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Mr Roast's BM6000 Quad

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Craig
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Craig Posted: Wed, Oct 11 2017 8:05 PM

After returning Dave's Beocord 5000 complete with new belts and pinch rollers he kindly dropped off his failing BM6000 for my further entertainment


Craig
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On disassembly the first thing to jump out is this nasty looking relay cover.....must have taken quite a bash to break this.


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None the less I gingerly powered her up and was greeted with a mostly functioning machine, only the slider panel lamps failed to come on....and of course there was no sound from three of the four channels when connected to a speaker in turn. Turns out four of the six 12v bulbs are goosed, will need replacements (Martin.....do you have any for sale?).

As all the power supplies seem ok I checked the quiet current settings for each channel, all where very close to 14.4mv....this is encouraging as it suggests the power amps are all functioning...I think, Though one was a bit flakey when the trim pot was touched...will need to be replaced.


Craig
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Further close inspection under the illuminated magnifying glass revealed a broken connection at the tone board, y/w wire feeding RR tone control and ultimately the RR power amp, soldered him back and I now have 2 channels functioning.


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MrRoast replied on Thu, Oct 12 2017 9:53 AM

My first reply seems to have disappeared into the ether Confused so I'll write it again!

A great start! Glad to hear it's not in too bad a shape - I'd forgotten about for the zip-lock on the relay! (not a factory part I assume Big Smile ). From what I believe it's had the same owner from new so would look to have been repaired (bodged) over the decades - certainly the glue holding the button panel down wasn't done with he greatest of care!

Also it was great to finally meet you at the weekend Craig - another victim of the beovirus!

I'll excitedly keep watching your progress!

Craig
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Had a scratch around in my bag of bits and found a selection of bulbs which where intact.....clearly not all the same rating as can be seen, but they confirm the wiring and power supply is good....still need half a dozen 12v replacements however.


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Craig replied on Thu, Oct 12 2017 6:50 PM

Dave.....did you replace the drive belts on these slider ribbons, I seem to recall your saying you had done so? If not I will acquire a set from Martin, along with bulbs if he has them, and replace them for you.

Craig
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Having now got two channels up I have dragooned my analogue scope into service once again, bit rough and ready but I'm looking for a signal not "the lost chord" so along with a free signal generator app downloaded to my Ipad I injected 4khz and went looking for it.


Craig
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This was what greeted me when I disconnected the four big output capacitors to check for my 4khz.....


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One of them had suffered a totally corroded terminal, the 4khz was present at the loose terminal. the two working channels both had the signal present either side of the capacitor and the remaining capacitor had it going in but not coming out.....so its 4 off 4700uf 100v caps on order.


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New cans arrived today, fitted them and now I'm up to 3 channels functioning, I was hoping for all four!.....the failing channel LF is the one which had the output capacitor with the corroded terminal. there is a faint sound from the speaker but nowhere near right, tried checking the quiet current again on all channels and that has remained stable around the 14mV level, as I have a date with a couple of cool fosters down in Yarm I will leave this for tomorrow, as ever any sage advice from "Mr Dillen" would be well received ;¬) 


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Dillen replied on Fri, Oct 13 2017 8:24 PM

In "missing channel"-cases like this, it's always a good idea to diagnose using "halfing".

Go in halfway through the signal path and look for the signal.

If it's there - look further downstream.

If not - look upstream.

In this case - is the signal present on the amp channel input?

If not - is it present at the balance potentiometer?

The tonecontrol?

The source selector switch?

etc.

 

Probably not the reason for one missing channel but worth checking to counter for many problems:

There is a central point in the Beomaster, where several grounding wires meet up.

They are stiff single-strand leads and one can easily break off unnoticed but still hang on just in

its insulation - pull them gently one by one to check.

 

Nice work.

 

Martin

Craig
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Craig replied on Sat, Oct 14 2017 2:38 PM

Right....following the advice of the great man I employed the analogue scope again and went in search of the missing signal, only to find it isn't missing after all....the 4khz signal injected at the AUX input can be found at all four outputs from the tone board....when I adjust the balance control all the way over to the missing speaker it can be heard but very attenuated, looking at the trace on the scope the poor channel trace at the output of the tone board is weaker than the three that are fully functional...but not by as much as I would have thought for the big difference in volume at the output.

I'm tempted to swap the left and right outputs from the tone board around and see if the problem moves from one side to the other, if it does then my issue must lie from the tone board back over, I have connected my scope to the tone board inputs but the speakers make a buzzing sound when I do this that is a little scary. Should I just disconnect the speakers and forge ahead blissfully unaware of the noise....or is this something I should take note of and take more care? this is very definitely a learning curve here ;¬)

I will rest for now and open a bottle of wine to help me gather my thoughts.......

Craig
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Craig replied on Sat, Oct 14 2017 3:50 PM

Ok.....as the wine isn't cold enough I have taken my own advice and swapped the left and right tone board outputs around and sure enough the fault has changed sides also...proving the power amp side of things is good.......I suppose the next move should be to swap the tone board inputs around and see if the fault  moves back to the other channel.

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Craig replied on Sat, Oct 14 2017 4:23 PM

done that.....swapped the tone board inputs over and sure enough the fault has gone back to the original speaker output....that effectively puts the power amp and tone board in the clear.....replaced all the wiring back to its original configuration, love the way those B&O boys followed the same colour code on the internal wiring throughout for each individual channel....obvious I know but I like it all the same.Balance pots next in the frame ;¬)

Craig
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Craig replied on Sat, Oct 14 2017 5:12 PM

Taking the great mans advice yet again I jumped to the Level board PC7 and swapped the RR and RF outputs and sure enough the fault moved with the change to the other speaker...this now eliminates everything downstream of the Level Board.....definitely time to open that bottle now, will resume tomorrow.

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MrRoast replied on Sun, Oct 15 2017 12:29 AM

You forgot to mention what kind of wine it is! Big Smile

 

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Craig replied on Sun, Oct 15 2017 1:03 PM

Sauvignon Blanc.....and it has to be suitably chilled Drinks....shifted my attention to the Pre Amplifer board No 6 this morning and swapped the RR and RL signals over and was rewarded by discovering that the fault stayed with the RR speaker and didn't move over to the RL side.....therefore my issue lies in the level amplifier board No 7. This is not such good news as this is a difficult board to access,

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Craig replied on Sun, Oct 15 2017 3:42 PM

Shopping list of parts I'm thinking should be replaced on the level amplifier to be sure I nail the problem, I'm hoping if I replace all components in the RR circuit I will solve the issue

7D4,

7TR4,TR22,7TR23

7C4,7C28,7C29,7C31

Not many parts really.........

 

 

 

Craig
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Craig replied on Mon, Oct 16 2017 8:22 PM

Checking my stock of miscellaneous parts I found I had all the above capacitors and even a high speed diode, had to order the 3 off transistors though, and as they will take a day or so to arrive and I know how difficult the level board is to work on I made a start and worked my way through the above list starting with 7D4, checking each component as I replaced them they all checked out good until the last one......isn't it always like that? 7C31 was open circuit.

4,7uf 63v replacement fitted, careful check of all the single strand wires as the boards where replaced and a quick test revealed all four channels now fully operational, very satisfying (shame about the 3 off transistors).

Mr Roast you need to make a decision on where to go next, it still needs some lamps and I still don't know if you replaced the rubber drive belts? it could do with a capacitor replacement, however you may decide to cut your losses and keep the machine in its current  working condition....there's always the chance I will spill a glass of wine over it ;¬)

Drop me an email and let me know what you think, I wont complete the reassemble until then

Craig

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Well.......Dave aka Mr Roast has decided to bite the bullet and go for the lamps, belts and caps, everything on order from Martin and RS respectively, the tone board is the easiest starter so will make a start there over the weekend,


MrRoast
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MrRoast replied on Thu, Oct 19 2017 11:02 PM

Yes - thumbs up

I believe the saying is "Go hard or go home" Big Smile - the question is do I start looking for some P50 Beovoxes to complete the look? Wink

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chartz replied on Fri, Oct 20 2017 4:02 PM

One day I will have one!

So far, the ones (an awful lot actually) I’ve come across have always had badly scratched panels and Perspex covers.

Impossible for me.

Very nice work Craig - keep it up!

Jacques

Craig
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Had a delivery from RS so cracked on with the tone board....no issues, having replaced all the capacitors I gave all the single strand wires a coat of looking at, sure enough one had became detached with all the pulling and tugging, soldered him back and powered up for a test....all still good.


Craig
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Martins Package arrived today, fitted the replacement bulbs into the volume panel and all looks good, hard to get a decent picture though.....


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decided to go for the belt replacement next, get it over with....always a bit disconcerting pulling the pots out and withdrawing the cluch shaft, especially when one knows single strand wires are letting go as you do so.


Craig
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And a bit of a relief when it all goes back together, will need a good inspection before powering up for testing....I know at least one of the single strand black ground wires which Martin described as coming together in a central point has broken loose.


Craig
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Had a good look and only found the one ground wire adrift, soldered him back in place and powered up, blue oyster cult "don't fear the reaper" played sweetly enough....in fairness the old belts were in reasonable shape, no signs of cracks or hints of becoming perished, the clutch shaft coupling rubber tube had gone quite hard though, and as I had to buy a meter of the stuff when I did Peters BM6000 I had enough left over for this unit too.


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MrRoast replied on Thu, Oct 26 2017 8:03 PM

Ahh that's handy - the belts on the 6000 I've got here are totally shot so will come in very useful!

And I've just acquired my first CD-4 Quadradisc ready to test out the awesome four channels of goodness you're working on. Im excited to see the little 4CH light up on the BG6000!

Craig
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Output amp next.....hauled it out as gar as I dare.


Craig
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Far even.........this board went quite well, still need to replace the trimmers though.


Craig
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the use of 25 turn pots requires modification to the connection legs, as per Rudi's blog, however I was a bit nervous about the two legs that need to cross over and although Rudi was confident in the fix I was inclined to slip a bit of sleeving over one leg as a precaution.


Craig
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Not the best pic in the world....here's the remaining three.........


Craig
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these turned out to be quite a tight fit between the two big capacitors.....


Craig
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and the other side, there are four really annoying tantals sitting between the selector switches that I simply cannot get too for the wiring looms behind.


Craig
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simply cannot get a soldering iron in there.........looks like a proper rats nest.


Craig
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Back to the level board.....have to do this with the output amp lifted up, makes it easier....still a tough old dog to work on.


Craig
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Sooner its started the sooner it will be finished, nerve wracking to work on.


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MrRoast replied on Sat, Oct 28 2017 3:06 PM

You have the dedication of I-don't-know-what given the tight conditions in there!

Craig
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Nailed it......it is a bit of a horror show to work on but worth the effort.


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