| Krossroad 500 [message #3889] |
Thu, 18 March 2004 17:58  |
solargrain
Messages: 2
Registered: March 2004
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Junior Member |
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My power amp smokes fuses, where should I start? Does any body have experience with this model?
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| Re: Krossroad 500 [message #3890 is a reply to message #3889] |
Thu, 18 March 2004 22:06  |
rodak
Messages: 516
Registered: October 2001
Location: Georgia
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Senior Member |
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Yes, ME! Mine turned out to be a couple of fried output transistors (Power MOSFET's). There are 5 pairs of them, arranged in 2 rows of 5. Unfortunately, the two that had fried were in different pairs, and burned away so much of the solder traces I could not have replaced them without some serious (and dicey) jumpering that I didn't want to mess with. Since my needs for power are not that great, I replaced only the 3 pair that I could.
In my research, I found that a possibility for the high failure rate of the 500's *might* have been a power supply that was a bit too beefy, voltage-wise, for the MOSFET's. So as part of my repair, I replaced the power transformer with another of somewhat lower voltage. So I suspect my actual power output is more like 250watts, but it's puh-lenty for my needs.
I'd be happy to help you with it in any way. I'm very familiar with the innards of the 500 (I even added a jewel lamp to it recently), and have done a schematic of the power supply and documented the wiring connections to the main board. (but I have no schematics for the actual wiring of the preamp/power amp board - I tried and tried and was unable to locate any)
Are you technically adept? Good with a soldering iron? Have you opened the bugger up yet? Lemme know where you stand and I'll do my best to get you going.
www.combo-organ.com
www.combo-organ.com
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| Re: Krossroad 500 [message #3895 is a reply to message #3893] |
Fri, 19 March 2004 08:46 |
rodak
Messages: 516
Registered: October 2001
Location: Georgia
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Senior Member |
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Excellent idea, Steven. In my case, I happened to have a transformer (two, actually - one for the high-voltage side, and one for the preamp) that fit (electrically and physically) perfectly. I'm using the old Krossroad transformer to build a power supply for one of my organs (overkill, power-wise, but it has the voltages I need).
www.combo-organ.com
www.combo-organ.com
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| Re: Krossroad 500 [message #3904 is a reply to message #3890] |
Sat, 20 March 2004 10:08 |
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Thanks much, I have something to go on, will let you know how things turn out. But might be a while as the amp is on the back burner. Again thanks, Grant
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| Re: Krossroad 500 [message #3905 is a reply to message #3893] |
Sat, 20 March 2004 10:12 |
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Hi, Thanks for the help, I will let you know how things turn out, but for now the amp is on the back burner. Again thanks much, Grant
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| Re: Krossroad 500 [message #3906 is a reply to message #3890] |
Sat, 20 March 2004 10:22 |
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Hi again, I have soldered, and own a volt ohmn meter, I have taken the amp out of the box and am looking at it. I may take it to a tech friend, but with help, I may tackle it my self. I am commencing to look for the power MOSFET's. I will let you know what I find, Also I read the other responses, and replied, and will follow any advice I get. Thanks Grant
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| Re: Krossroad 500 [message #3907 is a reply to message #3906] |
Sat, 20 March 2004 12:14 |
rodak
Messages: 516
Registered: October 2001
Location: Georgia
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Senior Member |
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As far as testing the MOSFET's, I don't know of any reliable way to test them in-circuit (there may be, I just don't know it). I actually pulled all 10 out, and built a simple circuit used for matching transistors, which does a fine job of testing them, too. Here's the link:
http://sound.westhost.com/transistor-matching.htm
Pulling the transistors out can be tedious, as they're soldered to both sides of the board. Solder wick works wonders! While they were out, I cleaned up all the solder traces (a can of spray flux remover and a small stiff brush were very helpful) and examined them closely, to see if any were unusable. (a magnifying glass was necessary in my case, since I'm over 40 ;-)
But before you go to that trouble, you might want to check the power supply first, to make sure you don't have a shorted wire or rectifier or something. Start with the transformer - unhook it from the circuit and make sure you have AC voltage at both windings (one will be about 120V, the other about 37V - both are center-tapped). Then unhook the rectifier and caps from the circuit board, hook the transformer back up, and make sure you have good DC at the caps - about 89V across each one(this will be only for the power amp supply - the rectifier and filters for the preamp supply are on the circuit board). It's unlikely you'll find any problems this way, but it's easy to do, and it's nice to finally see the pilot light come on and stay on. Make sure you know what you're doing with this, though - the voltages in there are pretty high, and can give you a good jolt, and make some real nice fireworks if you let the wrong wires touch. The power supply circuit is dead-simple to figure out, if you're familiar with basic power supply circuitry.
And one more thing I just thought of - make sure you have the right rating of fuse to begin with - I found that even with the amp working properly, anything less than a 3amp (fast, not slo-blo) fuse will blow instantly when you turn it on. The initial inrush of current to charge those big caps must be pretty hefty. I think the correct value for it is about 10 amps.
If you do want to replace the MOSFET's, I found them here:
http://www.bright.net/~robertso/
for about $5 each.
Oh, btw, I've forgotten the most important question of all - what color is it?
Hope this isn't information overload. I spent so much time working on mine, I'm glad for the chance to share the knowledge.
Robert
www.combo-organ.com
www.combo-organ.com
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| Re: Krossroad 500 [message #3914 is a reply to message #3907] |
Mon, 22 March 2004 07:50 |
stevem
Messages: 4775
Registered: June 2004
Location: NY
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Senior Member |
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Hi folks. The info I gave on the NTE zeners is somewhat wrong, I did not have the catalog in hand and was working from recall. The NTE parts numbers are in the 5200k range, such as a 50volt zener is NTE part number 5270k.If you only need to drop 3 to 5 volts a large wattage resistor may be better alot of these high current zeners need to mounted by a 5/8s inch hole in the amp chassie for a heat sink. The 5800 serise NTEs are zeners all so but I do not know how much current the largest in the serise will take. In tube amps when I do this mod the zener goes inbetween ground and the power trans center tap, in most solid state amps with a bridge rectifier, if you put the zener in the ground leg of the bridge you will drop voltage thru the hole circuit that supply feeds. So may be the whole amp.
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