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.