Luxman M-4000A Amplifier Restoration

A customer recently brought in his gorgeous matching Luxman M-4000A power amplifier and C-5000A preamplifier. Both pieces needed a thorough overhaul and the M-4000A needed repairs also.

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. Read on to find out more about some of the issues faced when rebuilding or restoring these classic Luxman amplifiers!

The Luxman M-4000A is a high-power amplifier. 180 watts per channel continuously is no joke! Hitachi MOSFET output devices – in this case 2SK175 and 2SJ55 – power this beast of an amplifier. 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 is great for 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 untouched.


Firstly, there is zero and I mean ZERO service data available for the M-4000A. I know a few people and I contacted them all about this problem. The service manual for this amp no longer exists. You might think that you could just use the data for the M-4000 but you would be wrong. I found, to my horror, that there are twice as many trimmer potentiometers in the M-4000A, and no sure way to work out what they all do, because there is no schematic.

The mystery deepens here though because there is a 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 job, critical details are missing. Trimpot functions aren’t labelled for example, nor are any voltages, so it’s of little use unfortunately.

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 idle current by using an infrared thermometer and monitoring power consumption. With many hours of adjustment and measurement, I was able to bring both heatsinks to exactly the same temperature.


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 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, so the equipment I service is not going to need to be back for similar work for a long time. 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 brand new silicone thermal washers 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.

Power Supply
This is the power supply board, feeding the driver stages. Someone has crudely replaced the 5 watt ceramic resistors and used poor-quality subsitutes. The original axial Nippon Chemi-Con capacitors are of good quality and tested fine. I don’t replace these unless it is really necessary, but 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 repairs made previously, poor soldering, lifted PCB traces and messy flux residue
This is how the board looks after I touched up a couple of joints and cleaned it
I did in fact decide to replace the capacitors on this secondary power supply board. 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.
Installed in the board, these 220uF capacitors will keep this baby going for many more years.
Finally, the smaller 100uF capacitors arrived and i was able to also fit those

All six high-spec Vishay capacitors have been installed and this secondary power supply will supply very clean, low-noise power to the driver stages of this amplifier now.

Protection board with original, heat-damaged capacitors in place
Here you can see the new, high-temp, low ESR replacement capacitors installed
Cleaned and de-fluxed rear-side of the protection board, complete with factory kludge wires – someone goofed on the layout!
This is the other protection board, or it might be the soft-start… Note the backplane connector bottom-right.
Close-up of the new high-spec replacement capacitors – these are rated for 105 degrees, over many hours
Another view of this board
And the rear of the board after cleaning
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.
In the middle of this image you can see what the factory 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.
You can clearly see the replacement capacitors here and the lovely clean board. One cannot underestimate the importance of cleaning everything, it greatly reduces that likelihood of other problems.
Same applies for the other channel driver board – cleaned and ready to go
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 cboards are identical
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, showing restored driver and protection boards. I reworked all boards, removed the remains of the corrosive glue, replaced all electrlytic capacitors and thoroughly cleaned and de-fluxed every board.
Output Modules
See if you can spot the very tired looking electrolytic capacitors on these output boards. I removed the amplifier modules, dissassembled and thoroughly cleaned them to get to these last few original parts…
This necessitated pulling the modules, then I always begin with a special foaming cleaner – amazing stuff for removing years of grease, gunk and dirt
Modules are then washed clean with water and allowed to dry under the Perth sun…
Next comes disassembly. Firstly we remove the die-cast heatsinks from the aluminium mounting plate/PCB assembly. Then we remove all the dried old thermal grease. Note how little grease is left – over the years, heat causes evaporation of the volatile components.
I remove the MOSFETs, along with the old thermal grease and mica washers. It’s best not to re-use mica washers, modern silicone replacements are more reliable and don’t require thermal grease.
This is a plan view of all the bits from one module – note the foul state of the PCB, and the cleaned coupling plate
This is a close-up, you can see the need for cleaning of the boards here
Cleaned, re-worked 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, that 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 have installed.
Cleaned aluminium coupling plate
Time to reassemble the module – I am using these new silicone thermal washers in place of the old mica parts
Progress in reassembly – it pays to work carefully here, no sense in rushing something involving high power, high voltage devices…
Properly torqued MOSFETs on a completed PCB/coupling plate assembly. All the power devices in this amplifier were quite loose. This is common in older amplifiers, where the thermal grease has dried up, leaving everything a little loose. The result is poor heat transfer from the output devices to the heatsink. Conductive interfaces must be clean and filled with the correct transfer material/compound. It’s important to correctly torque fasteners locating power devices, to ensure good thermal performance.
Another view of the restored output assembly
Next, after cleaning the old grease away, we apply new thermal grease to the heatsink…
…and then smear it around to evenly cover the mating surfaces
Next, we screw the board/coupling plate assemblies tightly to the heatsink. I also re-torqued the smaller TO-66 packages as they were quite loose.
Another view of the cleaned and refurbished output board
Two fully restored amplifier modules
With the amplifier modules back in the old girl, she is purring. Final stages involve adjustment of idle current and DC offset. The work on this M-4000A is done!

2 thoughts on “Luxman M-4000A Amplifier Restoration”

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

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