Intermittent faults are perhaps the most annoying type of faults one comes across when working on electronics. Whilst they occur in various fields, they are commonly found in the domain of electronics and can be a right royal PITA!
Intermittent faults are best described as faults that:
- Come and go, they are not always present
- Can be difficult to trigger or replicate
- Maybe modulated by triggers like temperature or voltage
Why indeed! There are a few reasons for the appearance of intermittent faults:
- Solder joints may degrade over time and become electrically or thermally intermittent
- Capacitors and resistors change both as they age and as they warm up during operation, altering their performance
- Transistors and diodes may develop internal leakage, causing gain changes and noise. This may present as popping, rushing, crackling, whistling and even silence. They may also cause DC offsets, tripping equipment protection circuits.
- Electronic parts don’t generally show obvious signs of deterioration as they age. They must be tested, often under dynamic conditions, to establish functionality and health. There may be hundreds of electronic parts in a piece of equipment…
Transistors are most likely to cause really painful intermittent faults and some types are more prone to doing so, a list of which I keep both electronically and in my brain! You don’t want to replace every transistor in the hope of resolving something like this. That’s a costly, wasteful process that may even change the sonic character of the equipment.
With the component level repair approach 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 always want to find the cause of the problem and eliminate it. Paradoxically, even reworking the 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.
Pain & Pleasure
Resolving these faults can be technically challenging, especially if the fault disappears for a while as they sometimes can. There are ways and means of ‘coaxing’ these faults out of hiding, but equipment will occasionally need a couple of visits for really problematic intermittent faults to be exterminated! Whilst these intermittent faults can be painful, they are also extremely satisfying to resolve and you have to resolve them to keep the gear that we love alive.