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Luxman M-4000A Amplifier Repair & Restoration

A customer recently brought in his gorgeous matching Luxman M-4000A power amplifier and C-5000A preamplifier for restoration. This article looks at restoration of 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: If you are interested, 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.

Trouble

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, nobody knows why. You might think that 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 they significantly redesigned the unit over the M-4000.

The mystery deepens 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 factory, 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 bias current using an infrared thermometer and monitoring power consumption. I verified this by measuring current drawn across a fuse holder with fuse removed. After many hours of measurement and adjustment, I was happy with my adjustments.

Repairs

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. 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.

Power Supply

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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.
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Here you can see the crude repairs made previously, poor soldering, lifted PCB traces and messy flux residue
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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

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

Driver Boards

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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.
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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.
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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.
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Same applies for the other channel driver board – cleaned and ready to go
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Ready to reinstall – note the two board-mounted potentiometers – the one near my thumb sets bias current for the four MOSFETs in that channel
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The other driver board is a mirrored opposite, whereas output cboards are identical
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Driver boards ready to reinstall. I love the removable cards in this amplifier, you rarely see this now as it increases manufacturing costs.
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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.

Amplifier Modules

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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…
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I always begin with a special foaming cleaner – amazing stuff for removing years of grease, gunk and dirt
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Modules washed clean and allowed to dry under the Perth sun…
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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.
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I removed the MOSFETs, old thermal grease and mica washers. I always use modern silicone replacements. They don’t require thermal grease and are therefore more reliable long-term.
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All the bits from one module – note the foul state of the PCB, and the cleaned coupling plate.
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You can see the need for cleaning of the boards here
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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, the channel may have been running a little more bias and therefore have been running hotter.
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This image shows how clean the top surface of the board is now, and the two new high-temp, low ESR capacitors I installed.
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Cleaned aluminium coupling plate
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Time to reassemble the module using new silicone thermal washers in place of the old mica parts.
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It pays to work carefully here, no sense in rushing something involving high power, high voltage devices…
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Properly torqued 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 filled with the correct transfer material/compound. It’s important to correctly torque fasteners locating power devices, to ensure good thermal performance.
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Another view of the restored output assembly
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Next, after cleaning the old grease away, I applied new thermal grease to the heatsink…
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…and then smeared it around to evenly cover the mating surfaces
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Next, I screw the board/coupling plate assemblies tightly to the heatsink. I also re-torqued the smaller TO-66 packages as they were also loose.
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Another view of the cleaned and refurbished output board
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Two fully restored amplifier modules
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With the amplifier modules back in the old girl, she is purring. Final stages involve adjustment of idle current and DC offset.

If you would like me to service or repair your Luxman amplifier, or any other Luxman piece for that matter, don’t hesitate to get in touch.

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

  1. 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 for several good reasons. Be very careful, there are around a dozen fuses in the M-4000A, many are glass fuses, many more are board mounted and must be checked after board removal. Fuses blow for a reason, so this reason 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 @ liquidaudio@iinet.net.au.
            Cheers.

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