|
Bronze Member Username: OccamPost Number: 30 Registered: Jul-05 | If you own an older receiver/amplifier is it a good idea to replace the capacitors? Do they naturally degrade with age and effect performance? |
|
Gold Member Username: Jan_b_vigneDallas, TX Post Number: 4645 Registered: May-04 | Yes, power supply caps first. |
|
Bronze Member Username: OccamPost Number: 31 Registered: Jul-05 | Are there signs that indicate you should think about replacing the caps? I do not play my stereo especially loud--is this when one would notice? |
|
Gold Member Username: Jan_b_vigneDallas, TX Post Number: 4651 Registered: May-04 | There are not really any signs that a cap will go bad, only signs that they already have. Any sort of leakage around the base or terminals of the cap means it has done its business and is ready to move on. Open circuits in caps are generally indicated by a 60Hz ground hum through the system. But a cap can go bad at any time with no signs of problems beforehand. I recently had that experience and described it on this thread https://www.ecoustics.com/electronics/forum/home-audio/139173.html at the post made on Sunday, April 10, 2005 - 03:11 am. In general any time an amplifier has sat unused for a few years, caps have a tendency to go bad from drying out. Otherwise, most people who deal in vintage gear consider a cap's useful life to be about 20 years. There are also some who feel no need to replace a cap until it goes out. That's living somewhat dangerously as a cap can take out other components when it leaves this mortal coil. |
|
Silver Member Username: EramseySouth carolina United States Post Number: 293 Registered: Feb-05 | Actually capacitors are in effect an open circuit. Current appears to flow through them as it migrates from one plate to the other during charging and discharging. A capacitor will always charge at an exponential rate which equates to five time constants per second. Polarized DC capacitors are used extensively in home audio equipment and most electronics for that matter are a "dry" type. Most are usually made of mylar or other plastic and contain borax powder that is held in place by a thin layer of gauze. The borax powder acts as the dielectric adhering to one plate or the other depending on the charge state. A capacitor is a voltage driven device that stores a charge in the form of an electrostatic field. AC capacitors,particularly oil filled ones, will definately leak over time. The greatest problem with DC caps is that over time the casing of the cap itself,as well as the dielectric, will begin to deteriorate from the constant cycle of charging and discharging,which of course produces heat, sort of like the tape winding in a transformer will over time begin to deteriote from the constant heating and cooling cycle from the flow of current and then being shut down when power is removed. Think of a capacitor as a bow string that is pulled tight then released over and over eventually over time the string or the bow will break. In an inductor the voltage leads the current and they are out of phase by 90 deg. In a capacitor the reverse is true. |
|
Bronze Member Username: OccamPost Number: 32 Registered: Jul-05 | I understand that the power supply cap removes the 60Hz hum before the current enters the amplifier tube, but are there further caps in the amplifier circuit? |
|
Silver Member Username: EramseySouth carolina United States Post Number: 294 Registered: Feb-05 | Neil: by the time current reaches the supply cap it has been changed into a DC current known as pulsating or ripple by a device called a rectifier. Usually hum in a cap is caused by it being improperly or poorly grounded. DC caps are used extensively in an amplifier for supply, filtration,coupling and voltage regulation. |
|
Gold Member Username: Jan_b_vigneDallas, TX Post Number: 4655 Registered: May-04 | The filter caps which remove 60Hz AC cycling are normally placed in parallel with the storage supply caps which act as a reservoir for the voltage and current the amplifier calls on to handle the dynamics of the signal. These large electrolytic storage caps are "wet" and will dry out or leak over time. Technically Eric is correct in his assessment of capacitors as "open circuits" since there is no physical contact within the cap from input to output. And I am incorrect to use the idea of an "open circuit" in a cap. Either way you look at it, the end result of a bad cap almost always manifests as 60Hz hum being present on the outputs unless the results are more catastrophic as in my amplifier. There are caps which act as filters, caps which act as coupling devices between the gain stages of the amplifier, caps which block DC from the outputs and, as Eric said, caps which act as voltage regulation in the power supply section of the amplifier. There are also caps which act as "bypass" capacitors when used in conjunction with larger, "slower" caps. This is what blew out in my power amplifier, a small bypass cap. A very small capacitor made a very large noise! In a twenty year old amp, I would just about always replace the large electrolytic power supply caps merely out of principle. If the amp has sat for any length of time these capacitors should be brought up to operating temperature and current slowly on a Variac. This will give the best chance of the caps surviving the large amounts of voltage they receive when the amp is turned on. It is not uncommon for an electrolytic power supply cap to seem OK for the first week or so after powering up from a long storage and then develop a problem as the interior of the cap deteriorates from the heat/cool cycle. Fortunately power supply caps will normally give a 60Hz warning signal which gets louder over time before they finally need to be replaced. Here's an article which might help you understand amplifier basics: http://sound.westhost.com/amp-basics.htm |
|
Silver Member Username: EramseySouth carolina United States Post Number: 295 Registered: Feb-05 | The US military did a study during the 1970's in which they found that an electrolytic cap,specifically a DC cap if operated at half or less of it's maximum rated voltage will increase it's life by 5 fold. |