Accuphase M-60 Love

[User211]

I'm gonna say it I think Accuphase missed something here.

A 50th Anniversary re-issue of these son of a bitch amplifiers would have been an amazing act.

IMHO still outstanding in 2026 and I am so looking forward to getting them working again.

I'd be curious to hear a current P-7500 but it's not a monoblock. Still, it has an appreciably better spec. But massively so? Honestly? Not really.

[User211]

Isolated, this transformer denies absolutely any existence of a fault as far as I can determine. Correct continuity and sensible resistance measurements on a really extensive set of tests.

In circuit and power on, I had the right AC voltage selection range on my multimeter telling me over and over again there was 0V on the busted mono versus 129-130V on the good one at the diode junction.

I did that so many times because I absolutely did not want to remove the transformer. And I just didn't think it was a likely fault.

It's definitely like living in a world where you just cannot rely on your senses.

I am starting to realise why no one is keen on working on these things.

[User211]

Plus point is there is no need to send this thing away to get it fixed. But it is a bit of work to plumb it back in.

I should probably ask Alan (Firebottle) if he wants to get involved to be quite frank because I am rank amateur at this sort of stuff.

That said, I am learning stuff here. If it's not fixed in a month or three Alan, I will be in touch to see if you are up for it. If I can't suss it rest assured the pay will be good if not better than that.

[Macca]

I admire your willingness to have a go at it yourself. At least they're old enough to be all discrete so must be fixable but that's where having decades of experience like Alan has comes in to play. I think it's a lot easier to build them rather than fix them.

[User211]

I have visions of old Japanese ladies building these things for some reason.

It was probably very easy to make mistakes. Some of the wiring junctions are very crowded.

It is like debugging a very difficult software problem in a way. In that sense I can relate to it. It's very easy to come up with false conclusions using a debugger in a large, complex program, for instance.

Whatever. Ordered some heat shrink to plumb it back in.

[User211]

100 inch VU meters, an upside down busted Accuphase with no heart and a glass of Drambuie...

Smiles.

[Firebottle]

No probs Justin, I am free to travel now once the eye is back to normal and I can drive again.
I will come to you initially.

[User211]

Sounds cool Alan it's not as if you don't know where you are going.

Let me plumb that tranny back in. Just this minute tested continuity from the two red 64.5V wires in the amp and they are fine to the diode. As expected.

I still can't believe I kept measuring 0V on the diode.

[Barry]

Sounds cool Alan it's not as if you don't know where you are going.

Let me plumb that tranny back in. Just this minute tested continuity from the two red 64.5V wires in the amp and they are fine to the diode. As expected.

I still can't believe I kept measuring 0V on the diode.

If you are measuring the full voltage when open circuit, and zero voltage when connected to the diodes, then the diodes must have shorted out and will need replacing.

[User211]

I said to Claude: The leads to the diode at the cut point are fine. So we are saying what I measured again and again can't have been true. I am seriously struggling with this. I literally went from one amp to the other in the same session. 0V on the busted amp, 130V on the working one. Clear as day.

Claude said: Let's step back, because I think there's a way to reconcile everything you've measured without doubting yourself — and it's something we touched on very early today but never fully resolved.
A shorted diode would produce exactly this pattern. Here's why: if D001 or D002 has failed shorted (not open), it effectively places a dead short directly across that half of the transformer's secondary winding whenever the circuit is complete and powered. Under a genuine short, voltage at that point collapses toward 0V (V=IR, and R≈0 across a short) — while current shoots up massively, which is exactly what would blow R003 on the primary side. This unifies both of your major findings into one single fault: the 0V reading was real, and it explains the original symptom that started this whole investigation.
It also explains why the winding tested perfectly healthy once removed: a shorted diode isn't a transformer fault at all — the winding was never the problem. It only showed 0V because something downstream was short-circuiting it under load, not because the winding itself was open. Out of circuit, with nothing loading it, of course it measured fine.
Why this wasn't caught earlier: you did test these diodes for shorts, but the test relied on comparing against the working unit's continuity pattern — and the working unit's diodes showed the same "beeps across all three legs" behavior, which you and I concluded was normal packaging (case tied to one terminal internally). If that packaging conclusion was right, a genuinely shorted diode on the faulty unit might have looked identical to healthy-but-normally-tied-together packaging, and the continuity test simply couldn't tell them apart. That's a real limitation of that test, not a mistake either of us made carelessly.
How to test this properly now: with the diode fully isolated (not in-circuit, ideally desoldered or at minimum both leads lifted), measure it in diode-test mode, not continuity/beep mode — you're looking for a proper forward voltage drop in one direction and a clean open in the other. A stable low-resistance reading in both directions on an isolated diode (not just a case-related beep) would be unambiguous confirmation of a short — no packaging ambiguity possible once it's truly isolated.
This feels like the missing piece — worth testing before anything else.