Electronics & Hi-Fi Problem Guide

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A few pointers on buying second hand electronic equipment that you need to be aware of:

 In general
Older equipment can fail whether it has been used or not! for instance:

1. All electronic assemblies age even when turned OFF, so unless it is of the highest professional standard do not buy if more than several years old unless you are prepared to deal with problems and do repairs.

2. Portable & transported Equipment can be wrecked inside, but still look great on the outside. Domestic equipment and the cheaper 'pro' portable sound equipment often have a similar (low quality) build standard so need to be handled very carefully. It is not uncommon for equipment to stop working after being transported so always check it asap after arrival. 

3. Due to European Directives, Lead may no longer be used for soldering except in certain types of equipment, so tin and other materials have to be used. These connections are generally less reliable as the regulations were brought into force before a satisfactory replacement for Lead could be found. For the consumer this means a much higher probability of soldered connection failure in 'modern' equipment.

4. Expected wear and tear on used electronic equipment could be:
'Noisy' controls - like a volume or tone control when rotated causing a rustling sound.
Switches - not operating reliably so need to be operated twice or more times.
Connectors - become intermittent and need to be 'moved' to bring back to 'life'.
Bulb Indicators - going dim or stop working if filament type.
LCD - displays losing contrast or fading.
Display Backlights - being dim as do computer displays in time.

5. Some problems that cause a lack of bass, punch and/or depth from speakers can be attributed to one or more of them being out of phase. The best way to check is to simply play music through one speaker and check the sound does not reduce in bass as any other speaker is turned up, a balance control can be used for this or reconnecting the other speaker before turning up the sound. If you suspect there is a problem the speaker phase may be tested with a small 1.5V battery if you can see the cone of the woofer. Disconnect the speaker from the amplifier and connect the battery directly across the speaker connection wires/terminals, the speaker cone should move forward with a 'click' as the battery positive is momentarily connected to the +/red terminal of the enclosure/speaker, all speakers must do the same. NEVER USE A HIGHER VOLTAGE AND DO NOT LEAVE THE BATTERY CONNECTED TO THE SPEAKER AS DAMAGE MAY OCCUR. It is possible the speaker terminal is marked incorrectly or confusingly as older JBL speakers may be identified on their +terminal with black marking, so this method provides a good way of being certain about the correct phase of the speaker.

Particular Problems with Electronic Equipment
The following problems are commonplace with (refurbishing) used equipment.

1. Electrolytic Capacitors - these 'dry out' and their value reduces. This can cause hum in power supplies, loss of 'punch' in amplifiers, frequency change in filters and oscillators, timing errors in 'delays'. When left unpowered electrolytic capacitors can even become de-polarized in time and short circuit.

2. All Capacitors -  Moisture can get into parts and with capacitors cause internal leakage. This is like adding a resistor in the part that can cause heating, if enough heat is generated the part may explode or burn, particularly with large 'can' types. This is why when refurbishing equipment all electrolytes are replaced. Old polyester parts also 'breakdown and suffer from high 'leakage' so these are normally changed as well. Other faults can cause sporadic noise and crackle from speakers, or evil smells emanating from the equipment.

3. Be VERY careful when cleaning electronic assemblies and use a  brush and light wind either blowing or sucking. Hi speed wind from aerosols can cause ultrasonic fractures in hermetic seals like those used with diodes and transistors. In time moisture can seep in and cause faults. Wind can also cause static charge build up  and these high voltages may damage parts.

4. Electrostatic discharge (ESD) can be lethal to electronic parts and assemblies so it is advisable to always 'ground' yourself and the workbench when handling them. Also wear cotton clothing/overalls as electrostatic charge can build up on nylon. Simply dropping a sensitive part can cause a high static charge to occur so in a professional working environment that part would be discarded. Sometimes the ESD only partially damages the ESD protection within a part, in which case that leaves it open to failure with the next ESD it encounters.
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