Musical Fidelity NuVista M3 Amplifier Repair & Service

I recently repaired and serviced this monster Musical Fidelity NuVista M3 integrated amplifier. Come and take a look inside.

The Musical Fidelity NuVista M3 Integrated Amplifier is a true two box monster. The NuVista M3 was one of Musical Fidelity’s statement products back in 2001 and cost $4500 USD, in 2001! If you’d like more of a review, try this article from Stereophile.

Design

Unlike conventional designs, this integrated amplifier consists of two large and heavy boxes. The smaller box is the power supply. It actually houses transformers and that’s it, so it isn’t really a power supply, more a transformer cabinet. Three heavy-duty cables link these transformers – left power, right power and preamplifier – to the main amplifier chassis.

The second, larger cabinet holds the rectifiers and filter capacitors, preamplifier circuit and power amplifiers. The circuitry is straightforward and the quality of the parts is quite basic, in typical Musical Fidelity style. Board quality is good and the modular design is again typical of MF. There is nothing unusual here, except maybe the Nuvistor tube buffer implementation, which is cool.

In terms of other design elements, the amplifier uses Sanken SAP15 Darlington output devices. Each unit consists of two bipolar transistors, a diode and an emitter resistor in a high-gain configuration that reduces the number of components needed in the drive circuitry. These devices are no longer available though and hard to match due to their inconsistently high gain. If this thing blows up, it could be a pain to fix now…

Specifications

Power Output: 250 watts per channel into 8 Ohms
THD: (Typically) < 0.003% at 1kHz (‘A’ weighted)
Frequency response: (Typically) 20Hz – 60kHz +0.25 / -3dB
Inputs: 1 phono 5 line level
Outputs: Speakers, Tape out, Preamp out.
Input sensitivity: (line) 300mV,  (MM) 3.5mV
Input impedance: (line) 100KOhm, (MM) 47KOhm
S/N Ratio: (line) (Typically) 102dB ‘A’ weighted
S/N Ratio: (MM) (Typically) 83dB ‘A’ weighted
Power consumption: 1200W max
Power requirements: 100/115/230V AC 50/60Hz
Dimensions: Amplifier 482 x 145 x 470mm (W x H x D)
Dimensions: PSU 365 x 145 x 250mm (W x H x D)
Weight: Amplifier – 23kg (un-boxed); PSU – 16kg (un-boxed)

Looking inside, we see densely-packed circuitry and a less than ideal heat-sinking arrangement for the output devices. Mating them to a small aluminium block, mounted to a larger heat-sink through a tiny footprint creates a thermal management problem.
Norte the modular layout, typical of Musical Fidelity. Of significance is the tiny footprint of the output device mounting blocks, screwed to the main heatsinks. This is not a sensible thermal interface and I would improve this with lapping and thermal paste during overhaul.
Protection relay and internal-RCA cable detail

Previous Work…

This late serial numbered unit had visited a technician generally known for poor workmanship, prior to its arrival here. This technician had replaced parts as part of an expensive repair job. In this case though and much to my relief, things were, for the most part, neat and tidy.

The issue, and I’m not sure if this had occurred to the owner, is that this work didn’t resolve the issue and was therefore likely unnecessary and definitely expensive. The channel drop-out problem with this NuVista M3 returned shortly after the work done a few years ago. My customer lived with this for some time before bringing the unit to me. Can you pick the new parts in the image below…?

One non-original group of SAP15N Darlington transistor packages. All the other output devices are also new. The clue lies in the date codes, plus also some witness marks on the fasteners and excess flux. These amplifiers were manufactured in 2001. They can’t contain devices from 2009 unless those devices are non-original.

So, I found a set of 16 SAP15 Darlington devices from 2009 in this amplifier from 2001. As I said, the work is neat so I have no problem with that, if it needed new output devices. The problem is, there is no sign it did need them though. This work cost the owner well north of $1000 AUD. I’m am going to go out on a limb and bet that the original devices were fine and should not have been replaced.

You see, the original fault returned shortly after the output devices were replaced. This tells us it’s highly likely that they were absolutely OK. The amp still ran at this point and these things usually explode when they fail, leaving signs that are visible even after replacement. These devices are used across a range of MF amplifiers and they generally fail catastrophically.

Issues

One of the first things I do with any piece of equipment exhibiting a recurring fault is to carefully examine every component possible. I’m looking for clues (thanks, Robert Palmer), signs of damage or overheating, poor soldering or anything that might exhibit intermittent poor contact, like a switch or relay.

I found three critical and two potential issues in this NuVista M3:

  • A damaged TO-126 device (inflicted during previous repair)
  • Poor soldering on a diode bridge
  • Excessive idle power consumption and asymmetrical bias
  • A remote-controlled mechanical input selector – potentially flaky
  • A full complement of crappy original capacitors
This immediately caught my eye, but obviously no one else’s before now…
The leg of this diode bridge might technically be connected to the board, but I’m not happy with the soldering. This has to be resolved.
This driver device also caught my eye. When I found it, the transistor lay almost at right angles to this correct alignment. The strain on the device when pushed (previous repairer) from above has bent it, causing the legs to almost fracture. This can be repaired in place. Removing this board is painful and I wanted to get the thing running reliably and then speak with my customer about further work.
The NuVista M3 contained all its original Jamicon capacitors. These are not great and not really appropriate for an amplifier like this. You can see from the next few shots that the filter capacitors have come away from the boards slightly. They don’t appear to be leaking but they are free to move and may have suffered heat stress. I told my customer that it is best to replace them.

Lack of bench space and my messy main repair area makes working on the NuVista M3 challenging…!
Idle power consumption is high. The real problem though is that one channel is dissipating most of this power, indicating an asymmetrical quiescent current condition. This may lead to one channel overheating, possibly causing a problem and definitely harming the sound.

Repair & Service

First, I repaired the poorly soldered diode bridge and the TO-126 transistor with the fractured legs.

I hit a couple of the legs on this bridge with fresh solder and a nice, hot iron, correcting the marginal soldering. There can be no thermal issues with the joints of this bridge now.
Likewise this driver transistor. Either of these issues could have lead to reliability problems. The device itself tested well and proper replacement is only possible by removing the entire board, hence this strengthening exercise. I’ll replace this device as part of a major overhaul to replace other worn parts.
I cleaned and re-made all internal RCA cable connections and serviced the input selector switch. These can be problematic and it’s very likely related to the problems my customer experienced.
I then ran the amplifier through an extended test cycle and very carefully adjusted the quiescent current to reduce power consumption to around 95W at idle, and completely symmetrical left and right channel current draw.

So what were the main issues here? When the output devices were replaced in the previous repair attempt, the technician failed to correctly re-bias them. He either misadjusted the quiescent current or never looked at it at all. This caused a thermal imbalance and poor sound and this, combined with a flakey input selector switch were the likely culprits.

Results and Suggestions

After addressing the problems, including the poor soldering and fractured transistor legs, extensive bench testing and adjustment, I gave the NuVista M3 a clean bill of health. Listening to the M3, she sounded lovely, particularly BIG, powerful and is clearly capable of driving some big speakers in larger rooms.

The only other thing I recommended to her owner is an overhaul/upgrade, using much better parts. This will transform the performance of this unit in sonic terms. To anyone owning a Musical Fidelity NuVista M3 integrated amplifier and wanting the best performance and reliability, a full overhaul including new filter capacitors, improved film capacitors and critical bypass and decoupling is the prudent way to proceed.

Feedback from her owner is very positive:

Dear Mike

I thought I would let you know that my amplifier has never missed a beat since getting it home, thanks so much I really appreciate it.

Dennis

My customer is bringing her back next year for this work and if you or anyone you know has one of these or any of the other big Musical Fidelity amps, I am very happy to offer service, repair and overhaul for these units.

The NuVista M3 is a seriously big unit
Looking lovely after a thorough detail

Feel free to share your thoughts and leave a comment!