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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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Static on amplifier, not volume related
Hi,
I have done a quick search but couldn't find a problem that was very similar to mine in previous posts, so here goes..... I have just bought a 30 year old amp (Rogers Ravensbourne Stereo) and it's got a bit of a problem. There is a continuous static noise coming from the speakers (one channel more than the other). You can also see the cones in the speakers moving in and out slowly along with the noise almost like applying a DC voltage. The amp works OK otherwise, the noise seems to be from the output stage as volume, bass, treble, input selection etc make no difference. I have whipped the top off and can't see any obviously fried components. There are some thermistors on the power supply that look pretty tatty but I shouldn't think these would introduce a static type noise. Thanks Dean |
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Dean wrote:
Hi, I have done a quick search but couldn't find a problem that was very similar to mine in previous posts, so here goes..... I have just bought a 30 year old amp (Rogers Ravensbourne Stereo) and it's got a bit of a problem. There is a continuous static noise coming from the speakers (one channel more than the other). You can also see the cones in the speakers moving in and out slowly along with the noise almost like applying a DC voltage. The amp works OK otherwise, the noise seems to be from the output stage as volume, bass, treble, input selection etc make no difference. I have whipped the top off and can't see any obviously fried components. There are some thermistors on the power supply that look pretty tatty but I shouldn't think these would introduce a static type noise. Thanks Dean It sounds like the amp is oscillating. Deteriorated capacitors could cause that, and wouldn't be surprising in a 30 year old amp. Got a 'scope? -- The e-mail address in our reply-to line is reversed in an attempt to minimize spam. Our true address is of the form . |
#3
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"Dean" wrote in message
om... Hi, I have done a quick search but couldn't find a problem that was very similar to mine in previous posts, so here goes..... I have just bought a 30 year old amp (Rogers Ravensbourne Stereo) and it's got a bit of a problem. There is a continuous static noise coming from the speakers (one channel more than the other). You can also see the cones in the speakers moving in and out slowly along with the noise almost like applying a DC voltage. The amp works OK otherwise, the noise seems to be from the output stage as volume, bass, treble, input selection etc make no difference. I have whipped the top off and can't see any obviously fried components. There are some thermistors on the power supply that look pretty tatty but I shouldn't think these would introduce a static type noise. Thanks Dean You didn't say wherther the amp is tube or solid state, but given the age, I'm assiming solid state. You also didn't state whether the transistors are in sockets or hard-wired. If they are in sockets, start there by removing the transistors and renew the contact surfaces by removing/inserting the transistors a few times to break up the contamination. If that doesn't solve the problem, there are a number of possibilities for the cause of the static. Electrolytic capacitors are always suspect in equipment that is old as your amp, especially if it has been in storage for more than a few years. Paper capacitors are also suspect. Resistors, especially carbon composition types are not immune from causing this type of problem. Even the transistors can become noisy and cause the static. Thermistors in the power supply?? Don't think I've seen that very often... more likely the thermistors are near the output transistors for bias stabilization. Other things to look for include weak or stressed solder joints and components that have gold plated leads. The plating is known to separate from the lead and cause a myriad of problems, including the type you're experiencing. -- Dave M MasonDG44 at comcast dot net (Just subsitute the appropriate characters in the address) Never take a laxative and a sleeping pill at the same time!! |
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"Tweetldee" wrote in message news:OP4wd.266447$R05.42256@attbi_s53... "Dean" wrote in message om... Hi, I have done a quick search but couldn't find a problem that was very similar to mine in previous posts, so here goes..... I have just bought a 30 year old amp (Rogers Ravensbourne Stereo) and it's got a bit of a problem. There is a continuous static noise coming from the speakers (one channel more than the other). You can also see the cones in the speakers moving in and out slowly along with the noise almost like applying a DC voltage. The amp works OK otherwise, the noise seems to be from the output stage as volume, bass, treble, input selection etc make no difference. I have whipped the top off and can't see any obviously fried components. There are some thermistors on the power supply that look pretty tatty but I shouldn't think these would introduce a static type noise. Thanks Dean You didn't say wherther the amp is tube or solid state, but given the age, I'm assiming solid state. You also didn't state whether the transistors are in sockets or hard-wired. If they are in sockets, start there by removing the transistors and renew the contact surfaces by removing/inserting the transistors a few times to break up the contamination. If that doesn't solve the problem, there are a number of possibilities for the cause of the static. Electrolytic capacitors are always suspect in equipment that is old as your amp, especially if it has been in storage for more than a few years. Paper capacitors are also suspect. Resistors, especially carbon composition types are not immune from causing this type of problem. Even the transistors can become noisy and cause the static. Thermistors in the power supply?? Don't think I've seen that very often... more likely the thermistors are near the output transistors for bias stabilization. Other things to look for include weak or stressed solder joints and components that have gold plated leads. The plating is known to separate from the lead and cause a myriad of problems, including the type you're experiencing. Rather than thermistors they're probably posistors, they have a higher resistance when cold which drops much lower as they self heat. Common in larger audio amplifiers to reduce the inrush current when first turned on. |
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