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.
-100dB or less (1 kHz) -80dB or less (20 Hz to 20 kHz)
100 (at 8 Ω Load)
Speaker protection circuit by DC drift of speaker terminal Amplifier protection circuit by overcurrent detection Amplifier protection circuit by detecting fuse blown for power transistor
Power 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 voltage
100 VAC, 50Hz/60Hz
Power consumption (Electrical Appliance and Material Control Law)
Width 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
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.