Accuphase M-60 Love

[User211]

How can you not love that?

What's more, when I put these monos up against a Luxman M-800A driving Apogee Duettas, they come out very well indeed. Better channel separation, higher power (half a Kilowatt into 4 Ohms).

But it's poorly.

Can I fix it? We will see.

[User211]

First off some Gemini AI derived info:

Based on your serial number K8X548, we can break down the vintage of your Accuphase M-60 quite accurately.
Using the classic Accuphase dating convention for that era:
Serial Decoding: K8X548
* K: This is the model series prefix used for the M-60 and other units from that specific mid-to-late 70s production run.
* 8: This represents the year of manufacture. For this series, it corresponds to 1978.
* X: This is the month code. In the Japanese system of that era, Accuphase often used X, Y, and Z to represent the final months of the year. X typically stands for October.
* 548: This is your specific unit's production sequence number.
Your amplifier was likely manufactured in October 1978.
Why this matters for your repair:
By 1978, the M-60 was at the peak of its production cycle (it was released in July 1975). This means your unit benefited from any "running changes" or minor circuit refinements Accuphase made during the first three years of the model's life.

[User211]

Soft start circuit. This supposedly 2.2 Ohm resistor is date coded 1993. Somebody replaced the original ceramic resistor with this. Granted it has lasted for some years, but this sucker is the wrong type. That's a metal chassis mount resistor designed to use the chassis for heat dissipation, I believe.

As it is no longer to spec, the switch on relay, which you can hear when it works, no longer works. So l am going to try putting a new ceramic one in.

[User211]

That LCR meter is ace. You can check an awful lot out with that with the amp off. £100 Ali Express.

Everything I have checked using it seems fine, apart from that resistor. Basically I am surprised at just how 'on spec' these old components seem to be, including the caps. They obviously used good shit.

[Macca]

That LCR meter is ace. You can check an awful lot out with that with the amp off. £100 Ali Express.

Everything I have checked using it seems fine, apart from that resistor. Basically I am surprised at just how 'on spec' these old components seem to be, including the caps. They obviously used good shit.

Funny that I was just admiring your meter

You can't beat the old amps, so much better than a biscuit tin with a power button and a blue LED.

[User211]

Resistor in. Did it work? Nope.

Last time I switched it on the circuit breaker tripped. Need to check that is still working. The switch on relay could also be roasted I expect.

But not today.

[User211]

Funny that I was just admiring your meter

You can't beat the old amps, so much better than a biscuit tin with a power button and a blue LED.

TBH I don't like the colour of modern Accuphase kit. At all. The silver facia of the M-60 is nice, though, I think, as is the simple styling.

Meters are lovely to watch in the dark and when cranked up it is quite disturbing to see how much the Duettas will drink.

[User211]

Quick continuity check on that circuit breaker says it's OK. Hm.

[User211]

Got it. For some reason that resistor broke. I bought 4 so will try another.

Edit: no I won't. I found the part in the manual it should be 20W not 10. Gemini had told me to order 10W but has admitted it was wrong.

Suspecting a relay fault or cap failure that is used as a timer to change its state, or the resistor itself. Maybe. Just guessing so far.

Resistor is part 15.

[User211]

This monoblock had a bit of a mash up in power transistors. No way on this planet were these matched. TBH I think one of the types in there wasn't right anyway - the AIs didn't like it. Can't remember which one it was.

So about a week ago I slammed a new set in. Photo of one side only for these. The AIs had better be right about these. Both Gemini and Grok think it is right.

I have also located the soft start delay cap plus two related transistors in the service manual to check later. Be good if something was obviously wrong with these items cos that probably means I have it sussed.