A compact Nikko TRM-50 amplifier from the early 70s arrived for repair where a previous repairer had said the mains transformer was faulty.
Failed transformers can be a problem in amplifiers as transformer specs are never provided and they’re custom made.
If you need an old amplifier or hifi repaired please contact me for a quote
A resistance check of the transformer primary and secondary windings suggested it was OK, although it had been unbolted and was loose in the chassis!
Suspecting the power supply in the amplifier I found a power supply rectifier diode had gone short circuit.
Looking on an oscilloscope the output of one channel was jumping about all over the place as the volume control was adjusted.
A squirt of contact cleaner into all the potentiometers and input selector switch sorted that out and the little amplifier started working properly.
This amplifier is an old design without any of the modern day safety features amps have. There’s no speaker protection relay for example so you get some speaker thump on power up and it carries on working when you switch it off while the smoothing capacitors discharge.
It does have what looks like thermal or over current trips on the power rails to the left and right amplifier channels that offer some protection in the event of something like a power transistor failure. Probably!
Below is a video of the Nikko amplifier repair I’ve put on my @orangevalleysystems YouTube channel. Please subscribe to the channel for new videos.
What does the Nikko TRM-50 sound like?
The Nikko packs a punch in the sound department as the loudness control has quite a dramatic effect. I’ve no idea what level of db boost it applies and to which frequencies.
The scratch and rumble filters when switched in deaden the sound slightly as you might expect.
This amplifiers output is 17w per channel into 8Ω which is typical of vintage amplifiers of the time and is plenty unless you want to shake the tiles off the roof!
A lot of the circuitry is in obsolete potted modules that would make replacing them difficult although I think they’re fairly simple so they could be replicated with individual components.