Amplifiers, Service

Rare Luxman M-4000 Power Amplifier Repair & Service

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Check out this beautiful Luxman M-4000 power amplifier I’ve just serviced. This classic vintage beast needed a little work, let’s take a look.

The Luxman M-4000 power amplifier is a compact, dense, 180 Watt per channel amplifier. It’s the predecessor of the M-4000A, which looks very similar, but is substantially different design-wise and even rarer. I completely restored an M-4000A for a customer a couple of years ago.

The M-4000 and others in this series share a lovely wooden case, out of which slides the immensely heavy for its size amplifier unit. This is a true dual-mono design, with two big transformers and separate filter capacitors and diode bridges for each channel.

Specifications (courtesy HiFi Engine)

Power output: 180 watts per channel into 8Ω (stereo)
Frequency response: 3Hz to 100kHz
Total harmonic distortion: 0.05%
Input sensitivity: 1V
Signal to noise ratio: 108dB
Dimensions: 485 x 390 x 175mm
Weight: 28kg
Year: 1975 – 1979

As always, there’s additional information on the Luxman M-4000 at The Vintage Knob.

If you would like a video summary, watch my M-4000 video on YouTube. Otherwise, read on!

Issues

This M-4000  presented with one channel out of service and I was able to get her working again without too much drama. In this case, I checked all of the 20+ fuses, and no I’m not kidding, there are actually at least 20 fuses in the Luxman M-4000. Many are standard M-205 glass fuses. Some are board mounted and some are soldered to boards, but there’s a lot of protection built into this design. Good work Luxman.

All fuses must be checked of course because any open fuse will stop the amp from running or at least part of it. This takes time because of the need to remove each board, pull the fuses, test them and the soldered fuses too.

Service & Repair

It’s good practice to remove and clean fuses, it helps re-make the connection between the fuse and fuse holder and allows you to feel if the end caps have come loose. Few people bother, but it’s good practice and if you are familiar with my work, you’ll know how much I like good practice.

I took the opportunity to clean the board card edge connectors whilst testing those fuses. After checking fuses and reinstalling cards, I powered up the amplifier with a variac for testing and to make some adjustments.

To my surprise, both channels now worked well and I spent some time setting DC offset and bias current. I also took the opportunity to clean the chassis, fascia and beautiful wood case.

I had to sit and admire the design for a few minutes. It really is a very solidly built amplifier and very powerful for the time.
Just taking the main chassis out of the wooden housing is a real chore for one. I use a method where I stand the amp on its face and slide the wooden surround off.
The Luxman M-4000 is beautiful to look at, even without the wooden enclosure. I like the mesh and it is good for cooling.
This view shows the modular and serviceable design of the M-4000. Each card slides out and can then be serviced, as needed. Note also the open fuse panel in the centre, holding four fuses. This opens with one screw, to reveal the power supply regulator board, which also pulls out.
This is that regulator PCB.
Here are just some of the fuses, labels clearly indicating the values. I often find repairers install the wrong fuses in equipment like this, though I’ve never understood why.
Here, I’m doing a final sound test in the Liquid Audio workshop. I’m lucky to have a pair of Yamaha NS-10 studio monitors as my test speakers. Prior to this, I set the DC offset and bias current in each channel. Bias current adjustment involves removing two fuses and measuring current drawn across the fuse holder in each channel.

Cleaning

Those who follow along will know I’m a little fussy about cleaning. This Luxman M-4000 needed a clean and didn’t escape my fetish. Here she is, after a clean and wax.

Wow, she looks really good after cleaning the fascia and wooden case.

The wood really pops after some beeswax.
Nice!

Results

This beautiful Luxman M-4000 power amplifier absolutely sang after a good clean and service. It just goes to show that some TLC, correctly delivered, can make all the difference to an old girl like this.

Don’t skimp on servicing your cherished hi-fi gear, especially at this age. You’d be amazed how reluctant some people are to have this sort of work done, yet if you knew what this amp might cost to replace, you’d absolutely need to spend around $10,000 AUD to get anything even close. Compare that to the few hundred dollars you might spend servicing it and it’s just a no-brainer,

If you live in Perth and you’d like me to service your Luxman amplifier, get in touch and we can arrange an inspection.

Feel free to share your thoughts and leave a comment!