I recently repaired and completely restored two stunning Accuphase M-60 monophonic power amplifiers. Read on to learn how these wonderful amplifiers were ruined by a local Perth electronics repairer, before they came to me.
The Accuphase M-60 is a monophonic power amplifier. It was Accuphase’s top of the range when introduced in 1975 and it was and is capable of driving almost anything. This beast can output 300 watts continuously into 8 ohms, more into lower impedances. Given that it’s monophonic, you’ll obviously need two for a stereo setup. There are very few loudspeakers that these amps won’t instantly submit.
If you’d like to find out a little more about the amazing Accuphase M-60, try this link first. Hi-Fi Engine has more here, and lastly, Accuphase’s own material on the M-60 is located in their wonderful Product Museum.
Accuphase M-60 Specifications
|Form||Monophonic power amplifier|
(20Hz – 20kHz, 0.03% or less of distortion)
|THD (20Hz – 20kHz)||0.03% or less (at Output power)
0.01% or less (at -3dB output)
0.05% or less (at 50mW output)
|IM distortion||0.01% or less (at Output power)|
|Frequency response (8-ohm load)||20Hz-20kHz, +0 / -0.2dB (at Output power)
2Hz-150kHz, +0 / -3.0dB (at 1W output)
|Dumping factor (8-ohm load)||120(40Hz)|
|S/N ratio (input short-circuit, IHF-A)||115dB (at Output power)|
|Subsonic Filter||17Hz, 18 dB/oct|
|Power output meter||Logarithm compression type
– It is a continuation Direct reading to 50dB (3mW) – +3dB (600W).
A peak level, with a volume level changeover switch
|The semiconductor used||Transistor: 47 pieces
IC: Three pieces
Diode: 51 pieces
|Power supply voltage||AC100V/117V/220V/240V, 50Hz/60Hz|
|Power consumption||At the time of no inputting: 65W
At the time of a 4ohm load Output power: 800W
At the time of an 8ohm load Output power: 540W
|Dimensions||Width 482x height 170x depth of 345mm|
|Option||A set of accessories (with a fan) O-81 (8,000yen)|
|Remarks||Standard rack (19 inches) attachment is possible.
Rack-mounted pitch: 100mm (4 inches)
Rack bore (horizontal): More than 430mm (16 15/16 inches)
A Sad Tale of Recapping …
My customer, Simon, purchased the pair of Accuphase M-60 power amps and enjoyed them for a while, before deciding to have them ‘refreshed’. This thought was perfectly reasonable, but unfortunately his first choice of repairer was not.
Simon chose a local, Fremantle-based repairer who specialises, and I use the term loosely, in guitar amplifiers. I won’t name the repairer here but if you contact me, I’ll be sure to pass the details on. This guy is, unfortunately, best avoided.
When I first inspected these M-60’s, it was immediately obvious that this repairer should not be working on hi-fi gear. In fact, I already knew this because I’ve repaired other stuff that he’s worked on. This guy should stick to guitar amps or possibly find a different profession altogether. He probably did his best, but is clearly incapable of executing the technical assessment and work necessary on complex equipment like the Accuphase M-60.
Anyway, the repairer decided both M-60’s needed a ‘recap’. Recapping seems to be all the rage right now, because some seem to think that it will fix any problem. Whilst it’s in-vogue, the average guy reading about this on a forum doesn’t have the technical knowledge, equipment or experience needed to determine if recapping is required. There is also a lot more to consider beyond the state of the capacitors. Such was the case with our guitar amp repairer.
He removed all the vintage, high-quality and electrically perfect Elna capacitors and replaced them with – you guessed it – Jack’con’ and Samxon capacitors! I’m not kidding, this guy ripped out premium Japanese parts that have a large bearing on the way Accuphase gear sounds and replaced them with literally the lowest quality parts you can buy – in the world! I’m not kidding. If he had simply tested them, he would have found they were all electrically perfect.
No One Rule
Trying to fathom the thinking here isn’t easy. Imagine you own a car, with four premium tyres with lots of life left on them, and you take it in for a service. When you get the car back, you discover that the mechanic removed the expensive tyres with plenty of life left, and replaced them with cheap, shitty Chinese tyres, because they are ‘new’. Happy? I think not.
At this point I know what you’re thinking: “Mike – how do you know the original capacitors were good?” Good question. My customer was smart enough to ask for all the removed parts and he kept them. He brought them to me when I repaired and restored his amplifiers. I tested them, all were perfect.
I’m often asked about my thoughts on recapping and if I have a ‘rule’ about this. The reality is that there is no rule regarding recapping older gear. Each piece should be assessed on its own merits, condition, type of equipment and type of caps. You need proper test and measurement gear to make these assessments.
Accuphase gear rarely needs recapping because of excellent thermal design and very high quality original factory parts. I selectively replace caps in Accuphase gear, based on testing them first. Testing avoids guesswork and there is no place for guesswork when restoring electronics.
Output Device Madness
Simon’s amps ran for a few weeks after their original Fremantle guitar amp guy ‘restoration’, then one of them blew up. Big puff of smoke. Then the other one blew up. This repairer charged my customer $3000 to ruin his amplifiers. It’s sad, but I hear stories like this quite often. Simon then found me and asked if I could help.
I love restoring audio equipment like this, and I could see immediately why these amps had died. No two output devices matched, all were from different batches, different years, different manufacturers. They weren’t even in complementary pairs, essential for stable operation. I’m not kidding, this is the worst case of transistor mismatching I’ve ever seen.
This terrible work may have also been done by the guitar amp guy or some other tech with no clue, I can’t be sure. The fact remains that no technician with any clue would knowingly install a range of incorrect, mismatched output devices from different years, like you see in these images, yet here they are, and Simon didn’t install them!
With all my restorations, things begin with a careful and thorough visual inspection, so that I can find out what I’m working with. I make note of critical issues and then proceed with disassembly.
Once I’d removed the modules from the first amp, I took the first chassis outside for a thorough wash. I use a process I’ve developed for washing amplifiers and it works incredibly well for removing years of baked-on dust and dirt. No other method I’ve tried cleans a chassis as effectively. The warm Perth sun and a towel took care of the drying. Whilst the chassis was drying, I disassembled and cleaned the second amplifier chassis and also set it to dry.
Restoring the Modules
Whilst the chassis were drying, I set to work on the modules, choosing the same modules from each amp, one at a time, to allow duplication of my workflow. I cleaned each board individually, starting with a wash to remove dust and dirt. I then removed all the cheap Chinese capacitors and replaced them with brand new laboratory-grade parts. Where possible, I re-used a couple of the original Elna bipolar electrolytics as they measured better than anything new I had to hand.
I carefully inspected each driver board and tested components where I suspected trouble. For example, some diodes had been replaced with incorrect parts. I replaced these with the correct value Zeners, and a couple of NOS vintage small signal diodes.
The secondary power supply boards also needed work. I reworked dry joints on these boards and a couple of heat-stressed resistors. Strangely, the previous repairer had used good quality vintage axial capacitors on these boards, so these stayed put. and matched the NOS vintage caps I installed on the driver boards.
New Output Devices
After parts replacement, I carefully reworked dry joints and flux-cleaned boards. I then turned my attention to the output devices. Never before have I seen such a mess. Someone had removed the presumably dead original output devices and replaced then with Motorola MJ15003 and MJ15004 devices – not really a match. Worse still, they were all from different batches and had wildly differing gain!
I decided to replace all of these with modern, high-speed, high-voltage MJ15024 and 15025 transistors. I used parts all from the same batch and I matched them for gain. After removing the old parts and the mica thermal washers and dried thermal paste, I cleaned the heatsinks and remounted the new devices on silicone thermal washers.
Reassembly & Final Touches
There were a few more steps. I replaced the meter illumination and the soft-start resistors. These resistors allow the current impulse drawn by the amp to be spread over time, reducing the chances of blowing a house fuse upon startup and making things easier on the amps. Again, these had been replaced with the wrong parts at some point in the past, so I ordered exact factory replacements and installed them.
The guitar amp guy replaced the main filter capacitors. This was totally unnecessary, I tested the original, beautiful Elna capacitors, they were perfect. The only saving grace is that he used very good RIFA capacitors. RIFAs are a premium spec part, so I left them. However, he used cheap hardware to complete the filter cap wiring and so I replaced this with stainless Allen fasteners.
Finally, I applied contact cleaner/enhancer to all board-to-board connectors switches and attenuators.
Testing & Adjustment
I powered the amplifiers up on a variac, one at a time, whilst closely monitoring current draw. The variac allows me to make sure everything is OK and that the amp is behaving itself during the gradual ramp up to full mains voltage.
Once they were up and running on 240V, I left each amp for a few minutes to stabilise and then carefully dialled in the correct amount of bias current. Once the bias had stabilised, I adjusted DC offset. Next, it was on to the protection circuit which you test with a 1 ohm output load.
Finally, I calibrated the meter drive circuitry, using a test signal and output loads. I discovered a little distortion in one amp. I traced it to of all things, an old staple which had found its way into the exposed output relay of one of the M60’s. It was happily part of the circuit until I removed it!
One of the things I enjoyed most was load testing each amplifier, into my 400 watt resistive load. With amps this powerful, I could hear each load resistor ‘singing’ to the 1kHz test-tone! Of course, the best part was connecting these babies to speakers and hearing how great they now sounded. Properly restored, they run stably, cool and without a hint of stress.
The last thing was to hand these gorgeous Accuphase M-60 monophonic power amplifiers back to their owner. Simon was shredded by the guy before me. I wanted him to be completely happy, and he was. He told me the amps sound incredible in his system, that they’ve never sounded better. Seeing the huge smile on his face when he collected them is the reason I love doing this!