Yes, in almost all cases transistors can be replaced. With what, though?!
The devil is always in the detail and the trick or the art is understanding transistor substitution. This means knowing what transistors can be used where and why and keeping stock of a wide range of premium substitutes. These details are where many go so wrong.
Substitution
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 a very common part of the work I do here at Liquid Audio and getting this part right, from a technical perspective, sets good repairers apart from bad.
There are thousands of different types, sizes and varieties of transistors. You can think of transistors as being like spark plugs or tyres. They all kind of do the same thing but each type is uniquely tailored to a specific job and use case. Substitute in the wrong type and the circuit won’t work, or worse still, may catastrophically fail.
Part of the art of repairing electronics lies in understanding these aspects and this means knowing what replacement parts to use, where and when, and if these parts aren’t available, how to select the most appropriate substitutes.
Transistor matching and substitution are technical topics and are generally poorly understood. I’ve lost count of the number of pieces I’ve repaired where the fault has related to incorrect transistors, rather than dead transistors. Incorrect or poorly matched parts can lead to excessive distortion, heat or premature failure. Therefore, a good working understanding of transistor specifications is important.
Stock
I keep a deep stock of NOS and high-spec modern devices, including output 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. A good repairer must have such devices on hand.
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, replacement parts from stock that worked perfectly. I have plenty more.
Note that eBay is the last place a professional repairer looks for parts and generally speaking you shouldn’t either. A quality-assured supply chain minimises warranty issues and maximises repair success rate. There is no point taking chances here.
Valuable 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 parts specifications, matching and substitution data. This allows us to replace old devices with new ones, often better than the original factory 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.