surfatwork
Well-Known Member
Just thought I'd share a little DIY repair story.
Some months back I had purchased an Audio Research LS2 from a FM. Not that I needed another pre, but you know how it is - and I have always loved the AR design aesthetic. It worked absolutely fine for many moons, until it developed an annoying habit of cutting out. It was an intermittent problem, so reproducing it was difficult. So it lost it's place in the rack.
This weekend I dug it out, determined to figure out the problem. Got the service manual, and spent some time poring over the schematics. I suspected the power supply (since both channels were affected), or the tube (since it was common to both channels). Thought it was probably the muting circuit cutting in because something else was wrong.
Took off the covers, connected it up and let it play on a test system. I was waiting for it to stop. But it went on playing....and playing.....and playing. Never so much as stuttered. All the voltages checked out. To be honest, checking voltages was a bit harrowing - this thing produces close to 400V! and no, the HT is not shielded.
So I hummed and hawed, and finally decided that nothing was actually wrong. The likely culprit was that the heatsink paste had dried out - a mosfet in the HT regulator was literally boiling. It is supposed to get hot (since there is a significant voltage drop across it), but this was really really hot. But all components around it were within spec, nothing looked overly heated up, all voltages were fine. The only thing to do was to replace the heatsink paste and put it back.
And it now works absolutely fine. I can feel that the mosfet in question is cooler now.
So the lesson is that sometimes it isn't as complicated as one thinks.
Some months back I had purchased an Audio Research LS2 from a FM. Not that I needed another pre, but you know how it is - and I have always loved the AR design aesthetic. It worked absolutely fine for many moons, until it developed an annoying habit of cutting out. It was an intermittent problem, so reproducing it was difficult. So it lost it's place in the rack.
This weekend I dug it out, determined to figure out the problem. Got the service manual, and spent some time poring over the schematics. I suspected the power supply (since both channels were affected), or the tube (since it was common to both channels). Thought it was probably the muting circuit cutting in because something else was wrong.
Took off the covers, connected it up and let it play on a test system. I was waiting for it to stop. But it went on playing....and playing.....and playing. Never so much as stuttered. All the voltages checked out. To be honest, checking voltages was a bit harrowing - this thing produces close to 400V! and no, the HT is not shielded.
So I hummed and hawed, and finally decided that nothing was actually wrong. The likely culprit was that the heatsink paste had dried out - a mosfet in the HT regulator was literally boiling. It is supposed to get hot (since there is a significant voltage drop across it), but this was really really hot. But all components around it were within spec, nothing looked overly heated up, all voltages were fine. The only thing to do was to replace the heatsink paste and put it back.
And it now works absolutely fine. I can feel that the mosfet in question is cooler now.
So the lesson is that sometimes it isn't as complicated as one thinks.
