Yes, the good news is that, in almost all cases, transistors can be replaced.
The devil is always in the details though and the bad news is that people often get these details wrong, mostly because they are unaware of or don’t understand them.
Part of the art of repairing electronics involves learning and understanding what replacement parts to use in various scenarios and how to select the most appropriate substitutes where original parts are no longer available (NLA). Good repairers know which transistors can be used where and stock a wide range of parts to accommodate most equipment.
Substitution
Transistors come in thousands of different types, sizes and varieties. You can think of transistors as being like spark plugs or tyres. They all serve the same role, but each type is uniquely tailored to a specific job and use case. Substitute the wrong type and the circuit won’t work, or worse still, may catastrophically fail, taking other parts out with it.
Transistor failures, in the form of noisy or drifty devices, dead shorts or open devices, are a common phenomenon in old and new gear. Resolving transistor failures is commonplace here at Liquid Audio and getting this right, from a technical perspective, sets competent repairers apart. Look at case # 19 in the Hall of Shame for example to see how badly this can go wrong.
I’ve lost count of the number of pieces I’ve repaired where the problem has been incorrect replacement transistors, rather than dead ones. Incorrect or poorly matched parts can lead to excessive distortion, noise, overheating or premature failure. Therefore, a good working understanding of transistor specifications is critically important.
Stock
To ensure I have parts to suit most of the equipment we work on, I keep stocks of NOS and modern devices, including devices in TO-3 or TO-3P packages, TO-126, TO-220, TO-66 and TO-18 drivers, through to tiny little TO-92 devices, and everything in between.
Good repairers must have a range of such devices on hand, sourced through quality-assured supply chains rather than eBay for example. A quality-assured supply chain minimises warranty issues and maximises repair success rate. There is no point in taking chances here.
It gets trickier when we consider MOSFETs and VFETs, many of which are no longer available and lack any suitable replacements. That being said, I’ve just repaired an amplifier with blown TO-3 MOSFETs and I used new parts from stock that worked perfectly. I have plenty more.
Data
Most older semiconductor devices have modern replacements and I keep a regularly updated database of cross-references. I also keep a ton of old data books containing invaluable and otherwise unobtainable parts specifications, matching and substitution data. This allows us to replace old devices with new and often better than factory original parts.
We have replacements for the NLA TO-66 bipolar devices and unobtainable JFET small signal devices used in Accuphase amplifiers, for example, and we keep stock of probably thousands of transistors including high-spec modern replacements for many vintage types that are NLA.
