Accuphase M-60 Love

[User211]

No real way of doing decent checks without that dim bulb tester, Barry, TBH. All reasonable static checks were performed.

Suspected the bridge negative rail DC reservoir cap but I swapped it for the other channel and there's still a short.

If you look at the 4 purple and white connectors on the cap connection ring, they all trace to the legs of output transistors - two on each heatsink. The first thing I did was replace them all with OnSemis. See way back in the thread. All brand new. At least one must be short you would think.

But if so, why? They were brand spanking new.

That's where I am at the time of this post.

This amp is good at eating transistors. The question seems to be why if indeed it has done it again in this instance.

[User211]

I am probably going to order a couple of TDK B41560A9229M reservoir caps.

It's about the only cap that looks a sensible fit. It isn't an audio cap but various AIs say they are excellent. They aren't cheap.

Why? On the working amp I see 175V across the bridged caps + and - terminals, and only 70V on the busted one. Everything elsewhere seems fine and I have a spent a good deal of time convincing myself of that. They test fine under no load. I have been aware the voltages on those caps have been wrong for quite a while but very unsure why.

If that sorts the amp, the other one will get the same treatment. If it doesn't, when I eventually fix the amp, everything that has changed will be mirrored in the other working amp.

Where there is a will, there is a way.

[Barry]

When you measure the voltages, are the rectifier blocks connected to the reservoir capacitors or not?

[User211]

When you measure the voltages, are the rectifier blocks connected to the reservoir capacitors or not?

Yup. Just the single bridge block rectifier in the non-working amp. The bridged capacitor - to + terminals across the two caps is what I am talking about. Both amps lacking boards and fuses in the same way to try and isolate the issue.

These caps have done amazingly well to be quite frank. 50 years ain't bad going but we have to get real and realise they aren't going to be in the best state. Even if I am wrong it is still an OK move to try and replace the pair in the non working amp I think.

Barry, the power transistors as I said are brand new and I did pull a few connected to the negative poll of one of the reservoir caps with nothing else in the line apart from wire to the transistor legs and no other capacitor terminal connected.

They are fine in isolation. In-circuit with no power the transistors I pulled seemed to exhibit the diode failure test that is easily applied to transistors. Pulled they did not.

Analogue circuits of this era and way earlier are voodoo.

[Barry]

Sound like one of the reservoir capacitors has become leaky, and definitely needs to be replaced. As you say 50 years is good going, so it would be best to replace all of them.

Once you get the power supply working on the broken amp, reconnect it to the amplifier board. If then you get trouble (fallen voltage, or even blown fuses) I would suspect the output transistor, but you say they are new, so it might be the driver circuitry that is at fault.

This is starting to exhaust my knowledge of power amp circuitry.

[User211]

It's a problem that does require a lot of wondering and guess work.

The best AIs aren't really sorting it, but they are good at coming up with ideas to explore.

I think it has to be worth £240 to dump some new ones in. We aren't talking average amps here.

[User211]

The bill was actually £288. Ouch!

The terminal spacing is slightly different at 31.7mm versus the ELNAs 35mm but hopefully I can work something out there.

I could have flipped the working monos caps in to see if it solved the issue but decided to go this way instead.

[Barry]

The bill was actually £288. Ouch!

The terminal spacing is slightly different at 31.7mm versus the ELNAs 35mm but hopefully I can work something out there.

I could have flipped the working monos caps in to see if it solved the issue but decided to go this way instead.

That is what I would have done just to check, but as you say the caps are 50 years old, so should be replaced. £70 each sounds very expensive. , what are their specs (i.e. their capacitance and maximum working voltage)?

[User211]

That is what I would have done just to check, but as you say the caps are 50 years old, so should be replaced. £70 each sounds very expensive. , what are their specs (i.e. their capacitance and maximum working voltage)?

Barry they were £144 each. There's a Chinese website doing them for $75 dollars but to be honest that does seem a little too cheap.

https://www.digikey.co.uk/en/product...3YO3abJLkz27zv

I do quite like the fact they are TDK, though. It is definitely in 1975-77 Accuphase spirit. There's a lot of TDK snubber caps in here.

[User211]

That said I did realise today that over the capacitor bridge on the original circuit diagram there aren't any.

Very curious.