I recently restored this gorgeous Luxman M-4000A power amplifier and matching C-5000A preamplifier. Let’s start with the beautiful M-4000A.

Apparently, the legendary Tim De Paravicini designed the original M-4000. The improved and even rarer Luxman M-4000A probably also came from De Paravicini’s hand, but nobody seems to know for sure.

UPDATE: Check out my recent service of a Luxman M-4000 power amplifier.

The Beast

The Luxman M-4000A is a very powerful amplifier. A massive 180 watts per channel continuously is no joke, even now. Hitachi MOSFET output devices – in this case, 2SK175 and 2SJ55 – power this beast. These devices are bolted to large die-cast aluminium heatsinks, via a precision-milled aluminium coupling plate.

Card-style boards house other circuits and these attach to a backplane. This system is often found in test gear and computers and really enhances serviceability. Someone had been into this amp before me because I found replacement capacitors on the driver and power supply boards. The rest of the amplifier was original.

Luxman M-4000A

Luxman M-4000A Specifications

Courtesy Audio Database

TypeDuo Beta Circuit Stereo Power Amplifier
Continuous effective output (20 Hz to 20 kHz ;
Simultaneous operation for both channels / single channel operation)
270W + 270W (4 Ω)
180W + 180W (8 Ω)
Total Harmonic Distortion Factor (8 Ω, 20 Hz to 20 kHz)0.008% or Less (180W)
Intermodulation distortion factor (8 Ω, 60 hz : 7 khz = 4 : 1)0.008% or Less (180W)
Frequency characteristic3 Hz to 100 kHz – within 1 dB
Output bandwidth10 Hz to 100 kHz -3dB (0.1%)
Input Sensitivity / Impedance600mV/50k Ω
Signal-to-noise ratio (IHF-A corrected)116 dB or more
Residual noise0.2 mV or less
Crosstalk (Volume Max)-100dB or less (1 kHz)
-80dB or less (20 Hz to 20 kHz)
Damping factor100 (at 8 Ω Load)
Protection circuitSpeaker protection circuit by DC drift of speaker terminal
Amplifier protection circuit by overcurrent detection
Amplifier protection circuit by detecting fuse blown for power transistor
AttachmentPower meter
Peak Indicator (With Peak Indicator On / Off Switch)
Meter Sensitivity Selector Switch (0 db, -20dB)
Input level set with 1-dB interval 22-point / detent volume
Power supply voltage100 VAC, 50Hz/60Hz
Power consumption (Electrical Appliance and Material Control Law)520W
External dimensionsWidth 491x Height 181x Depth 390 mm


Firstly, there is zero, and I mean ZERO, service data available for the M-4000A. I contacted everyone who knows about Luxman gear regarding this problem. The service manual for this amp no longer exists and nobody knows why.

You might think you could use the data for the M-4000 but you would be wrong. There are twice as many trimmer potentiometers in the M-4000A and it is significantly redesigned over the M-4000. The mystery deepens though because there is a Luxman M-4000A schematic, kind of…

The problem is that whoever drafted this schematic used old mechanical drafting techniques. It’s hard to follow, there are errors and because it’s not a factory document, critical details are missing. Trimpot functions aren’t labelled for example, nor are any voltages, so it’s not super helpful and may not even be correct.

In the end, I kind of reverse-engineered the potentiometer functions by taking very careful measurements of circuit parameters as I made small adjustments. I was able to deduce what half of the potentiometers did, critically bias and DC offset servo adjustments.

I set the bias current using an infrared thermometer and metering power consumption. I verified these measurements by measuring the current drawn in each channel. After hours of measurement and adjustment, I was happy with where I arrived.


Anyway, as I hinted above, someone had previously repaired this Luxman M-4000A. Whilst some capacitors had been replaced, others subject to very high temperatures had been left in place. This is most likely because they are difficult to get to, located on the output boards. They don’t lift out like the others and come as an output module, complete with the heatsink. The only way to get to these four capacitors is to remove and completely disassemble the output modules.

Part of my repair ethos is to make sure my work is reliable over the long term. This means I go further than other repairers, paying more attention to the details, using better parts and going deeper into the disassembly to get the bits others leave. To that end, I decided I would completely rebuild the amplifier modules, which meant removing them from the chassis and stripping them.

Doing this allowed me to use new silicone thermal pads to re-make the critical thermal interface between the output devices and their coupling plates. I was also able to use fresh thermally conductive grease between each coupling plate and its heatsink.

What follows is a photo journey through the restoration, with comments where applicable.

H/V Power Supply

Luxman M-4000A
This is the power supply board, feeding the driver stages. Someone has crudely replaced the 5-watt ceramic resistors. The original axial Nippon Chemi-Con capacitors are of excellent quality and tested fine. I don’t replace these unless it is really necessary, or I am asked to. I am doing so here, to maximise the benefits of this restoration and minimise the chances of future trouble.
Here you can see the crude previous work, poor soldering, lifted PCB traces and messy flux residue.
This is how the board looks after rework and cleaning.
You can see the premium BC / Vishay replacements in the background.
The new BC parts are somewhat smaller than the originals but otherwise a perfect fit. They are superb quality as well, this series is super-low ESR, high temperature and ultra-long life rated.
Luxman M-4000A
Luxman M-4000A
Installed in the board, these 220uF capacitors will keep this baby going for many more years.
Luxman M-4000A
Luxman M-4000A
All six high-spec Vishay capacitors have been installed and this secondary power supply will now supply clean, low-impedance, low-noise rails to the driver stages of this amplifier.
Luxman M-4000A


Luxman M-4000A
Protection board with original, heat-damaged capacitors in place
Luxman M-4000A
Here you can see the new, high-temp, low-ESR replacement capacitors installed
The cleaned and de-fluxed rear side of the protection board, complete with factory kludge wires. Someone goofed on the layout!
Luxman M-4000A
This is the other protection board, or it might be the soft-start controller… Note the backplane connector bottom-right.
Close-up of high-spec replacement capacitors – these are rated at 105 degrees C, over many thousands of hours.
Luxman M-4000A
Another view of this board
And the rear of the board after cleaning and flux levelling.

Driver Boards

Luxman M-4000A
Driver boards had been worked on by someone before me. They at least used high-temp Jamicon capacitors to replace the old ones and cleaned most of the corrosive glue from the board. I saw no need to replace these caps but did remove a little more corrosive glue and cleaned the boards front and back.
Luxman M-4000A
In the middle of this image, you can see what the factory’s corrosive glue has done to the copper lead of this board-mounted fuse. After cleaning some more of the glue away, this board was good to go.
Luxman M-4000A
You can clearly see the replacement capacitors here and the lovely clean board. One cannot underestimate the importance of cleaning everything, it greatly reduces the likelihood of other problems.
Luxman M-4000A
The same applies for the other channel driver board – cleaned and ready to go
Luxman M-4000A
Ready to reinstall – note the two board-mounted potentiometers – the one near my thumb sets bias current for the four MOSFETs in that channel
The other driver board is a mirrored opposite, whereas output boards are identical
Luxman M-4000A
Driver boards ready to reinstall. I love the removable cards in this amplifier, you rarely see this now as it increases manufacturing costs.
Luxman M-4000A
Luxman M-4000A, showing restored driver and protection boards. I reworked all boards, removed the remains of the corrosive glue, replaced all electrolytic capacitors and thoroughly cleaned and de-fluxed every board.

Amplifier Modules

Luxman M-4000A
Spot the very tired-looking electrolytic capacitors on these output boards. I removed the amplifier modules, disassembled and thoroughly cleaned them to get to these last few original parts…
Luxman M-4000A
I always begin with a special foaming cleaner – amazing stuff for removing years of grease, gunk and dirt
Luxman M-4000A
Modules were washed clean and allowed to dry under the Perth sun…
Next, disassembly. Firstly I removed the die-cast heatsinks from the aluminium mounting plate/PCB assembly. Then I removed all the dried old thermal grease. Note how little grease is left – over the years, heat causes evaporation of the volatile components.
I removed the MOSFETs, old thermal grease and mica washers. I always use modern silicone replacements. They don’t require thermal grease, are not susceptible to the effects of grease aging, uneven application and variable thickness mica washers, and are therefore more reliable and consistent over the long-term.
All the bits from one module – note the foul state of the PCB and the cleaned coupling plate.
You can see the need for cleaning the boards here
Luxman M-4000A
Cleaned, reworked and de-fluxed board. Note the difference in how it looks compared to the image above. There were a few dry joints on one of these boards, the channel may have been running a little more bias and therefore have been running hotter.
This image shows how clean the top surface of the board is now, and the two new high-temp, low-ESR capacitors I installed.
The cleaned aluminium coupling plate
Time to reassemble the module using new silicone thermal pads in place of the old mica/dead thermal paste.
It pays to work carefully here, no sense in rushing something involving high power, high voltage devices…
Properly torqued ultra-rare and sublime-sounding MOSFETs on a completed PCB/coupling plate assembly. All the power devices in this amplifier were loose. This is common in older amplifiers, where the thermal grease has dried up. The result is poor heat transfer from the output devices to the heatsink. Conductive interfaces must be clean and fitted with the correct transfer material/compound. It’s important to correctly torque fasteners locating power devices and use premium thermal pads to ensure good thermal performance.
Luxman M-4000A
Another view of the restored output assembly
Next, after cleaning the old grease away, I applied new thermal grease to the heatsink…
…and then smeared it around to evenly cover the mating surfaces
Luxman M-4000A
Next, I screw the board/coupling plate assemblies tightly to the heatsink. I also re-greased and torqued the smaller TO-66 packages. These days I would also use new silicone TO-66 thermal pads.
Luxman M-4000A
Another view of the cleaned and refurbished output board.
Luxman M-4000A
Two restored amplifier modules
Luxman M-4000A
With the amplifier modules back in the old girl, she is purring. The final stages involve the adjustment of quiescent current and DC symmetry.


I think it’s fair to say that this was a stellar result and that lots of hard work and attention to detail paid off here. Of course, we must lose sight of the fact that all the original service data is gone, unobtainable, and this makes working on these tricky. I have done all the heavy lifting now for anyone else who ever looks at this lovely old girl, I only wish I had been so lucky when she first came to me!

It goes without saying that a heavyweight MOSFET power amplifier like this is always going to sound great and this one does. These fetch a pretty penny now but it’s well worth it for the sheer performance on tap here and the classic retro looks.

If you would like me to service or repair your Luxman M-4000A, M-400 or any other lovely Luxman piece for that matter, don’t hesitate to get in touch.

Discover more from LiQUiD AUDiO

Subscribe to get the latest posts to your email.

16 thoughts on “Luxman M-4000A Amplifier Repair & Restoration”

  1. Mr. A. Cerqueira, from Lisbon, Portugal, owner of both a M-4000 and a M-2000.

    Very interesting, indeed. Really a nice job done. Congratulations.

    1. Hi, sorry to hear you are having trouble with your M-4000A. I don’t generally provide this sort of technical advice. Be careful, there are around a dozen fuses in the M-4000A, many are glass fuses and many more are board mounted and must be checked after board removal. Fuses blow for a reason, so this must be explored. There is no service manual, so fuses must be compared to board and chassis designators to check they are correct. The unit must first be removed from the wooden chassis, a tricky operation in itself. Are you anywhere near Perth..?

      1. No, I am in Greece and I am sorry for my previous mistake but the amp is an M4000 not the M4000A model (my bad). I have an expert that’s been servicing the unit since it came in the country back in the 80s, and he is the one who has also retrofitted proper audio out jacks.

        I can’t seem to get a hold at him (maybe he retired) so my options are somewhat limited. I noticed that there’s no apparent easy way to open the amp up. I have the original manual and service (?) schematics (which I am willing to scan and sent over email if anyone’s interested).

        I was thinking to check just the fuses, yet if it’s the power supply or anything else I won’t risk doing anything. Like I said the amp enters the checking cycle but doesn’t pass by it so it doesn’t power up. Thanks for your time.

        1. Hi Jason, no problem at all, yes the 4000 and 4000A are very similar in terms of layout and construction, so my previous comments apply here also.

          There are loads of fuses, in fact I’ve never worked on amplifiers with more fuses than these, so you’ll need to check a lot of them. You’ll need a DMM for this, and need to know where they are. This is not immediately obvious and impossible to describe unless we are both sitting with the thing in front of us. If you open her up and remove each board though, you should find them.

          The most obvious group though is under a little panel in the middle of the amp, on top of a power supply module. One screw and the lid lifts, then you’ll see what I mean. Just be aware that there are a bunch more than this, including soldered, board mounted fuses. You must check all of them to be sure the rails are intact. Be sure to replace fuses with the correct speed ie fast or slow blow, this is important.

          The wooden case must first come off of course, be careful as the amp inside is extremely heavy and it’s easy to cause damage at this stage. I usually place the amp on its end, and lift it out of the wooden case, so that nothing is damaged.

          I have a scan of the service manual but its low resolution, so a better scan would be very helpful for sure.

          1. Hi, I couldn’t find the service manual but found a schematics page and the original owners manual. I’m quite certain that there’s a service manual here somewhere but atm I wasn’t able to find it. I will try to sent you some iPad (12megapixel) shots (including some closeups – even though printing of that era wasn’t exactly stellar) of what I got hoping that you might find it useful for your work. Check your email.

  2. The amp keeps blowing out all 6 output transistor on one side. It may be as I turn up the volume, or time (a couple minutes).

    But it does come up working. Then all of a sudden boom. The 100 ohm resistors start to smoke on the pre-driver board, and its over.

    I ran the unit without the outputs to monitor everything going to them. I looked for DC on from the drivers, the power supply DC, and other things that may take out the final drivers. I compared everything to the working channel, NOTHING wrong!

    I am in about $320 in parts and many hours of time. The last try, one of the transistors on the pre-driver board and a 500ma fuse burnt out. The outputs were OK. When powered up the fuse and transistor went again.

    What I did notice and what I think may be the problem: I noticed AC voltage at about 40 volts was on the chassis when compared to earth ground. Very low current, but it is there. I am thinking this voltage is somehow getting back into the amp. I also believe that the power supply on that side may have a transformer issue. I didn’t go any further, and returned it to the owner. If it is the transformer getting one and paying for it may prove to be a bad idea since these parts are hard to find and expensive.

    I traced all the signal on the good side with a DVM and a scope. Compared them with the bad side and found 100% match. But the chassis at 40 volts can not be correct. It is on the RCA jacks too.

    1. Hi Joe, thanks for writing and yes it seems you definitely have a problem there. Some transformer leakage is very common with older units like these and is generally not a concern. I doubt it’s the issue here, just a red herring. This will be a component level fault, or faults, most likely related to some of the many fusible resistors, or a semiconductor. There are many variables potentially at play and I’d really need the unit here for some serious troubleshooting to sort this out. Does the owner live locally? I can very likely fix this for him if so. I have fixed issues like this previously, with various causes relating to incorrect semiconductors, poor previous work, hard to find diode and transistor faults and usually a combination of these. Anyway, I’d be very happy to take a look. With the right test set-up, output devices shouldn’t be failing either, so it’s well worth reconsidering your set-up. You might like to give me a call to discuss this one!

  3. I can’t imagine the cost of this work but well worth it I am sure. Very impressive. Mike, do you work on Perreaux gear? Thanks. Bernard

    1. Hi Bernard and thanks for your comment. This type of work is time-consuming but surprisingly affordable, given the true value of gear like this and the cost to replace it. Interesting you should ask about Perreaux because yes I work on a lot of it and love older Perreaux gear. I own an SM-2 and PMF-5150 and they form part of my main system. Just this year I’ve repaired and serviced a PMF-5150, EP-2 and E-200 for various customers and I have an SM-2 and SM-3 waiting for work!

  4. hey mike im a tech at just audio im servicing a m-4000 that goes into standby after being on only afew minutes theres a voltage thyat starts out at 1.5 but when it hits 3.6 it goes into standby problem seems to be on the driver board yet ive changed all the semis an the caps any ideas?? thx for your help mike

    1. Hey Edward, welcome and thanks for your question. My first question would be why have you changed all the semiconductors on the driver board and what substitutes have been used in each case? This is critical. Rather than get into all that here, technical queries are best addressed via my contact page. You can purchase consult blocks which are perfect for enquiries like this. Very happy to chat further via phone or email. PS: Knowing that my replies to these sorts of questions are often ignored, you are most welcome, in advance!

Thanks for visiting, comment, like, share and subscribe. Our trusted advisory service is available via the contact page.

Discover more from LiQUiD AUDiO

Subscribe now to keep reading and get access to the full archive.

Continue reading

Scroll to Top