Hi all!

Just a little story to share with you...

Ive recently bought a bm8000 in "need for repair" state.  However when I recieved it everything was loose inside, a good sign of a technichian who gave up repairing it, but the exterior was in good shape.

At this state I didnt dare to power it up since some repair attempts were done on the output amplifiers and the power supply and I was afraid of making more damage than necessary. The pic shows the right hand output amp hanging loose in the cables.

Further examination showed a rusty bottom plate, so I decided to strip everything down to be able to clean and repaint.

The top left pic shows the cleaned plastic part of chassis.

The 2 pics on the top right shows repainted and refitted bottom plate and rear mounting frame.

After that it was time for reassembling the powertransformers and the bottom PCBs, all cleaned and with new caps. Preamplifier module also got new trimmers for setting input sensitivity as the old ones were worn.

The power amplifier PCBs and the display panel had been "over repaired" and got lots of burn marks from hot components.

Some tracks of the power amp PCB were loose or broken, overall in bad shape. The display panel was cracked.

I didnt want to keep it that way so new PCBs were made to replace the old broken ones.

Top left pic shows original power amp board with burn marks

Top right pic shows the 3 new replacement boards (2x module 5, 1x module 8)

Bottom pic shows new and old poweramplifier board

After testing the power amps on the workbench it was finally time to fit them in the chassis. A little tricky and time consuming work.

Most tricky part is to route the cables right so theyre not get damaged in the small amout of place inside the beomaster.

The pic shows the power amplifiers mounted in the chassis.

This is where I am at the moment. More information will come soon as I make more progress with it.

One day it will come alive again

/Fredrik

Very impressive job, you do here !

  !

Martin

Thanx Martin :D

I just felt the need of saving it from the electronics recycling, hoping that I can get it up and running again...

/Fredrik

How many times I have wished the original PCBs were fibre epoxy instead of pertinax!

That's one determinate repair undertaking

Wow, great job on those PCB's, Mohawk! Looks really neat!

Must be an exciting project to work on! I dream of having my own BM8000 someday.

I wonder what it's going to sound like with all these modern components in the power amp instead of the old carbon comp/film resistors and ceramic/electrolytic caps. Do you have an 'original' BM8000 for comparison?

I'll be following this great thread!

Speaking of resistors; The ones you have mounted looks like metalfilm resistors.
I think, I would have preferred to stay with carbon types in the amplifier sections.

Another thing is the power rating of the resistors. Several of the original resistors are 1/2 Watt components and
they ran hot (your board is not particularly burned!). Your resistors look like 1/3W or so, which could prove a problem.
I suggest you replace them with 1/2W or maybe even 1W, at least the ones in the "hot" areas.

Martin

Hi Martin,

Why would you prefer carbon types? Closer to original?

Is a higher power rating the reason B&O used the big carbon comp resistors? I always wondered why they were still used even in 1980's equipment as they are said to be rather unreliable and lossy in comparison to carbon film or (said to be even better) metal film.

That said, B&O used components regarded 'not good enough for hi-fi' a lot, like electrolytic caps in crossovers. Yet their products are of great sound and quality so I'm not complaining!

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:

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.

Another step forward.....

Now its been time to assemble my new displayboard (module 8). I used the 7 segment displays from the old board but I had to take them apart to change the segment LEDs inside them. It is a well known problem that the segments go out one by one as time flies. This is discussed in another thread in this forum. The LEDs I have used will light up at a smaller current than the old ones so I needed to change the resistor values to the segments both on module 8 and module 9 (CPU) to get the display brightness right.

When I examined the old displayboard I saw that there was a burn mark on connector P75 pin 12 which is the power feed to the display board. These connectors will wear out and loose their contact pressure and this leads to bad connections here and there. Its always a good idea to replace those connectors to avoid display trouble in the future. The same applies to the CPU module with all its connectors. Bad connections here will lead to control problems and/or intermittent function.

You can see in the pic that I have fitted new Molex KK connectors all the way around module 8 and 9. I have also made a new cable harness with new connectors to module 9 instead of the old and often faulty flat cables.

The top pic shows the old display board.

The bottom pic shows the new display board together with the CPU module.

Restoring the keypads

I have discovered some problems with my keypads on the main control panel and the secondary control panel.

Some of the old metal discs have lost their tension and do not "click" like they used to do when depressed.

The solution I came up with was to use the discs from new tactile keypad switches like MEC 3CSH9 with a disc diameter of 6.5mm ( http://www.mec.dk/switches/switches_under_foil/3c ).

I simply took them apart and removed the little metal disc inside and fixed the discs on the keypad PCB with clear tape. Its a very fiddly job to get them right but its worth it to get the "click" feel back on the keypad.

The results for the main control keypad are shown in the pics.

/Fredrik

Hi all - What a great day!

Been spending several evenings in my workshop lately...

Now it was finally time to do a test run of my restored and rescued bm8000...and guess what...it really works!!!

Felt great!!!...

Now I just have to reassemble the rest of the cabinet and bring it to my living room to see how it performs...

/Fredrik

Great photo thread Fredrik,

I can't wait to see the finished article, your dedication is staggering.

Chris.

Thanx Chris

It will soon be time to reassemble everything. At the moment Im going through all the adjustments in the service manual to make sure everything is properly set.

/Fredrik