Saturday, October 24, 2009

The Sounds of Failure

As the title suggests this will be an attempt to document the sounds of failure in hifi gear. This will no doubt be just a start, so I will revisit this entry as I learn more. This is not a simple topic. In our discussions Mike emphasized there is even variability between manufacturers, eg. failing preamp transistors sound a bit different in Marantz vs. Pioneer receivers.

Perhaps the most common sound of failure is that scratchy noise you hear when you adjust your volume, balance, bass or treble control. You turn the knob and it sounds like gravel in there. The good news this is caused by the accumulation of oxidation on the contacts and this is easily eliminated by a product such as Caig's Deoxit. The bad news is that your controls are not always easily accessible. Many integrated amps and receivers require peeling the face of the unit, which means pulling all the knobs and removing the retaining nuts.

Oxidation is not limited to potentiometers. Switches (eg. the push button FM mute or rotary function selectors) suffer the same problem, but the characteristic sound tends to be lower volume, distortion or silence. The solution is the same: douse the switch with a product such as Deoxit.

The whole process of cleaning the controls of a typical receiver is worthy of a post of its own.

One switch that is not obvious to the typical listener is the relay. Relays are circuit protection devices and are typically buried in the heart of the amplifer and they are prone to oxidation. This can be manifested in a weak, distorted or missing channel. When working properly they make that click a few seconds after you turn on your receiver. They are essentially little copper fingers, at least one per channel, that make the final connection after the protection circuitry has determined all is well with the unit. Relays are typically plastic enclosed cubes soldered to the circuit board and are not dousable with Deoxit. They typically need to be removed from the board and the plastic cover removed, and then they are burnished with a very fine file or sand paper. Again, this process is worthy of a post of its own.

Next up: silence. Oxidation as described above can be severe enough to kill one of both channels. I have encountered several slightly corroded fuses that have silenced a my gear. Don't forget to check your fuses!

More seriously, two dead channels suggests something amiss in the power supply. This is typically capacitors, but can be any number of devices. The power supply takes AC from the wall and transforms it into the various DC outputs that drive each section of the receiver, so trouble here and take the whole unit down.

One channel of silence can still be the power supply, but can indicate a problem anywhere in the preamp or amp.

In a comment Kenny asked about 'rushing and hissing' in a vintage European integrated amp. Rushing and hissing can be caused either by faulty caps or transistors, especially in the preamp. Mike and Robert have been preemptively replacing the preamp low noise-high gain transistors in many of their restorations in addition to the usual cap replacement. Robert describes the sound as 'spitty', but Mike emphasizes the wide variation in the sound of preamp transistor failure.

A very striking sound of failure is that of the differential transistors in the power amp. Mike calls this tiny pair the keystone of the power amp. Their failure results in a very distinctive and violent spike of noise.

When I get a chance I will update this post with a discussion of hum (fascinating!), DC in the output and much, much more...

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