Question about Amplifiers & Preamps
I have a SP500 amplifier that needs fixing as a favour for a communial centre, they had put two speakers into one output. Two of the main mosfets blew in the full bridge circuit. I replaced them but only got sound from one channel, but then the other two blew. Is there a special way to set this amp up when replacing the mosfets? I noticed a pot that could be adjusted, but without a circuit diagram I'm stuck. I do have access to test equipment.
Any help appreciated.
I strongly recommend that you replace all output transistors at once, and the drivers also. Also look for arcing across small low value resistors about the output stages, and small signal diodes also for 0.6 volts barrier potential voltage(they often go short circuit also). soft start the amp with a variac if you have one, or an incandescent 60watt 240v(110v if in the US) lamp in series with mains power to limit the current that can flow to protect the amps circuits if there are still faulty parts about. Check
DC conditions everywhere you can. If both channels appear to be ok, Then some careful observation of voltages with a DMM will help you to find any more problems.
Amps by design with high potentials and direct coupling ascociated with DC feedback loops, make this sort of work hard to do even for an experienced tech as it is very easy to damage the parts you have previously replaced if you do not check things well enough before firing the unit up. Softstarting with a variac or some sort of limiting in the power supply are generally the safest ways to approach this sort of repair. If you dont think you are up to it, refer the work to an experienced audio tech, or hitme back here and I can assist you if you wanna have a crack at it. Cheers. A "fixya" rating would be great for my 2 cents worth here. :)
Posted on Jan 19, 2008
I've got one on the bench at the moment which has all four output devices shorted on one channel - I found this site while looking for a schematic without success. I've traced out the main elements of the circuit so I can understand it better and I can tell you the trimmer is to set the standing bias current on the output stage ( this should be about 40mA, measured where the positive fuse would go if the amp was working OK ).
This is a pretty awful circuit, using all N type Hexfets, ( IRFP240 ) which shouldn't really be used for audio at all. I assume it's just for cheapness. These devices are notorious for thermal runaway, so make sure you keep the ventilation clear when you rebuild it and make sure the fan works properly.
I agree with the advice about 'soft-starting' the unit - especially if you haven't replaced all the other devices at the same time. I use a couple of 40Watt light bulbs wired across some old blown 20mm fuses so I can quickly insert them in the circuit of anything I'm repairing.
They only drop a few volts if the circuit is OK, but they limit the maximum current to half an Amp or so if there's a problem. That's usually enough to let you turn it off before the smoke gets too bad...
Obviously, they'll do the same if you try to drive a signal through the amp once it's fixed, so it's best to check the circuit with a line-level sine-wave signal and an oscilloscope with no load to make sure the waveform isn't clipped or distorted before you fit real fuses and connect a dummy load.
The signal test should then be repeated so you can check the current drive is OK. It's also handy on the Soundlabs to check the 'PEAK' indicator lights just below the level where the signal gets clipped. Driving speakers with a clipped signal greatly increases the power they receive and is the biggest cause of early failures.
Hope this helps, Graham.
Posted on Mar 24, 2008
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