Yamaha MX-1 Statement Power Amplifier Repair

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 it’s made of lead!

A big and very dirty power amplifier at this stage. Hard to get a sense of the sheer weight of this thing from the photo..
Much cleaner after vacuuming
A lot of the weight is located right here – dual mono power transformers
And a lot of energy is stored right here – in these four very large filter capacitors. We aren’t talking Levinson or Krell here, but these are BIG for mainstream Japanese equipment.

This is a beautifully built and very cleanly laid out amplifier. The result is that it’s breeze to work on. Like all well-engineered equipment, each area is easily accessible and each module is easy to remove. Modern gear is just not made this way. How much of what you buy today do you think will will still be working 25 years from now…?

Layout is very neat, wiring twisted to reject noise and most connectors are screwed or soldered into place. There are lots of solid copper buss bars in the power supply and each amplifier block.

Note the each channel features two heatsinked diode bridges. This is at least twice the number you would normally see, some amps only have one diode bridge to rectify AC into DC for the whole amplifier!

Yamaha MX-1 specs, as always, courtesy of the HiFi Engine:

Specifications:

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
Weight: 24kg
Year: 1993

The Problem

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.

Soft start or remote turn-on circuit. All the trouble was on this board.

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.

See the three large joints in the middle of frame – these are the power transistor joints and the device was just flapping around in those holes. There was zero electrical conductivity here!
And here is the second source of trouble – note the completely vaporised solder blob. This isn’t a fault of the circuit, there are no overcurrent faults here. Poor thermal design means that this pad just gets too hot, but there is a way to fix this…
The whole board, note that vaporized pad! It literally has zero solder anywhere!
The Repair

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.

Then prepared the area around the relay leg to accept much more solder, for better heat-sinking and no chance of joint erosion like before.
I also took the opportunity to open and clean the relay. I refurbished the contacts with very fine emery paper and some DeoxIt.
With the relay back in place, I resoldered the joints and paid special attention to the problem joint. As you can see, this will conduct heat away from the joint much better now.

I resoldered the power transistor back in place and flux cleaned the board. Out of curiosity, I quickly replaced the board without putting the amp back together and tested her – perfect! The amp powered up, no problem.

Resoldered transistor joints and clean, defluxed board

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.

Owner is a smoker…
Nice, clean Yamaha MX-1 ready to go home to her owner.

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