ds/5109/l6561.htm
It looks as though the burnt resistor (2W not 1/2W as previously said) was
1K from the bands so dropping from 1K to about 100 may be the initial
problem.

 

[N Cook]

Curioser and curioser
Replaced 1K, 2W and 2nF,1KV cap and powered up via Variac.
At about 60 per cent mains much increased amps taken and then out-gassing or
something
and crackles near the 2 x 200V, 200uF electrolytics.
Across each cap is a 330K resistor in parallel with something designated on
pcb as ZNR.
One of these ZNR discs had blown the ceramic covered end faces off, the
other looked
OK. Removed the apparently OK one and it measured 260 ohms at room
temperature
and reduced ohmage on increased temp so presumably neg coef. thermistor but
260 ohms
over a 200V cap ? ? ?
Removed the other 'ZNR' and held the end pieces back to the core with some
aluminium foil for
better contact and squashed together gave a minimum of about 3K which also
is too low.

 

[N Cook]

Someone's been here before, ZNR = Zinc oxide Non linear Resistors

http://groups.google.co.uk/group/sci.electronics.repair/browse_thread/thread
/c5e60f0c64c0e3c2/6cd6580e5fcdf9d7?lnk=st&q=znr+transient&rnum=2&hl=en#6cd65
80e5fcdf9d7

 

[CJT]

Curioser and curioser
Replaced 1K, 2W and 2nF,1KV cap and powered up via Variac.
At about 60 per cent mains much increased amps taken and then out-gassing or
something
and crackles near the 2 x 200V, 200uF electrolytics.
Across each cap is a 330K resistor in parallel with something designated on
pcb as ZNR.
One of these ZNR discs had blown the ceramic covered end faces off, the
other looked
OK. Removed the apparently OK one and it measured 260 ohms at room
temperature
and reduced ohmage on increased temp so presumably neg coef. thermistor but
260 ohms
over a 200V cap ? ? ?
Removed the other 'ZNR' and held the end pieces back to the core with some
aluminium foil for
better contact and squashed together gave a minimum of about 3K which also
is too low.

Much as I believe in recycling, etc., you've probably already spent more
time and money on it than it's worth.

 

[Franc Zabkar]

It is a power factor corrector sub-circuit, would seem.
Does that mean all those millions of PC SMPS without this corrector are
getting free electicity off the generating company?
Until I got oblique lighting just right very difficult to read IC number
and it is L6561
http://www.st.com/stonline/products/literature/ds/5109/l6561.htm
It looks as though the burnt resistor (2W not 1/2W as previously said) was
1K from the bands so dropping from 1K to about 100 may be the initial
problem.

At the risk of defying EU standards (which mandate PFC in new
equipment), I would remove the PFC circuitry. You will gain
reliability without increasing your power consumption. It will also
cost you nothing other than a little time.


- Franc Zabkar