Rare Luxman M-4000 Power Amplifier Repair & Service

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 fairly compact, dense, 180 Watt per channel bipolar amplifier. It’s the predecessor to 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.

Issues

This M-4000 was presented to me with one channel out of service. I was able to get her working again by attending to the basics. 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. A few are board mounted and some are soldered to the boards.

All the fuses must be checked of course because any one open fuse will kill the deal. This takes time because of the need to remove each board, pull the fuses, test them and the soldered fuses too.

It’s good practice to pull 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 necessary.

I took the opportunity to clean the board card edge connectors whilst testing those fuses. After checking fuses and reinstalling cards, I gradually powered up the amplifier 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.

Service & Repair

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

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 center, holding four fuses. This opens with one screw, to reveal the power supply regulator board, which also pulls out.

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 little fetish. Here she is, after a clean and wax.