Hey guys,
A friend's dad just gave me a Sherwood MOSFET power amp. It's been broken for a while now, as he replaced it several years ago with a Hafler DH-200 amp. Prior to replacing it, he got it serviced from an authorized service center who replaced all of the output MOSFETs in one channel. But now it's toast, apparently...
I would love to get this amp working again, but I have read that the MOSFETs are hard to find. The symptoms are not numerous: all of the outputs have several (between .5V and 3V) volts of offset. Not too good...I haven't checked it's actual capability as far as driving a load, as clearly there is a problem. (and I don't want to risk burning out some speakers!)
Where should I start with this amp? I have plenty of voltmeters, an oscilloscope and a signal generator. I only have one probe, though.
Where can I find a schematic for this beast?
Thanks!
Kyle
A friend's dad just gave me a Sherwood MOSFET power amp. It's been broken for a while now, as he replaced it several years ago with a Hafler DH-200 amp. Prior to replacing it, he got it serviced from an authorized service center who replaced all of the output MOSFETs in one channel. But now it's toast, apparently...
I would love to get this amp working again, but I have read that the MOSFETs are hard to find. The symptoms are not numerous: all of the outputs have several (between .5V and 3V) volts of offset. Not too good...I haven't checked it's actual capability as far as driving a load, as clearly there is a problem. (and I don't want to risk burning out some speakers!)
Where should I start with this amp? I have plenty of voltmeters, an oscilloscope and a signal generator. I only have one probe, though.
Where can I find a schematic for this beast?
Thanks!
Kyle
Yeah, I measured the seemingly appropriate rail voltage across them, with 3.029V (or something like that) on each gate. They would "seem" okay, I guess.
Without a schematic, however, I am pretty much hopeless. I see a few pots, but I'm not going to go messing with them until I know what they are.
Also, how much DC bias is too much? My McIntosh MA 6100 has <20mA per channel...is that okay?
Kyle
Without a schematic, however, I am pretty much hopeless. I see a few pots, but I'm not going to go messing with them until I know what they are.
Also, how much DC bias is too much? My McIntosh MA 6100 has <20mA per channel...is that okay?
Kyle
I wouldn't mind tracing it. However, I am definitely not knowledgeable enough to do it on my own. I wouldn't know where to expect one voltage or another, to be honest. Unless something is getting burning hot, I wouldn't really know what's wrong or not. And it seems as though I don't have a good channel to compare it to!
Could I get a few resistors and use them as dummy loads just to see if the thing even amplifies? Measuring across a dummy load with my 'scope would seem like a good start.
I'll try and get some pictures soon. One thing I do know about this amp: it was NEVER abused, and I got it from the original owner. He takes very good care of all of his equipment, and it really does show! This thing (except for a lot of dust on the inside) is absolutely beautiful!
Kyle
Could I get a few resistors and use them as dummy loads just to see if the thing even amplifies? Measuring across a dummy load with my 'scope would seem like a good start.
I'll try and get some pictures soon. One thing I do know about this amp: it was NEVER abused, and I got it from the original owner. He takes very good care of all of his equipment, and it really does show! This thing (except for a lot of dust on the inside) is absolutely beautiful!
Kyle
Yep, in fact a dummy load is recommended 
You could get some 4.7 ohm power resistors, mount them to a heatsink or even a bit of metal, blow air across em to keep them cool. Mount in series for 8 ohms ive seen dummy loads built with 4x4.7 ohm 50W resistors, the aluminium clad types, with sockets so you can put them in series and parallel combinations to get 4, 8, 16 etc.
The scope will then show you if the waveform is good. Squirt a sine wave into it and see how it comes out. If it's distorted this can often be a clue as to where the problem is.
You could get some 4.7 ohm power resistors, mount them to a heatsink or even a bit of metal, blow air across em to keep them cool. Mount in series for 8 ohms ive seen dummy loads built with 4x4.7 ohm 50W resistors, the aluminium clad types, with sockets so you can put them in series and parallel combinations to get 4, 8, 16 etc.The scope will then show you if the waveform is good. Squirt a sine wave into it and see how it comes out. If it's distorted this can often be a clue as to where the problem is.
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