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!
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…?
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.
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.
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.
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.
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.
What re-cap kit beside the 4 Main cap for the MX-1 do you recommend? Thank you!
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.
Can you please recommend the re-cap kit for the MX-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.
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!
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.
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
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.
I have one but i can undecover, can someone tell me how please
Hi Jesus, I’m sorry but your question is unclear. What exactly would you like to know?
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!
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!
I have 2 of these in need of repair. Would you be interested in fixing them?
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.
Beautiful amp and nice work Mike!