I’ve just finished a repair on an amazing power amplifier – the Yamaha MX-1. You can skip to the video now, or read on for more details…
The Yamaha MX-1 was the best and most expensive amplifier in the Yamaha line-up for 1993. I truly had to wrestle this thing around the workshop, it’s so heavy for its size, it feels like its made of lead!
This is a beautifully built and very cleanly laid out amplifier and a breeze to work on. Like all well-engineered equipment, each area is easily accessible and each module is easy to remove. Modern gear is in a different universe. How much of what you buy today do you think will will work for 25 years before failing…?
Yamaha MX-1 specs, as always, courtesy of the HiFi Engine:
Power output: 200 watts per channel into 8Ω (stereo)
Frequency response: 20Hz to 20kHz
Total harmonic distortion: 0.008%
Damping factor: 250
Input sensitivity: 1.46V
Signal to noise ratio: 125dB
Dimensions: 438 x 116 x 486mm
The problem with this MX-1 was that the amp would turn on, but not really turn on. Let me explain – push in the power switch and a red LED lights up. Cool, except that there were no voltages present anywhere that I could measure, except on the primary side of the mains transformer.
It took a little thinking time and looking at the schematic to realise that this thing has a soft or ‘remote start’ arrangement. This explains the very light power on button ‘feel’. This secondary power supply is behind the front panel.
The board shows severe signs of solder joint erosion around all of the switching power transistor legs and one leg on the relay that it switches. The power transistor was literally completely disconnected from the circuit, so bad were the three dry joints. The eroded relay joint was so bad that all solder around the leg was gone, along with the copper.
This part was straightforward. I remade the eroded relay joint by removing solder mask from adjacent copper to allow for a really large soldered area, making a really nice, big joint. Then I refurbished the relay by opening it and cleaning the contacts. The new joint is electrically and thermally very sound, so this fault won’t occur again.
I then resoldered the power transistor back in place and flux cleaned the board. Out of curiosity I then quickly replaced the board without putting the amp back together and tested her – perfect! The amp powered up no problem.
Last things to do were to reinstall the front facia, power the amp up and leave it on for a while to stabilise and then set the bias current. Both channels needed adjustment, so I set those to 14mV and then tested the amp with some music.
I made a short video about the repair, you can watch it on my YouTube channel.