Lafayette LR3000

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par4ken

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Nothing visible there, a good sign. Blown fuses are caused by short circuits, so it could be a shorted diode in the power supply circuit. It could also be a shorted capacitor or less likely a shorted primary or secondary winding in the power transformer. I would lean toward the diode theory because of the way that it happened with a sudden large current draw on the power supply.

You will have to find what component is causing the short and replace it to get the power supply up and running and then go from there. Hopefully all else is fine.
 

DuncanS

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I was wondering if an input voltage spike from connecting the pre-amp has taken out an amplifier's output transistor or both output transistors, as the transistors are in different coloured packages I'd assume one is an n-type the other a p-type (it was common to just have n-types in an output stage). If the transistor(s) have gone short on their Collector-Emitter connections then that would cause a high current to flow, collapse the supply and cause a fuse to blow. Is it possible to lift the supply wires from each amp, and see if the supply/fuse is ok, then solder the amps power supply wiring back in one by one?
 

DudeRocks

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I was wondering if an input voltage spike from connecting the pre-amp has taken out an amplifier's output transistor or both output transistors, as the transistors are in different coloured packages I'd assume one is an n-type the other a p-type (it was common to just have n-types in an output stage). If the transistor(s) have gone short on their Collector-Emitter connections then that would cause a high current to flow, collapse the supply and cause a fuse to blow. Is it possible to lift the supply wires from each amp, and see if the supply/fuse is ok, then solder the amps power supply wiring back in one by one?
Do you have an example of where I am looking for the fuse inside the supply? Is it literally inside the black box? Otherwise the expected fuses are accessible on the back of the unit.
1694553285370.jpeg
 

DudeRocks

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View from the back. 5 fuses. The main one is the one that blows.
 

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DuncanS

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Do you have an example of where I am looking for the fuse inside the supply? Is it literally inside the black box? Otherwise the expected fuses are accessible on the back of the unit. View attachment 96206
The black box looks like the mains transformer. So if the input supply fuse (i.e. the one you have shown) then the whole PSU is taking too much current, which is why I think you have a supply rail-to-rail/ground short on the supply due to output transistors blowing. This could blow the fuse you indicate.

Its difficult without a schematic to figure things out - not aided by plus a few thousand miles!

If you can identify the power supply wires to each amplifier and then unsolder them, then power up and see if the fuse still blows. If it still blows then the fault is possibly in the Power Supply. If not solder the supply back to an amp at a time and see if the fuse blows. If it does, then that last amp has a fault - most likely its output transistor(s) on the heatsink.
 

mdevries

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I took one of your pictures and marked up the wires on the solder side of the board. The -VCC is connected to the one of the boards on Pin 13 or 14 and there is a short jumper wire that connects to the other board. If you remove the +VCC and -VCC wires from the amp boards and it still throws a fuse that would rule out the amp boards. If it powers up then at least 1 of the boards has an issue. I noticed some of the transistors in the corner of one of the boards are showing quite a bit of corrosion going up the leads.

I have the same issue I am trying to figure out on my LR3000. I narrowed mine down to the rear channel amp board.
 

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par4ken

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Do you have an example of where I am looking for the fuse inside the supply? Is it literally inside the black box? Otherwise the expected fuses are accessible on the back of the unit. View attachment 96206
Looks to me like the power supply filter capacitor is bad. Look above the rectifier diodes (to the right of the transformer, below the 27峏 resistor) the top (bottom actually) of the capacitor looks black, also the diode solder connections look as if they have melted. Diodes are likely shorted out as well the one farthest to the right looks brown and the insulated tubing looks partially melted. The second diode as well both connect to the blackened capacitor.
 

quadsearcher

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The bottoms of the two main filters should be black, that's a rubber covering. One lug is painted black, that's negative. Solder looks bad because someone had the primary leads disconnected and reconnected, possibly without removing the old solder, and overheated the diode leads. Also the diode on the left has been disconnected at the cathode B+ side but it doesn't look reconnected properly, maybe just tacked on without good solder flow. Maybe that's part of the problem, or just created by someone trying to track down the problem. A lot of guesswork, but add a meter and the knowledge of what to look for and the odds of success go up immensely. But if it's a fun rabbit hole, then so be it. Just keep one hand in a pocket and respect the danger of electricity. Those filter caps might hold a charge when the unit is unplugged, on bigger amps I've seen the charge blow a chunk out of a screwdriver.

Also, overdriving the first stage might not have caused the blown fuse. It's been worked on before right at the main bridge.

Stuff like this is why I use a $50 Sherwood 5.1 receiver from a flea market, put the money saved there into a Surround Master, and spend more time listening to music.
 
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DudeRocks

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The bottoms of the two main filters should be black, that's a rubber covering. One lug is painted black, that's negative. Solder looks bad because someone had the primary leads disconnected and reconnected, possibly without removing the old solder, and overheated the diode leads. Also the diode on the left has been disconnected at the cathode B+ side but it doesn't look reconnected properly, maybe just tacked on without good solder flow. Maybe that's part of the problem, or just created by someone trying to track down the problem. A lot of guesswork, but add a meter and the knowledge of what to look for and the odds of success go up immensely. But if it's a fun rabbit hole, then so be it. Just keep one hand in a pocket and respect the danger of electricity. Those filter caps might hold a charge when the unit is unplugged, on bigger amps I've seen the charge blow a chunk out of a screwdriver.

Also, overdriving the first stage might not have caused the blown fuse. It's been worked on before right at the main bridge.

Stuff like this is why I use a $50 Sherwood 5.1 receiver from a flea market, put the money saved there into a Surround Master, and spend more time listening to music.
I wonder if you apply a turned on preamp to a surround masters inputs if you could blow it up as well. 不不不 So far the price of one SM I've bought 3 SQ-W working units. I spend plenty of time listening to music all while troubleshooting this one. Was planning to hook this up for a second room. None of this would be any better with a SM. Not sure why every SM owner when talking SQ-W turns their SM into a contest... in every single thread! I don't care if a 700 dollar SM sounds marginally better than a 200 dollar vintage hobby. 仄
 

DudeRocks

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The bottoms of the two main filters should be black, that's a rubber covering. One lug is painted black, that's negative. Solder looks bad because someone had the primary leads disconnected and reconnected, possibly without removing the old solder, and overheated the diode leads. Also the diode on the left has been disconnected at the cathode B+ side but it doesn't look reconnected properly, maybe just tacked on without good solder flow. Maybe that's part of the problem, or just created by someone trying to track down the problem. A lot of guesswork, but add a meter and the knowledge of what to look for and the odds of success go up immensely. But if it's a fun rabbit hole, then so be it. Just keep one hand in a pocket and respect the danger of electricity. Those filter caps might hold a charge when the unit is unplugged, on bigger amps I've seen the charge blow a chunk out of a screwdriver.

Also, overdriving the first stage might not have caused the blown fuse. It's been worked on before right at the main bridge.

Stuff like this is why I use a $50 Sherwood 5.1 receiver from a flea market, put the money saved there into a Surround Master, and spend more time listening to music.
Appreciate the advice!.I'll take a look at what you are pointing out when I get home.
 

DudeRocks

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Looks to me like the power supply filter capacitor is bad. Look above the rectifier diodes (to the right of the transformer, below the 27峏 resistor) the top (bottom actually) of the capacitor looks black, also the diode solder connections look as if they have melted. Diodes are likely shorted out as well the one farthest to the right looks brown and the insulated tubing looks partially melted. The second diode as well both connect to the blackened capacitor.
Appreciate the advice!.I'll take a look at what you are pointing out when I get home.
I took one of your pictures and marked up the wires on the solder side of the board. The -VCC is connected to the one of the boards on Pin 13 or 14 and there is a short jumper wire that connects to the other board. If you remove the +VCC and -VCC wires from the amp boards and it still throws a fuse that would rule out the amp boards. If it powers up then at least 1 of the boards has an issue. I noticed some of the transistors in the corner of one of the boards are showing quite a bit of corrosion going up the leads.

I have the same issue I am trying to figure out on my LR3000. I narrowed mine down to the rear channel amp board.
Coincidentally somebody was selling an LR3000 " that needs a little work but comes on " for 75 bucks, 50 bucks shipping. I went ahead and grabbed it. Figure if anything between the two of them I can get one up and running for the other room and have spare parts. My next project after this is trying to get the CD4 module to sound good. I got one that came in a LR5000 that wouldn't work at all, I put it in this LR3000 and it worked for separation but sounds distorted like FM being out of tune between a station. Learning from your suggestions on this has been really cool. I'll see what I can see on this when I get to it tonight.
 

quadsearcher

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I wonder if you apply a turned on preamp to a surround masters inputs if you could blow it up as well. 不不不 So far the price of one SM I've bought 3 SQ-W working units. I spend plenty of time listening to music all while troubleshooting this one. Was planning to hook this up for a second room. None of this would be any better with a SM. Not sure why every SM owner when talking SQ-W turns their SM into a contest... in every single thread! I don't care if a 700 dollar SM sounds marginally better than a 200 dollar vintage hobby. 仄
I'm sorry, I did not mean to imply that I thought it was a contest. I do not think so.
I could've used the money for more records, for example.
I meant only that old receivers often need work, and I grew tired of that and now have something old but not quad-era old.
I actually wanted a SQ-W for some time, but found I like QS type decoding better.
 

mdevries

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Appreciate the advice!.I'll take a look at what you are pointing out when I get home.

Coincidentally somebody was selling an LR3000 " that needs a little work but comes on " for 75 bucks, 50 bucks shipping. I went ahead and grabbed it. Figure if anything between the two of them I can get one up and running for the other room and have spare parts. My next project after this is trying to get the CD4 module to sound good. I got one that came in a LR5000 that wouldn't work at all, I put it in this LR3000 and it worked for separation but sounds distorted like FM being out of tune between a station. Learning from your suggestions on this has been really cool. I'll see what I can see on this when I get to it tonight.
I saw that one on E-Bay!!

The LR-3000 I am working on was the victim of a small toad getting inside it somehow and the previous owner believed it may have peed when it was inside the unit. She pointed out a general area and sure enough when I removed the metal shield that is above the amp boards one of them had quite a bit of corrosion on the components. I removed the board and tested the receiver with it removed and the front channels work. I cleaned up the corroded board with some Windex, a small bristle brush, rinsed, and blew it dry. Then I cleaned it with some electrical contact cleaner and the little brush and blew it dry. I changed most of the electrolytic caps with ones I had on hand. I put it back in to see if maybe just the corrosion and urine were creating a short that blew the fuse, but no luck.

So far, I removed the small green transistor on the side of the board where the corrosion was and found it was bad. Then I connected the lead from my meter to the -VCC pin on the board and probed the leads on each of the power transistors. You should get similar readings on each pair of the power transistors. I found one that tested as a direct short to the -vcc on all 3 leads. I removed it from the board and checked it and it checks out as bad. I plan on replacing all of the power transistors on this board. The other board already has different power transistors on it, so I will likely use the same ones that were used on the other board. I will use the symmetry to do similar testing of the other transistors. Switching to the +vcc pin and ground pin and checking the resistance to different nodes will work as a double check.
 
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