I have a pile of suspect Lateral mosfets and I need to find a way to easily test them for damaged gates.
I have an old beat up Sound Code Systems 2350A amp. it's basically the same circuit as a Soundcraftsmen PCR amplifier minus the goofy power supply. Same circuit topology anyway. It exhibits an odd problem that I suspect is due to bad/damaged mosfet gates.
At 1watt into 8 ohms 1khz the distortion in each channel is .003-ish percent. but. at a very specific repeatable point, each channel will show oscillation on the waveform. it's equal to about 25vrms or 83 watts. below that point, and it is a sharp break over point, the amp works flawlessly. I don't particularly care about fixing the amp. I am only interested in the mosfets from the amp.
Now in the past when I have had old haflers and the like that were abused and had damaged gates on the mosfets, they would just oscillate all the time over a signal at any level. SO, I am looking for a way to build a simple test jig to test each mosfet as well as the pile I have on hand and determine if they are ok. I do not have access to a curve tracer at this time so I need to devise a test jig and I am looking to those with more knowledge then I to offer suggestions as such.
Please advise.
I have an old beat up Sound Code Systems 2350A amp. it's basically the same circuit as a Soundcraftsmen PCR amplifier minus the goofy power supply. Same circuit topology anyway. It exhibits an odd problem that I suspect is due to bad/damaged mosfet gates.
At 1watt into 8 ohms 1khz the distortion in each channel is .003-ish percent. but. at a very specific repeatable point, each channel will show oscillation on the waveform. it's equal to about 25vrms or 83 watts. below that point, and it is a sharp break over point, the amp works flawlessly. I don't particularly care about fixing the amp. I am only interested in the mosfets from the amp.
Now in the past when I have had old haflers and the like that were abused and had damaged gates on the mosfets, they would just oscillate all the time over a signal at any level. SO, I am looking for a way to build a simple test jig to test each mosfet as well as the pile I have on hand and determine if they are ok. I do not have access to a curve tracer at this time so I need to devise a test jig and I am looking to those with more knowledge then I to offer suggestions as such.
Please advise.
Laterals are not supposed to do that. Verticals will often fail that way...
It depends on the circuit.
I have always used zeners on the gate to source to limit current through them.
ohh I like that method. I will experiment with that!
After further testing. I found the problem was with my test setup. I found out that this particular amplifier does not like having one phase of the balanced input shorted to ground. it is an old SCS 2350A amplifier with 1/4" balanced inputs. I was connecting the output of my AP from the BNC output to the 1/4" input with a Mono plug adapter and it did not like that. as the BNC output is floating. changing how things were wired corrected the problem.
I just recently acquired two 2600A amplifiers in good working condition (so far, about 10 hours on them).
I've had a technician friend due a cursory check and all caps are spot on. We changed the XLR mono input connector on each amp and check some soldering points.
As I am using these with my main speakers, is there anything I should have checked/done to ensure these "very vintage" amps don't hurt my speakers?
Thanks for any feedback!
I've had a technician friend due a cursory check and all caps are spot on. We changed the XLR mono input connector on each amp and check some soldering points.
As I am using these with my main speakers, is there anything I should have checked/done to ensure these "very vintage" amps don't hurt my speakers?
Thanks for any feedback!
I have an scs2350 here that has the problem breaking into oscillations/massive distortion after just a watt or two. From the melted condition of the capacitors on the output zobel, I assumed that the gates were blown, but haven’t confirmed that.
An added external speaker protection relay would be a good idea on these amps.
An added external speaker protection relay would be a good idea on these amps.
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