The BM8000 was a no-compromise receiver.
If metal resistors were better, they would have been used - and they are
actually used in selected positions here and there.
25 years have passed and lots of new technology is available now, but I'm not sure
all of it improves performance just because it's new.
Clearly the hybrid amplifiers did not improve anything soundwise.

I hardly ever use metalfilm resistors in amplifiers.
It's possible that they make them a different way today than
they did 20 years ago but back then I had problems with
self-oscillation in some amplifiers and the cause proved to be the resistors.
Metalfilm resistors (at least the ones I had problems with) had
a slight amount of inherited induction, depending greatly on their resistive values, as
in wirewound resistors, and that was enough to set the whole thing in motion.
Metal resistors can fail short. It's rare but it does happen.
A coal resistor will never short, it will burn and go open circuit.

Others may like metal resistors, some claim they can hear the difference, some claim
that the coal resistors are noisier.
I never observed a particularly noisy BM8000 amplifier stage anyway and I tend to like
the sound of the BM8000 circuits as produced and intended by B&O so I like to mount
components similar to the original as far as possible.
We had the same issue with regards to the opamps in the pre-amplifiers recently. Here
I also tried mounting "better" opamps but as a couple of other members I reverted back
to the original when I found the sound reproduction too cold and sterile for my liking.
In Pentas, however, they do improve performance a lot.

That's probably just me. I'm getting old, the family radios were valve things when I
was a child. I liked the warm sound and today I care more about sound pleasant to my ears
than for 100% linearity and/or costly "improvements" that I cannot hear and
remain theory.
My ears are luckily still fine. I can still tell the difference between most B&O
amplifiers and I can detect 0.5% distortion on a 1KHz sine but on some setups, like my
own BM8000, I like to use Loudness.

I'm sure that it will work fine with metal resistors.

Martin

Interesting thoughts about resistors...

I know there are two schools in that issue, some prefer the old carbon types and some prefer the metal film.

I personally prefer metal film resistors because they have higher stability, low capacitance (<0,2pF), lower noise than carbon composite, small size and  they will not get into fire the way carbon composite resistors do in case of failure in the circuit.

On the other hand carbon composite is superior when it comes to pulsed applications, and they will withstand an intermittent overload.

The carbon types used in bm8000 have a power rating of 0,5w and the metal film I use 0,6w.

/Fredrik

Hey Fredrik,

Amazing job your doing there, never seen one stripped down like that .... OK, so, I have a question from a complete novice, I'm fascinated...how did you get new circuit boards to match the originals? Would have thought these were customised... but what do I know?

She'll be a beauty when your finished ..... just like new literally!

Laurence.

tournedos wrote:

That's one determinate repair undertaking 🤒

Indeed!

Nice work Fredrik!

Looking forward to seeing it complete! 🤒

Hi Laurence

Thanx :)

I have drawn the PCBs by myself in the computer with the help of the old ones and the schematics in the service manual. Then I had them produced for me at a professional PCB manufacturer. I have done the 2 power amps and the displayboard. I will probably make a new CPU board too after finishing my bm8000 as a future spare since many bm8000 have trouble with the thru plating on the original CPU boards.

I hope she'll be a beauty when finished 😁

/Fredrik

Very impressive Fredrik. Some serious attention to detail there, in particular the new PCB's.

Craig🙂

0,6W resistors should be fine.
And I noticed you lifted them up from the board a bit, which will also
improve cooling.
The extreme left driver transistor (the left one on the left output board) could
need a bend to the cooling fin.
Maybe both of them.
They end up very close to the black metal tray that goes under the connector
bay (when mounted) and the risk of shorting the cooling fin(s) to the grounded
metal is high.

Martin

Thanx all of you believing in the project 😁

Martin has mentioned a very good point here when working on the power amps of bm8000.

It is very easy to forget the tabs on the driver transistors. They need to be bent to make sure that the dont make a short to ground. Its also the same for the 3 large caps on the amplifier boards, they cant be higher than approximately 20mm. Keep that in mind all of you who recap your bm8000s.

/Fredrik

Hi Martin:

When I was younger I built power amps as a hobbyist.

There was one design that I had a lot of problems with becaused I used metal resistors which caused lots of oscillation.

It was rather catastrophic as it blew itself up.

Derek

Hi Fredrik,

Impressive indeed... never knew you could do that get one or two PCB made like that, and drawn up by your own hand... real dedication to the restoration process ..... So I guess all is not lost if you want to commit to a complete rebuild of these wonderful machines if they have major issues like burnt out / ruined PCB's!

As a novice just observing your and many of our fellow members dedicated restorative works has given me a greater understanding of how these machines work.... altho I'll may never be able to do the actual work, it gives me valuable insight...

She will be a beauty, looking forward to the rest of your rebuild diary with lots of the great photos!🙂

Keep up the good work.

Laurence.