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

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?

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

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Reply to
N Cook
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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

Reply to
N Cook

On Tue, 16 Aug 2005 11:13:11 +0100, "N Cook" put finger to keyboard and composed:

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

- Franc Zabkar

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Reply to
Franc Zabkar

side

value.

doubled

barrel

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

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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.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

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Reply to
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.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

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Reply to
N Cook

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

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Reply to
N Cook

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

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Reply to
CJT

On Wed, 17 Aug 2005 09:39:36 +0100, "N Cook" put finger to keyboard and composed:

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

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Franc Zabkar

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