I picked up this vintage Trio KA-4000A amplifier to use on my Desktop PC (DAW) to drive a small pair of book shelf speakers. However, the amp had a fault on testing so out came the iron and a box of spares. The fault was noise (hiss) and crackling on one channel. This turned out to be a faulty transistor, namely a 2SC458. This particular transistor is quite well known for failing and is a common part in many amplifiers and other audio equipment of this era. Therefore changing these is a must. If you have a vintage solid state Trio amp with a noise problem and it is not just a simple volume pot cleaning process then check for any of these 2SC458 transistors. These should be substituted for 2SC2240 types but I also found that the more common BC182B or BC182LB worked reasonably well in my unit.
At the same time I decided to renew many of the small caps on the pre-amp stage PCB's. Most electrolytic caps dry out with age so changing these is good practice. I checked the large grey Elna PSU caps and these tested ok so I left these alone, but many small caps were quite off value on testing. Anything around 15+ years old will have electrolytic caps that are drying out and most likely carbon resistors high in value. It is possible to replace most of these with modern parts that remain in keeping with the amp. New old stock parts are also available with some searching on eBay for example.
After fixing the fault and replacing all the caps and resistors that had drifted over time, I then turned to a few simple upgrades. I added classic green Mylar caps as by-pass on all electrolytic PSU caps along with some additional smoothing caps at key stages in the supply line. Build quality is good with an engraved aluminium front, solid aluminium knobs and resistor network based Treble and Bass controls etc. I would describe the sound as classic warm and very easy to listen to even when fed with poor FM radio broadcasts.
** Transistor datasheets download attached **