Accuphase M-60 Power Amplifiers Repair & Restoration

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
FormMonophonic power amplifier
Output power
(20Hz – 20kHz, 0.03% or less of distortion)
450W (4ohms)
300W (8ohms)
150W (16ohms)
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 distortion0.01% 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)
Input sensitivity/impedance2.0V/100kohm
S/N ratio (input short-circuit, IHF-A)115dB (at Output power)
Subsonic Filter17Hz, 18 dB/oct
Power output meterLogarithm compression type
– It is a continuation Direct reading to 50dB (3mW) – +3dB (600W).
A peak level, with a volume level changeover switch
The semiconductor usedTransistor: 47 pieces
IC: Three pieces
Diode: 51 pieces
Power supply voltageAC100V/117V/220V/240V, 50Hz/60Hz
Power consumptionAt 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
DimensionsWidth 482x height 170x depth of 345mm
Weight27kg
OptionA set of accessories (with a fan) O-81 (8,000yen)
RemarksStandard 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.

Amps as they came to me, dirty, and in need of some real care and attention.

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.

Jack’con’ capacitors have absolutely no business in any decent piece of electronic equipment, let alone Accuphase monoblocks.
Why..?
And what the heck is this?! This is what some people do when they don’t have the correct parts in stock. They probably thought this was OK, because they used Dale resistors…
More weirdness here with the series resistor networks they guy has made here at the top of the image. You just don’t do this if you have access to a decent selection of parts and care about your work.
Recapping ‘Rules’

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 is sad – here we see one amplifier’s output devices, plus one original Toshiba transistor used in the power supply. The rest are replacement Motorola devices, all from different batches and different years. This is terrible practice and will almost always result in an amplifier blowing up.
More mismatched outputs. Note that the M-60 is asymmetrical in its physical layout. The two images above show each side of the amplifier. The side above has a bit more power supply stuff on it, the image above that is mostly output devices.

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!

Cleaning Away the Grime…

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.

This series of images shows the amplifiers in their original condition, as delivered to me. Note the layers of dirt and grime on everything and bear in mind that these have just been ‘restored’ by someone before me! You’ll see a finger swipe on the chassis, just to the left of the mains transformer, showing clean metal beneath the grime.

Closer in on that grime and also note the row of Jackcon axial capacitors to the left and right. These should NEVER be installed in an Accuphase amplifier.

The second amplifier chassis is just as bad. You’ll perhaps also note here the bodged resistor array the previous guy has installed, because he didn’t have the right parts.
This image shows the meter-drive circuitry.
The first chassis being cleaned using my wet method. My method is based on the way test and measurement is cleaned when used in environments that cause high contamination. I first wet the chassis, then use a special foaming cleanser, then rinse then dry..
The gorgeous Perth weather in March is perfect for drying. We had about 35 degrees Celsius on this day, with warm breezes. If the weather is not conducive to drying, I use a drying oven to make sure every last bit of moisture is removed.

View of the now clean chassis before re-installation of boards.
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.

Boards are thoroughly washed and dried, again starting with foaming cleanser.
Boards are then set outside to dry in the warm sun. On days like this, drying takes a matter of an hour or so.

Here we have an original board-set from one M-60. Both boards contain the Chinese caps installed before they came to me, but not for long.
Both boards now with some excellent vintage axial capacitors and couple of the original Accuphase parts re-installed. Boards have been cleaned top and bottom and de-fluxed, as well as having any dry joints repaired.
Another shot of these boards. Astute readers will note that I’ve installed one temporary cheap capacitor, bottom right, in orange. This was just for testing purposes, whilst I waited for more caps in a parts order.
Here we can see the replacement resistors I’ve installed, after pulling those bodged resistors someone installed previously. Note that these are again period correct and of the correct power rating. I have large stocks of vintage NOS parts for use in equipment like this.

Same two boards, from the other amplifier, prior to restoration.
Same boards, again after restoration. Again, I’ve re-used some of the original and perfect Accuphase axial bipolar capacitors. I’ve also used some premium vintage axial caps to match others in this amp.

View of the backside of a pair of boards, after re-work and cleaning. They didn’t even come from Accuphase looking this good!

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.

These high-voltage capacitors test good and can stay put.

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.

A sad lineup of grossly mismatched semiconductors, on original mica thermal pads and dried silicone paste. I removed and replaced all of them bar one device per amp, and remounted all TO-3 devices on new silicone thermal pads.
Original mica pads and dry grease – this together presents a high thermal impedance and will cause TO-3 devices to run hot.

After removing the devices, I thoroughly cleaned each heatsink to remove remnants of old paste.
Beautiful new devices, all courtesy of ON Semi. These devices are not cheap, at $10 – $15 each, but they are the best replacements in this instance. My motto, as you all will know, is do it properly, once.
New devices mounted on silicone thermal pads, attached again to the amplifier heatsink. The only original part is the red Toshiba transistor which was perfect. This arrangement will work reliably now for many years.
And here we see the other side, also restored with all new devices. These are all output devices.
Same here with amplifier two.
And again, here are the new output devices.
These are all the TO-3 semiconductors I pulled from the pair of M-60’s. Every piece marked with a black ‘X’ is a dead device, the others are just grossly mismatched. Note the vast range of parts used from across two decades!
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.

Someone, maybe our guitar amp guy, had soldered these replacement bulbs in place. I removed them and replaced them with proper fuse-style incandescent bulbs.
Soft-start resistors should also be replaced if we are doing this properly, and we are. The bottom parts came out of the amp and is a cheap replacement and was bodged in place. The top part I sourced and is an exact factory replacement that fits perfectly.
this is how they should look installed.
And in amplifier number two.
Cleaned chassis before installing restored boards.
This image and those following show a number of views of the cleaned amps with boards reinstalled.

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.

Installing new stainless Allen fasteners and star washers. This is a much better arrangement than the previous Philips head screws with no star washers. They can be torqued properly and will make a better, lower impedance electrical connection.

Nice and neat, both sides of one amplifier done.

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.

Very useful device, plugged into my variac, allows me to monitor mains voltage, current draw and therefore power consumption. I can bring the device under test up to voltage gradually, whilst checking for shorts or other anomalous behaviour.

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!

Adjusting bias current by monitoring voltage drop across an emitter resistor. Note that I am also looking at power consumption and comparing this to the manufacturer’s rated power consumption.
Load Testing

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.

Load testing is great fun. Here the M-60 is outputting 40.5V into 8 Ohms. A quick calculation using P = V2 / R gives us around 205 Watts RMS, continuous. Not bad, but this was just heat soaking the amplifier. This dummy load I put together is good for 2 x 200 Watts or 1 x 400 Watts, @ 4 or 8 Ohms.
This was the indicated power consumption when the amp was delivering full rated power into 8 Ohms.
A day later I think and the second amplifier from memory. This time I was adjusting bias using my Keithley 199 bench meter.
These last two images show calibration of the power output meters. There were various steps involved and I had to monitor waveforms with my Tek 3012 as you can see here, plus other voltages, whilst making small adjustments to the meter tracking.

Handover!

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!

Pretty, aren’t they?!

Detail shot shows how nicely these amps came up with some TLC.
You can’t beat big, bold VU meters!
Trust me, they need these rack handles, they are damn heavy amplifiers.

Simon sent me these two images of the Accuphase M-60’s back in service in his system. Look great don’t they?!

6 thoughts on “Accuphase M-60 Power Amplifiers Repair & Restoration”

    1. Thank you and yes, this is my philosophy with everything I do. You may not be aware that resistors are non-polarised components, so their orientation is not important in electronic terms. Having said that, you are mistaken, I purposely oriented the few resistors I replaced in the same direction as others nearby. The tolerance bands are at the left-hand end of the resistors in the images, and all are correctly oriented when referenced to their board designators. I’d suggest also that the attention to other details, like the power rating of the resistors I used, their composition and the way that I spaced them equally, up off the board is more important. Thanks again for reading!

  1. Wow! This is the most detailed post I’ve seen yet. I’m so happy you were able to save those wonderful amplifiers. The more I see on your site about electronics repair the more fascinated with it I become. Mike, I fear I may get sucked into learning some of the trade as a hobby once I retire…(assuming I can ever acquire some skill with a soldering iron)LOL! Could you elaborate on what electrical variables exist in premium caps as opposed to the cheap kind that the previous repair worker used?

    1. Hi Tim, thanks and I’m glad you enjoyed the post. Lots of variables exist between electrolytic and other caps. The main properties to consider include ESR, impedance, ripple current rating, rated and working voltage, rated life@temp, leakage current, temperature rating, frequency response, dissipation factor (tan). Impedance, frequency response and resulting distortion are particularly important in audio gear. ESR and leakage current are important everywhere. There are lots of good articles out there on capacitors, perhaps some good bedside reading..?! I suggest articles by the manufacturers, Vishay / Roederstein have some excellent material out there to read, as do the others. Cheap caps have higher impedance, higher ESR, poorer frequency response, lower ripple current, lower rated lifetime, higher leakage current etc.

  2. Top job Mike! Reminds me of a mate of mine, ran the old Alberts HiFi in Fremantle many years ago. A pair of M100’s, a Michell GyroDec and a pair of Gales.. OMG pure ear candy ! …took a couple of decent blokes to shift those Amps as well

    Cheers
    Mal

    1. Thanks Mal, this was a very involved job but very rewarding. Also a timely reminder of why many repairers out there need to stay away from equipment like this!

Thanks for reading, leave a comment and let me know what you think!