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Old 22nd April 2008, 02:51 AM   #1
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Default Philips FR-70 power supply problem.

I inherited a broken receiver from a friend and figured I'd give a quick attempt at fixing it. Its not a great receiver but my parents need something to power some in-wall speakers and this is more than enough for that.

The problem is that it won't power up, my friend found the fuse had gone and so he fed it another one which did the same thing.

The sub-PSU that feeds the electronics seems to be at fault. There is some light burning marks on the PSU near a TO-220 part that I believe is a voltage regulator. I figure I could put a replacement in but I'm weary of just feeding it more parts. How should I be diagnosing the problem?

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Old 22nd April 2008, 10:39 AM   #2
Mooly is offline Mooly  United Kingdom
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Hi, Need more info than that i'm afraid. You are probably going to need a circuit. Which fuse blows ? Whats it's rating ? Is it on the primary or secondary side. Check for any short circuit diodes on the secondary side of the transformer. That's a small transformer in your pic, is it just used for a standby function. The discoloured print may be quite normal by the way.
Regards Karl
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Old 22nd April 2008, 04:23 PM   #3
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Thanks for the reply. The fuse seems to be on the primary side of the small transformer pictured. My friend also mentioned that the transformer (the large one, not pictured) made a noticeable loud humming for the 1/4" second or so before that fuse blew.
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Old 22nd April 2008, 04:43 PM   #4
Mooly is offline Mooly  United Kingdom
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If the large transformer made loud buzzing before fuse blows it does point to a short on a rail. Usually this would be an output stage that is short circuit (is it discrete or an I/C), or maybe a rectifier if you are lucky. Does that fuse feed only the small transformer or the large one as well. What rating is it. Have you tried replacing the fuse with a mains bulb, say 60 watt so you can work on it powered up. If the bulb just lights brightly then you have a short somewhere. (Filament bulb, not an energy saver )
Regards Karl
Edit, have you any test gear, meter etc.
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Old 22nd April 2008, 04:49 PM   #5
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The fuse feeds only the smaller PSU that drives the front panel electronics it seems. The amplifier is a discrete configuration.

Could it still be an output short circuit, the location of the fuse seems to imply that the electronics section is at fault? I do have a multimeter and general soldering knowledge. I also have 3/4 of a EE degree but it never seems to help on stuff like this, maybe if I were trained as a technician
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Old 22nd April 2008, 06:03 PM   #6
Mooly is offline Mooly  United Kingdom
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WHAT RATING IS THAT FUSE ? I am asking because if it is a 63 or 100ma or so it will just feed the small auxiliary supply, if it's a 2amp or 4 amp it feeds other things. I am a little suprised that the primary fuse blows to be honest, a small transformer usually would not be able to draw that kind of current even with a short on the output. There is usually more than enough protection on the secondary side, safety resistors and so on that would go up first. It could even be a faulty transformer you know, unlikely but possible. You have a meter. Check on a low ohms range for any short circuit diodes in the power supply.
In your pic there is a large black object toward the front, is it a relay ? Does this power up the main amp when you take it out of standby. If the fuse really does supply just that small transformer it might be worth unsoldering all the secondary pins if you can not measure any shorts (use braid and a hot iron). If the fuse still blows it has to be the transformer or something directly on the primary side.
Regards Karl
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Old 22nd April 2008, 06:12 PM   #7
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Ah good point. I just found it's a 5 amp fuse, certainly too high for the front panel electronics. I'll check out that black box could be a relay, something is definitely clicking when I try to take it out of standby (even without the 5A fuse in place). I'll also get a couple overview shots so perhaps things will make more sense.
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Old 22nd April 2008, 06:29 PM   #8
gain is offline gain  United States
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first, disconnect the PS from the rest of the receiver circuitry. on some receivers you can just disconnect a wire harness and on others you will either have to cut/desolder the wires themselves.

after you have the PS isolated, put a fuse and apply power. if the fuse still blows, the problem is the PS, if not, the problem is most likely in the output stage of the amplifier board.

if the problem is the PS, check the rectifier diodes, snubbing caps (small disc caps in parallel w/the rectifier diodes), and resevor caps (the large caps on the PS board) for shorts. also check the primary and sec windings of the transformer.

if the problem is NOT the PS, check the output devices for shorts, especially between the collectors and emitters. also use the diode function on your VOM to check the BE junction of all the output transistors as well (the outputs are the ones that will be mounted on the main heatsink)

if none of this reveals the issue i recommend you take the unit to a repair shop or scrap it for parts.

regards
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Old 22nd April 2008, 06:31 PM   #9
Mooly is offline Mooly  United Kingdom
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It's going to be the output stage. If it's "clicking", thats probably the relay. The standby supply will be O.K. Measure those output transistors, any short ?
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Old 22nd April 2008, 06:35 PM   #10
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Click the image to open in full size.

I'll check the output for shorts right now.
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