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Yamaha MX-1 Statement Power Amplifier Repair

I’ve just finished a repair on an amazing power amplifier – the Yamaha MX-1. Read on for more details about this stunning, heavyweight amplifier.

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!

Yamaha MX-1
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..
Yamaha MX-1
Much better after a deep chassis clean…
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A lot of the weight is located right here – dual-mono power transformers
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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 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 just not made this way. How much of what you buy today do you think will still be working 25 years from now…?

Yamaha MX-1

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The 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.

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Note, 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!

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

Yamaha MX-1 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.

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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.

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See the three large joints in the middle of the frame – these are the power transistor joints and the device was just flapping around in those holes. There was zero electrical conductivity here!
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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…
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The whole board, note that vaporized pad! It literally has zero solder remaining!

Repairs

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.

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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.
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I also took the opportunity to open and clean the relay. I refurbished the contacts with very fine emery paper.
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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.

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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.

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Resoldered transistor joints and clean, defluxed board

The 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.

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

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Liquid Mike

As a kid, I cherished my Tandy 200-in-1 electronics project lab, Dick Smith electronics kits, my Dad's hi-fi and my own first proper system. Later, I created Liquid Audio to help keep classic hi-fi gear alive and well. Our mission: to deliver TLC for classic Japanese, American and European hi-fi stereo equipment.

This Post Has 17 Comments

  1. Darvin Sparks

    Beautiful amp and nice work Mike!

  2. Jeff

    I have 2 of these in need of repair. Would you be interested in fixing them?

    1. Mike

      Hi Jeff, thanks for getting in touch and yes I’d be very happy to look at your MX-1 power amps for you. The clincher will be whether you are a Perth local, otherwise its probably not viable. Regards, Mike.

  3. Midimaniac

    I am scouring the internet trying to figure out what class of amp this is. I have the original manual for it but it does not specify what amp class it is. I doubt it’s class D, probably class A. I have one of these which I bought new in 93 and it is still working fine and without a scratch on it. I also have the matching CX-1 preamp. Please help!

    1. Mike

      Hi, it’s a regular class AB amp, like 99% of other solid state amplifiers from this era. Much better than class D which was introduced as yet another way to reduce costs. You are very lucky to have a pair of these!

  4. Jesus

    I have one but i can undecover, can someone tell me how please

    1. Hi Jesus, I’m sorry but your question is unclear. What exactly would you like to know?

  5. Nagarian

    Mike I have same issues and I see no issues as you found on the board. The unit has not been plugged in for id say at least 5 years and its been properly stored in its original box

    1. No problem, I suggest this is a good example of a unit that needs to come in for testing, diagnosis and repair. If you are local to Perth, you are most welcome to bring her in.

  6. Darlig H

    What would you recommend to replace Main filter caps? Only I can think of is Mundorf M-Lytic, but it’s like $500 for the 4 caps. Wonder if it’s even worth!

    1. Liquid Mike

      Hi Darlig, lots of options there besides Mundorf, some that would be better, but why do you need to replace them? Have they been carefully tested? Really good main filters like say RIFA/Kemet will make a difference for sure, but they are expensive as you say.

  7. Taylor

    Can you please recommend the re-cap kit for the MX-1?

    1. Liquid Mike

      Hi Taylor, I’ll provide some additional observations you might find useful. As I mentioned, I don’t use or recommend kits for many reasons, but for someone working without parts to hand or the knowledge to help choose and order them, a kit may be an option. From a specialist’s perspective, the bigger questions are: does that person have the knowledge, skills and tools necessary to do the work well and what caused them to look for a kit in the first place? If there are issues, it’s worth addressing those directly. Shotgun cap replacement introduces many potential points of failure and rarely solves most faults. If you are looking to improve performance, capacitors can definitely help, but they must be carefully chosen and installed to avoid board damage and introducing other problems.

  8. Taylor

    What re-cap kit beside the 4 Main cap for the MX-1 do you recommend? Thank you!

    1. Liquid Mike

      Hi Taylor, thanks for your enquiry. I don’t recommend any ‘kits’ as such, it’s better to replace parts as needed, based on testing and measurement, with the right parts for the job and budget. This is done case by case and in terms of what I do, with parts from stock. Let me know if you have any other questions.

  9. Nikolay

    Both MX-1 and MX-2 are fantastic amps. The sound is nice, warm and delicate. Perfectly capable of driving “difficult” speakers like Dynaudio. I picked them both while they were still cheap, however as the time goes, I see more and more of their kind turn into e-waste due to the inevitable IC201 failure. A custom-made, obsolete IC at the heart of the amplifier. 🙁

    Great work on restoring the MX-1 and the owner is one lucky guy!

    1. Liquid Mike

      Hi Nikolay, thanks, glad you enjoyed the article and yes they are very nice amplifiers, especially the MX-1! Most regular consumer speakers will be no problem for this old girl. I’ve worked on many Yamaha amps and have rarely seen a BA3122N fail. Maybe I’ve been lucky, some fail no doubt, but I suspect some amplifiers are incorrectly diagnosed by people who stumble across forum posts about this issue and aren’t able to correctly diagnose the fault. Anyway, yes the owner is a very lucky guy!

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