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Yamaha MX-1 Statement Power Amplifier Repair

I’ve just finished repairing an amazing Yamaha MX-1 power amplifier. Read on for more about this stunning, heavyweight amplifier.

It’s fair to say that Yamaha, along with Sony, soldiered on with high-end consumer hi-fi gear for longer than many others and this beautiful Yamaha MX-1 power amplifier proves it.

The Yamaha MX-1 was the best and most expensive amplifier in the Yamaha line-up for 1993, and what a beast it is. 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!

As usual, I made a video about the repair, check it out:

Inside

The Yamaha MX-1 is a very serious design and immensely heavy for its diminutive size. It appears initially to be a dual-mono design but upon closer examination, the filter capacitors are shared between the channels and two sets of power supply rails are derived from these, allowing ‘rail switching’, a feature designed to increase efficiency.

Besides that, the twin transformers, twin diode bridges per channel and separate heatsinks and driver boards almost qualify this as a dual-mono design, but not quite. The best and really only true dual-mono designs have two separate amplifiers in one box, including power supplies and even power cables, like this BAT VK-500 I repaired.

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, in this case, my proprietary chassis wash process.
Yamaha MX-1
A lot of the weight is located right here – dual power transformers, one for each channel.
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And a lot of energy is stored right here – in these four very large filter capacitors. We aren’t talking about Mark 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, the wiring is twisted to reject noise and most connectors are screwed or soldered into place. There are lots of solid copper bus bars in the power supply and each amplifier block.
Yamaha MX-1
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Heatsinked diode bridges are a really nice feature. 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 Specifications

As always, courtesy of 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
Weight: 24kg, good Lord this is a heavy thing…
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 main 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 remaining solder!

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.

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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 service the relay. Relay service is a lost art, they don’t always need to be replaced, actually, they very rarely need to be replaced.
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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 reworked the remaining joints including the remaining gnarly one near the top and put her back together.

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.

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The owner is a smoker, never a good thing for hi-fi gear.
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Nice, clean Yamaha MX-1 ready to go home to her owner.
Yamaha MX-1

So, this was a very successful service and repair job, as usual. If you own a Yamaha MX-1, or any other beautiful Yamaha amplifier for that matter, don’t hesitate to get in touch regarding maintenance.

As always, thanks for visiting!

28 thoughts on “Yamaha MX-1 Statement Power Amplifier Repair”

    1. Hi Mike
      Thank you for your report. I have two MX-1’s. One had exactly the same error.
      I find that some transistors like the Q226A get very hot. Should I mount small heatsinks there to extend service life?

      Best regards Ivan

      (sorry google translator)

        1. Thank you.

          I’ve found that the transistors that get hot often have cold solder joints. They might be cooled by the circuit board.

          1. Correct, excess heat causes gradual deterioration of the joint and evaporation of metal atoms, reducing thermal performance, accelerating the problem. This is why I make the repair the way I described in this article, it works well to resolve the issue.

      1. Dear Mike, thank you very much! On my Mx-1 the transistor Q226A left channel and the Q232C right channel always get very hot. On my second MX-1 they are just a little warm. What could be broken? I can’t find the error. I really like the MX-1 and don’t want to give up. Kind regards Iven

        1. Hi Iven, I don’t know, there may have been some iterative changes to parts values and/or the schematic which explain the higher operating current of the devices. Given the asymmetric thermals you’ve observed though, measurements will tell the story, especially around the devices in question regarding their biasing and power dissipation.

    1. 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.

  1. 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. 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!

  2. 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.

  3. 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. 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.

    1. 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.

    1. 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.

  4. 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. 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!

  5. Hi Mike, Just one thing … the MX-1 is not a dual-mono design. Take a look at the schematc and you’ll see that, although the transformers each serve their own pairs of rectifier bridges, the resulting +75/-75(HB) and +33/-33(LB) supplies for the output stages (which are dual voltage class G) share main capacitors wired in series. I do love this amp. John

    1. Hi John, I’m glad you enjoyed the article and thanks for pointing this out. I didn’t say the amplifier was a dual mono design in the article, but I see that I wrote “dual mono transformers”, which is ambiguous and needs rewording. The main filters are indeed shared, with rail switching to increase efficiency, something I will clarify in the article. Apart from the shared main filters, everything is dual mono, but the distinction is worth making and rules it out as a true dual mono design. These are certainly a favourite of mine to work on!

  6. Hi, maybe a difficult question to answer so just an opinion would be appreciated. I have an opportunity to buy a CX-1 & MX-1 in 8/10 condition for $1000 AUD. For that price would you say there are ‘better’ options or would it be hard to beat if I’m after a system that offers a clean neutral sound. Thanks. Ps…. Great site.

    1. Hi Guy and thanks for your question. We have an advisory service specifically for digging into these sorts of queries and accessible via the contact page. Regarding price though, for those two pieces, if working well, it’s crazy cheap!

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