Intermittent faults are the most annoying type of faults exhibited by electronic equipment and the most challenging to resolve.

Features of intermittent faults include:

• They come and go, they are not always present
• They are modulated by certain triggers like temperature, voltage and humidity
• They can be difficult or unreliable to trigger or replicate
• When they don’t present, they are essentially not traceable
• They can be elusive and difficult to resolve

Causes

Intermittent faults can arise for various reasons:

• Solder joints degrade over time and may become electrically resistive and intermittent
• Capacitors and resistors change as they age and as they warm up during operation
• Customer equipment set-up may cause or trigger intermittent faults
• Transistors and diodes may develop leakage, causing gain changes and noise such as popping, rushing, crackling and whistling, or silence
• Semiconductor faults often cause DC offsets, which may trip protection circuits.
• Mechanical signal path contacts can oxidise and degrade, most commonly in switches, potentiometers and relays

Transistors are most likely to cause tedious intermittent faults, and some types are more prone to doing so. Electronic parts don’t generally show visual signs of deterioration, though; they must be tested, usually under dynamic conditions, to establish functionality and health.

You certainly don’t want to replace every transistor in the hope of resolving something like this. That’s a costly, wasteful process that may change the sonic character of the equipment and cause other issues when carelessly done, as it often is.

With the component-level repair necessary when working on older hi-fi equipment, one should always find the culprit/s and replace them, and others like them, where necessary. In other words, we want to find and eliminate the cause of the problem. Recapping almost never solves intermittent faults.

Paradoxically, even reworking dry joints associated with a bad transistor can sometimes ‘cure’ it for days or weeks, only for the problem to reappear. Fixing these problems is where we earn our money and lose our hair.

Fixes

Resolving intermittent faults can be challenging. There are ways and means of coaxing faults out of hiding, but equipment will occasionally require a couple of visits for the most problematic intermittent faults to be resolved. Whilst intermittent faults are tedious, they are also extremely satisfying to fix. Some of my best repairs involve resolving intermittent faults, like with this Technics SE-A5 power amplifier.

So, remember this, before blaming your technician for not fixing an intermittent fault: The intermittent and problematic nature of these faults is no more your technician’s fault than it is yours, or your mum’s! They are what they are, and whilst good technicians will be able to solve most, if not all, intermittent faults, customers need to work with their repairer to resolve them. Patience and understanding are helpful!