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Taking on the BM8000

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krais
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krais posted on Sat, Dec 28 2019 11:56 AM

Hi all, 

I've recently started restoring an old BM 8000 (type 1901) that I bought second hand years ago. The seller told me that the BM would not turn on (repeatedly clicking relays). I did not try to power the unit up to avoid blowing the output stages so not sure what the actual symptoms are.

After not being able to find a B&O technician in the Netherlands that is willing to take on a BM8000 due to the labor involved I decided to have a go at it myself. I know, I know, these are complicated amplifiers, not really stuff for an inexperienced hobbyist with little knowledge of electronics... I just cannot resist the challenge so decided to give it a try with the help of information available online (beolover website). 

Any help along the way would be appreciated. I love this design so would be awesome to get it working again (or at least take care of the laborious tasks such as recapping and rebuilding the LED displays).


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sonavor
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Replace the insulators with new mica insulators and grease. You can opt for sil pads instead of the mica/grease insulators if you want. I have restored using both methods without any problems. I just wouldn't re-use the old insulators at this point on yours. 

-sonavor

Beo_Jean
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Beo_Jean replied on Sat, Jan 11 2020 10:34 PM

Good choice on the Nichicons, I like them!

Personally, I'm using thermal pads instead of the micas, much cleaner work and one versus the other, a matter of preference.  Now, if you had multiple layers of mica, I would give the edge to the pad for heat dissipation performance.

I'm also concerned about the cooking PCB.  Mounting the resistors with a gap from the PCB is a good practice if you have clearance on top.  Consider adding cooling for that transistor that seems to suffer from heat too!

Dillen
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Dillen replied on Sun, Jan 12 2020 7:44 AM

"Also, I swapped the 0.5W resistors for 1W carbon film versions to alleviate some of the heat issues"
The same amount of energy will be dissipated, regardless ot the components wattage range.
You could do as Jean suggests and lift the resistors a milimeter off the board, though it's usually not a problem - unless completely burned - in which case something else will most likely be amiss too.

Lifting components without fitting some kind of relief under them puts stress on their solder joints, which must now carry
the components weight - "upwards".
The lifted components will easily be pressed down, forcing the solder pads from the board. Vibrations will also be much higher and will eventually cause cracking to the solder joints.
Using radial capacitors instead of axials will also do something like that. Why not use axials?

Martin

krais
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krais replied on Tue, Jan 14 2020 8:43 AM

Thanks for the great feedback all, I have ordered new mica insulators and will finish the output boards when they arrive. 

Dillen:
You could do as Jean suggests and lift the resistors a milimeter off the board, though it's usually not a problem - unless completely burned - in which case something else will most likely be amiss too.

Yes, indeed the resisters are raised slightly from the board.

Dillen:
The lifted components will easily be pressed down, forcing the solder pads from the board. Vibrations will also be much higher and will eventually cause cracking to the solder joints.

Good to know, I will be more mindful of minimizing vibrations when replacing components. I'll see if I can add some stress reliefs here and there (on the radial caps for example).

Dillen:

Using radial capacitors instead of axials will also do something like that. Why not use axials?

The reason I used radial capacitors for C205/C211 is that I couldn't find axial capacitors with -10%+50% tolerance as specified in the service manual. Maybe this is not a strict requirement?

Dillen
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Dillen replied on Tue, Jan 14 2020 10:10 AM

krais:

The reason I used radial capacitors for C205/C211 is that I couldn't find axial capacitors with -10%+50% tolerance as specified in the service manual. Maybe this is not a strict requirement?

It's not that critical. 20% tolerance caps will be fine.

Martin

krais
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Worked this weekend on replacing the mica insulators and performing the no-load current and DC offset adjustment procedures. Here are the completed output amplifier boards. 


krais
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I followed the adjustment procedure from beolover.com (link). Here's the setup using a small collection of vintage linear power supplies (one PSU each for -55V +55v power rails with 150mA current limit, one for the +15V line).


krais
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Setting the no-load current test voltage to ~18 mV


krais
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The +55V/-55V power rails were drawing around 100mA. 


krais
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krais replied on Mon, Jan 27 2020 8:22 AM

I was a bit nervous when trying this procedure for the first time but it all worked out. The no-load test voltage was a little difficult to get right as the voltage was drifting. It steadily increased from ~15mV to around 18mV after a few minutes. However, the voltage kept creeping up to 19.5-19.7V after 45 minutes as the components were getting warmer.

No issues with setting the DC offset voltage. After warming up the voltage kept drifting between -1.5mV and +1.5mV which I assume is ok. 

chartz
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chartz replied on Mon, Jan 27 2020 8:28 AM

I prefer adjusting when everything is back in place into the BM.

Do check your voltages when the amp board are back in place.

Jacques

krais
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Moving on to the power supply. Here is the board just after removal from the chassis.


krais
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Notice that C40 has bursted? This was one of 3 capacitors along with C39 and the one on the board with the voltage regulators that are not original. The guy who replaced those caps made a mistake and put in C40 with reversed polarity which caused it to fail.

 


krais
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Here's the recapped board. I also reflowed all connector solder joints.


Dillen
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Very nice.
MKS is probably not the worst choice here, but I really would have chosen tantals for the two in the remote receiver amplifier.
B&O chose to use tantals here for a reason.
We talked about tantalum capacitors vs. electrolytics in a thread here some time ago, but I can't seem to find it right now.

EDIT:
Found it

https://forum.beoworld.org/forums/p/25190/202897.aspx

Martin

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