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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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krais
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So far I have disassembled the BM and cleaned out most of the dust and debris. Here's the beast with all the boards removed and cables marked.


krais
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I'm now in the process of rebuilding the output stages (replacing caps and trimmers), cleaning and applying new thermal grease to the Darlington transistors. Here's the right output amplifier removed.


krais
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First question I have is on the trimmer replacements. I ordered Bourns 100ohm 12-turn 1/4W trimmers (PV37 Series). However they seems smaller than most of the trimmers most others are using. Would these be ok or should I really use 0.5W trimmers instead? 


Dillen
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Dillen replied on Sat, Dec 28 2019 4:37 PM

Use the good old black Piher PT-10. It'll be fine for the job.

But - I really would recommend diagnosing and repairing before making any "updates" and "improvements".
The initial problem may not be capacitor or trimmer related (indeed I don't think it is in this case) - and by doing a lot of work there's a huge risk of
introducing more faults, - including faults that have no "natural" cause, - and that could make diagnosing very difficult.

THEN - do one module at a time. Adjustments and all.

Now, when you've messed with the idle current settings, you will have to set those right away (both of them!), when the Beomaster starts up (if it will start), leaving you no time to do any other diagnosing at start-up, which could have been of interest with regards to the initial problem.
If you don't set the idle currents (and DC-offsets) right, the Beomaster may not stay powered on long enough for you to do the settings, or if set all wrong it may overrun and do enormous damage to one or both output stages.
Keep an eye on the current monitor of your variac the first time you start up the Beomaster - and cut the power immediately, if the power consumption is not normal.

Martin

Beo_Jean
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I would advise the same as Martin for the trimmers, PT-10 or PTC-10 for the cermet version for better thermal drift.

As for the capacitors, the red ones must go out, from past experience.  Actually, one looks dirty on top.  Might be a leaked cap or liquid with dust mixture...

Craig
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Craig replied on Sat, Dec 28 2019 5:35 PM

I have to applaud your ambition....and the amount of work you have already done is quite impressive too. Myself I would have chased down the actual fault before going to town on the whole unit....but hey you are where you are....this is going to be an interesting project, keep the pictures and dialogue coming Smile

Craig

krais
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krais replied on Sun, Dec 29 2019 8:58 PM

Thanks Martin, 

It was my understanding that it would be wise to change the trimmers before powering up. So figured I should start with the output amplifier boards first and it would make sense to replace the capacitors on those boards as well. In fact most of the caps I checked were either out of spec or completely gone.

I'm planning to set the idle current and DC offsets with the output boards out on the workbench and refrain from making too many updates.   

Dillen
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Dillen replied on Sun, Dec 29 2019 9:30 PM

krais:

I'm planning to set the idle current and DC offsets with the output boards out on the workbench and refrain from making too many updates.   

Great idea.
- But how?

And I agree on replacing the caps, but they will not be the reason for the present fault.

Martin

sonavor
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Hi,

I use this method to do my initial restorations of the Beomaster 8000 output amplifier assemblies. I have also successfully reworked the assemblies while they are still inside the Beomaster but I have found that I like reworking boards outside the cabinet better. Besides making the desoldering and soldering work easier, a lot of the units I have come across have required a lot of cleaning (dust, dirt, debris, rust, etc.). In addition, more than one output amplifier board has required some repairs to burned/damaged traces. 

-sonavor

krais
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krais replied on Mon, Dec 30 2019 10:21 PM

Thanks Sonavor, that's exactly the guide I was planning to follow. Those blog posts are so incredibly helpful.

Quick question on the dummy speaker load: I assume that for the DC offset adjustment procedure output power is limited so using two 50W 4ohm resistors in series would be ok? Thanks

 

sonavor
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That should be fine. The audio inputs to the output amplifier are shorted in my test setup so the audio signal is zero. After the output amplifier assembly is complete I recheck the no-load current adjustment and DC offset adjustment again when the assembly is back inside the Beomaster. 

-sonavor

krais
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Finally found time to make some progress on the output amplifiers. Replaced the elcos and trimmers (used the Bourns after all as my local supplier did not have the Pihers in stock). Also, I swapped the 0.5W resistors for 1W carbon film versions to alleviate some of the heat issues (the boards had significant burn marks).

I'm currently working at cleaning the dried out thermal paste and mica insulators on the Darlingtons. However, I noticed that although most transistors had a single mica installed, some had multiple (thinner) mica sheets. Does anyone know if there is a logic to this (there does not seem to be consistency between the right and left heatsinks)? 


sonavor
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The original mica insulators were probably flaking apart. Using mica and thermal paste only requires one mica insulator (plus the thermal paste).  Use thermal paste designed for transistor heat sinks. Not the paste used for computer processor cooling.

I like those Bourns multi-turn trimmers as well.

The bank of resistors on the output amplifier board do get hot. The discoloration on the PCB from their heat is normal but shouldn't be black...as I have seen on a couple of boards. I don't think increasing the wattage rating will reduce the heat. Were the original resistors still in tolerance?

-sonavor 

krais
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krais replied on Sat, Jan 11 2020 7:44 PM

It's weird, on one transistor I even found 3 thin mica insulators, one of which fell apart when cleaning. Do you think it would be ok to reinstall the micas even when using the thin sheets (I'm using Wakefield 120 for thermal grease btw)? Or would it be better to use sil pads instead? . 

The boards were quite dark but not black. Replacing the resistors was probably overkill although about a third of the old ones measured out of spec.

 

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