Dr. Hi Fi

By Appointment Only.
Level 8, Suite 29, 100 Walker Street North Sydney


Service Bulletin

  1. Amplifiers
    1. How Amplifiers are Damaged
  2. Speakers
    1. Perished Foam Rubber Surround
    1. Poling Speakers
    1. Speaker Damage from Amplifiers
    1. Warranty of Speaker Repairs
  3. Compact Disc Maintenance
    1. Caring for Compact Discs
    1. Warranty of Compact Disc Repairs
  4. Turntables
    1. Balancing the Tone Arm
    1. Hum Caused by an Earth Loop
    1. Acoustic Feedback
    1. Stylus Cleaning and Checking
  5. Cassette Decks
    1. Chewing of Tapes
    1. Inability to Record on a Cassette
    1. Connection of a Tape Deck to an Amplifier
  6. Radio Reception
    1. FM Reception
    1. AM Reception

Amplifiers

How Amplifiers are Damaged

The most common form of damage to an amplifier is caused by a short circuit of the speaker wires. The damage will often be extensive.

Two wires go to each speaker. A short circuit occurs when two wires touch each other. The short circuit need only be momentary, with perhaps only two strands of the multi-strand cable.

Internal image of a fried audio device.Some of the most common occurrences of short circuits are where:
  1. - frayed wires touch each other at where they connect to either the amplifier or the speaker
  2. - speaker wiring has been joined
  3. - thick "monster" cable is connected to the amplifier or speaker terminals which cannot readily accept such thick wire
  4. - a staple (or other such device) retaining the wire has gone through the plastic insulation of the cable
  5. - constant pressure on the wire (from treading, heavy pieces of furniture, door jambs etc.) damages the plastic insulation
  6. - pets have chewed through the wires

IMPORTANT: If a short circuit is suspected for any reason, replace the entire length of speaker wire.

Short circuits can be avoided by extremely neat termination of the wires to both the amplifier and speaker. Remove not more than one centimetre of the plastic insulation from the cable and then tightly twist all strands together so there are no loose strands. Having connected them to both amplifier and speakers, ensure there is no exposed wiring.

Do not join speaker wiring. The cost of replacement is much less than that of repairing a badly damaged amplifier. If using thick "monster" cables, use the appropriate connectors to terminate them.

Internal image of a fried audio device.

IMPORTANT: It is imperative that should a short circuit on the speaker wiring have caused damage to your amplifier necessitating repair, the short circuit MUST be corrected BEFORE re-using the amplifier. If not, the amplifier may again be damaged and this is NOT COVERED BY ANY WARRANTY. As such, the above instructions must be followed without fail.

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Speakers

Perished Foam Rubber Surround

Most of us never realise that our speakers may be performing well below their ability. This is because many speakers deteriorate very gradually over time, so we do not notice the performance degrade.

The common problem is that the foam rubber that delicately suspends the cone perishes. This causes flabby and reduced bass with little definition.

Image of a perished speaker surround.

The coil (attached to the cone) moves inside a very small gap within the magnet. When it is not suspended correctly, it can rub against the magnet causing potentially serious damage to your amplifier and risks permanent damage to the speaker.

Fortunately, at Dr. Hi Fi we can usually do a very thorough job in repairing speakers. Rather than just replace the foam rubber (which some firms do), we first remove the cone and spider entirely to inspect the voice coil. Any damage to the coil is usually repairable with some delicate surgery (it may be rewound by hand). The cone is then re-mounted and centred precisely with a new high quality foam rubber surround.

It is a great joy when you get the speakers back home. You will be astounded at how much better they now sound (and realise how sick they were!).

The deterioration is primarily caused by humidity. Foam rubber is generally used on better quality speakers as it provides ideal suspension which is critical to a speaker's performance. Some types of foam rubber can deteriorate significantly in only a few years, so it is a good idea to check speakers regularly. Sometimes only one may clearly show deterioration, but it is always recommended to do both speakers.

Image of a perished speaker surround.

Checking most speakers is easy. The front panel is designed to be removed by gently pulling it away. Look for cracks in the foam rubber around the outside of the larger speaker cones. Sometimes the foam rubber will become tacky to the touch. If you detect a fault, have it repaired quickly as this will minimise further damage (that can be permanent).

Poling Speakers

If your speakers sound distorted at low levels, or slightly "squawky", then they may be suffering from a fault called "poling". This is where the coil rubs against the magnet as it moves in and out. It can occur to any speaker.

The best way to check for poling is to listen to some solo piano music, at a very low volume. Put your ear up to each speaker in the box and listen for distortion.

There are many causes of poling. Bring the speakers into Dr. Hi Fi for repair as soon as possible, as use risks damage to the voice coil.

Speaker Damage from Amplifiers

Speakers themselves rarely develop problems other than those described above. The other main cause of damage is of course playing them too loudly! Speakers that suffer from this "disease" are easily diagnosed when the voice coil is removed and inspected.

It is a misconception that if a 50 watt speaker is driven by a 25 watt amplifier, that the speaker can therefore handle all of the amplifier's power. When the volume is turned up to a level where the amplifier is running out of power, the amplifier distorts the signal. The distortion component of the signal can then damage the speaker, especially tweeters. Of course, an amplifier of 100 watts can damage a 50 watt speaker by overload.

Put simply, any amplifier can damage any speaker, irrespective of power ratings. Ideally, an amplifier should be as powerful as possible, and controlled by a sensible user! When the volume is turned up, there will come a point of audible distortion. Whether the distortion is caused by the amplifier or speaker is irrelevant - the maximum safe volume for the hi fi system has been reached, and the volume should be turned down to safeguard against damage.

Amplifiers can develop a fault that can destroy a speaker in a brief instant. This occurs when the amplifier blows up and goes what is known as "D.C." (often a loud hum is heard). Most amplifiers have a protection system designed to shut down the output of the amplifier to the speakers when this problem occurs. Sometimes it doesn't work quickly enough. It is a common misconception that amplifier protection systems are designed to protect the amplifier. They are designed to protect your speakers from the damaged amplifier. If an amplifier has gone "D.C." it must be repaired, often with the speakers.

Warranty of Speaker Repair

As with all repairs at Dr. Hi Fi, they are guaranteed. Naturally, that does not include speakers damaged through excessive volume levels or if an amplifier has caused the fault and not been repaired at the same time.

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Compact Disc Maintenance

Caring for Compact Discs

The main advantage of Compact Discs over vinyl records is that a healthy player itself will not wear or damage the compact discs. In most other respects, they are the same.

Compact discs must be handled with much care. They should be kept very clean, free of dust and finger marks. The player itself must not be allowed to collect dust.

Close up of a CD player.

Dust is the enemy of compact discs. Being plastic, CDs often attract dust. Care is needed in handling them. They can scratch easily, causing problems that may be worse than a scratched vinyl record.

When dust gets into a compact disc player (through its ventilation holes or via the disc), the damage can be permanent. Dust on the laser's lens can be removed; dust that settles in the cavity in which the lens sits can often not be removed. The lens is delicately suspended within this cavity, and moves up and down to focus on the disc, and sideways to track the disc.

Warranty of Compact Disc Repairs

With compact disc players, a variety of faults can cause the common symptom of mistracking or inability to correctly read the compact disc. Diagnosis can be an involved procedure as each possible cause must be assessed and repaired.

The problem may be simply dust on the lens. Next time, however, the laser itself may produce the identical fault.

As such, with these symptoms, repairs of compact disc players are only guaranteed if the laser is replaced.

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Turntables

Balancing the Tone Arm

Usually a dial at the rear of the arm indicates the tracking weight in grams. For this to be accurate, it must first be calibrated. This is simply done by adjusting the weight so that the arm floats or is balanced. This represents zero weight, so without further moving the weight, adjust the dial to read "0". It is now calibrated. Then adjust the weight (which moves the dial) to the desired tracking force for the stylus, commonly around 2 grams.

Hum Caused by an Earth Loop

The signals from the output of a phono cartridge are minute in level and require many more times amplification than those of tape decks, tuners and C.D. players. To reduce 50 cycle mains "hum" being picked up by the turntable wires (and thereby greatly amplified), special attention is needed to earth the turntable and amplifier together. This is done usually by a single wire from the turntable which connects to an "earth" or "ground" terminal on the amplifier, normally located near near the phono input of the amplifier.

Acoustic Feedback

Just as a microphone held close to a speaker causes a high pitched "squeal", the same scenario can occur if a turntable is close to a speaker. This is called acoustic feedback and is usually a low pitched sound which may increase in volume. It is caused by the speaker's output directly vibrating the stylus which then sends that signal back to the speaker through the amplifier, causing a feedback or loop effect.

The vibrations from speaker to stylus may be transferred not just through the air but also through the floor and cabinet to which the turntable is attached. This is particularly so with wooden floor boards or where the speaker sits on the same shelf as the turntable.

To reduce acoustic feedback, keep the turntable and speakers as far apart as possible. Avoid the speaker facing the turntable. A turntable should sit on a solid surface, preferably a shelf mounted to the wall, not on floor standing cabinets. Close the lid on the turntable and avoid playing records loudly. Severe damage to speakers (and possibly the amplifier) can result from excessive acoustic feedback.

Stylus Cleaning and Checking

A worn stylus can irreparably damage records, with the damage cost far exceeding the cost of replacing the stylus.

A stylus will soon become putrid as it "scrapes" along the many metres of record grooves. The build up of rubbish will severely affect the sound quality.

When replacing your stylus, have it inspected to see how it has worn. This provides an indication of whether the turntable's arm needs adjusting, especially the anti-skating force.

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Cassette Decks

Chewing of Tapes

In more than 50% of cases, the cause of a chewed cassette is the cassette itself, not the machine. This is because the mechanics within the cassette (usually the spools) may become stiff. When this occurs, even the best cassette deck will still chew the tape.

Unfortunately, all too often pre-recorded cassettes utilise inferior mechanisms. With blank cassettes one should avoid cheap ones; expect to pay at least $7+ for a C90 cassette. Do not expose cassettes to heat or sunlight (common in cars) nor moisture.

Close up of a Cassette Deck.

As a cassette is being chewed, the sound will usually become garbled and the pitch may seem to alter. Try to detect this and immediately stop the machine and remove the cassette to avoid damage to the machine itself. If the cassette will not come out do not force it as this may compound the damage.

Inability to Record on a Cassette

At the rear of all cassettes are two plastic tabs, which, if removed, prevent recording by not allowing the record button or key to be engaged. (The front of the cassette is where the tape is exposed.)

The tabs are designed to be removed in order to prevent accidental erasure of a valued recording. Each tab relates to one side of the cassette. Looking at the cassette with the front towards you, the tab on the left governs recording on the side of the cassette facing up.

The tabs are often accidentally broken or pushed in, thereby preventing recording. This can be remedied by covering the slot with cellophane or vinyl tape.

Connection of a Tape Deck to an Amplifier

Tape DeckAmplifier/ Reciever
Line Out, Play Back or PB (connects to)Play Back, Tape In or Mon
Line In, Record or Rec (connects to)Record, or Tape Out

IMPORTANT: Care should be taken to observe left and right channel connections. As a standard, red is usually connected to the right channel. The other colour is therefore used for the left channel.

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Radio Reception

FM Reception

FM radio reception is similar to television reception. A good "clean" signal is needed for quality stereo sound.

FM reception can suffer from the effects of "ghosting" where the same signal is received twice - once directly from the transmitter and the second signal a fraction of a second later after it has bounced off a building for example. This is also called Multi-Path reception. A "clean" signal is where only one signal is received.

If the signal strength is good, yet there is distortion, then the chances are multi-path is to blame. If moving the antenna does not help, then a directional antenna is needed, which is pointed at the transmitter, just like a TV antenna. Quite often using an existing VHF TV antenna will solve the problem. In the worst cases a separate external antenna for FM may be needed.

Many are fortunate in that the supplied internal or indoor FM antenna is sufficient. These come in various forms such as a one metre length of wire, a "T" shaped antenna made of flat ribbon cable or a telescopic metal whip antenna.

Should the internal antenna not be satisfactory, then the next step is to try connecting the receiver to an external VHF TV antenna. This requires normally a connection to a wall socket using coaxial cable (a "splitter" may be needed). Failing that, then an external antenna for FM may be required.

It should be noted that antenna boosters are useless if you are have receiving multi-path signals. Boosters are rarely required in metropolitan areas.

Digital Radio, whilst it may provide better reception in some problem areas, is no substitute for the quality of the FM analogue signal. Possibly, this may change in the future.

Close up of a recording panel on an audio device.

AM Reception

As AM radio reception is directional, the orientation of the receiver's antenna will significantly affect reception quality.

With most modern tuners or receivers the aerial is an external device, often connected to the receiver with two wires (one goes to the "Ant" terminal and the other to a "Gnd" terminal). The antenna can then be moved for good reception and hopefully placed in a convenient spot. Sometimes the aerial is a rod like device on the back of the receiver. This can usually be rotated for optimum reception. If the antenna is within the receiver then you must position it for good reception (often inconvenient!). An alternative is to run a length of wire (at least five metres; any type of insulated wire is fine) which connects to the AM Antenna terminal on the back of the receiver.

AM radio is more susceptible to interference than FM. This can be generated by large electrical appliances (fridges, lifts in nearby buildings). Changing the direction of the antenna can reduce the level of interference.

Digital Radio may provide superior reception (as well as variety) than conventional AM analogue signals.

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