Yes, transistors can be replaced in almost all cases, with excellent results when done correctly.

As with anything technical, though, the devil is in the details, and people often get these details wrong. Part of the art of repairing electronics involves understanding what replacement parts are optimal in various scenarios and how to select the most appropriate substitutes when original parts are no longer available (NLA).

Good repairers stock a wide range of parts to accommodate most equipment/replacement scenarios. Good repairers do not source transistors on eBay. Read this FAQ to learn why.

Substitution

Transistors come in thousands of different types, sizes and ratings. Transistors are like spark plugs or tyres. They all serve the same general roles, but each type is tailored to a specific use case and set of parameters, like operating voltage, frequency response, gain, and power handling. Substitute the wrong types and the equipment may perform poorly, exhibit distortion, noise or high temperatures, or worse still, may catastrophically fail, taking other parts 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 was incorrect replacement transistors, rather than dead ones. Incorrect or poorly matched parts can lead to 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 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 can be replaced with modern parts, and I keep a regularly updated database of cross-references I implement. I also keep many old data books containing invaluable and otherwise unobtainable parts specifications, matching and substitution data. This allows the replacement of old devices with new and often better-than-original parts.

I stock replacements for the NLA TO-66 bipolar devices and unobtainable JFET small signal devices used in many older Accuphase amplifiers, for example. We also keep stock of probably thousands of transistors, including high-spec modern replacements for many vintage types that are NLA.