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BM3000 for something to do

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Craig
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Craig Posted: Wed, Mar 28 2018 8:34 PM

Acquired this from an ebay seller, said it "worked fine for a couple of mins then  the left speaker made a bit of a rustling sound, its probably something very simple to repair"............


Craig
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Cant say I am over enthusiastic about this model, for no particular reason, and therefore never had one apart as yet....and as i'm short of a project right now I thought for 40 quid I would give it go. Looks like it may have had a bit of a hard life, the lid fell off the case when I picked it up.....could have been damaged in transit.


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Went ahead and connected up the Variac, multimeter in series, and slowly powered up.....very little current at 240vac however no signs of life.


Craig
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Replaced some lamps (12v @ 30ma) and things looked better.


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Plugged the Ipad into the TAPE input and sent it some Floyd.....right channel responded, left channel " made a rustling sound that will be something simple and easy to fix" ;~)


Craig
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The Floyd could still be heard in the left channel with the rustle overlaying it, when all inputs are switched OFF and no inputs connected the "rustling@ sound is still present......to be honest its more of a roar, bit like the noise a wave makes breaking on the shore.....but prolonged, none the less I think shes worth saving and she'll keep me out of the pub in the evenings Huh? 


solderon29
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solderon29 replied on Thu, Mar 29 2018 12:42 PM

Nice inside though isn't it?It doesn't seem to have been thrashed or contaminated with gunge like some!!!

It should scrub up well too,and with one of Dillens preset cover's in place,it will look a treat.

Shame about the casework.If they are clean breaks,you should be able to glue it back together?

The "something simple etc"causing the missing channel is almost certainly a dodgy tantalum bead coupling cap on the main board.A re-cap etc of that pcb is a must as a matter of course.

If the mains transformer is nice and quiet  too,the whole unit should be well worth restoring.

Get some can's in and give the pub a miss.I'd rather listen to the Floyd,but then I probably need to get out more!

Nick

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Krolroger replied on Fri, Mar 30 2018 11:30 AM

My BM3000-2 (S/N 59488) has a reversed polarity and placing of C472/473 and C561/568 compared to yours.  Other than that, it looks the same.

I wonder if there was a design change somewhere along the line?

As a matter of interest, how do you determine polarity of colour coded tants?

Simon

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Dillen replied on Fri, Mar 30 2018 12:08 PM

Krolroger:

My BM3000-2 (S/N 59488) has a reversed polarity and placing of C472/473 and C561/568 compared to yours.  Other than that, it looks the same.

I wonder if there was a design change somewhere along the line?

Yes.
The preamp board was also changed - more than once.
- And some have RCAs and DIN while others only have DIN.

Krolroger:

As a matter of interest, how do you determine polarity of colour coded tants?

Pins down, facing the spot, right pin is positive.

Martin

 

 

Craig
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Here's a curious thing, the manual I have, and the one in the library here, states the quiet current should be set to 100mA. As a matter of course before I start to look around I decided to check it out.....both channels are currently set around 21.0mA......I'm reluctant to change this without a second opinion.....any offers?

Craig


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Beo_Jean replied on Fri, Mar 30 2018 3:51 PM

Craig:

Here's a curious thing, the manual I have, and the one in the library here, states the quiet current should be set to 100mA. As a matter of course before I start to look around I decided to check it out.....both channels are currently set around 21.0mA......I'm reluctant to change this without a second opinion.....any offers?

Craig

Craig,

100mV seems way to high IMO, I read somewhere else that it should be around 10-15mV so at 21, you're not far off I think.

Got into another one already Wink

 

 

Søren Mexico
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On my BM 4400 it is 10-15 mV according to manual, I have it at 11 mV after warm up

Collecting Vintage B&O is not a hobby, its a lifestyle.

Krolroger
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Krolroger replied on Fri, Mar 30 2018 5:50 PM

100mA not mV.  I think it's measured by putting an ammeter in series with the 60 volt feed to each side of the board.

Across a single emitter resistor, I seem to remember that might equate to a 10-15mV voltage drop.

Anyway, I'm a novice round here so I'm happy to be corrected.

Simon

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Craig replied on Fri, Mar 30 2018 6:21 PM

It is mA.........the manual says 100 between the removable red/wht wire and the PCB pin......seems a bit too hight to me, seeing as both mine are currently set around 21mA

 

Søren Mexico
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Check mV across emitter resistor

Collecting Vintage B&O is not a hobby, its a lifestyle.

Craig
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Craig replied on Fri, Mar 30 2018 7:40 PM

Don't you just love all this chipping in with info.......everyone like to get involved, love it ;~)

Craig

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Beo_Jean replied on Fri, Mar 30 2018 8:00 PM

Craig:

It is mA.........the manual says 100 between the removable red/wht wire and the PCB pin......seems a bit too hight to me, seeing as both mine are currently set around 21mA

 

 

Sorry Craig, my bad, I was to suggest you use this formula to find the mV value A=V/R.  R is the emitter resistor value.

 

Craig
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Craig replied on Fri, Mar 30 2018 8:07 PM

hehe....commonly known as ohms law, quality 

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Craig replied on Fri, Mar 30 2018 8:11 PM

However.....getting back to the original question, the manual does state 100mA in series with the red/wht removable wire and the pin, and I have 21mA....something is amis here, I'm not about to adjust anything just yet Wink

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Craig replied on Fri, Mar 30 2018 8:11 PM

However.....getting back to the original question, the manual does state 100mA in series with the red/wht removable wire and the pin, and I have 21mA....something is amis here, I'm not about to adjust anything just yet Wink

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Beo_Jean replied on Fri, Mar 30 2018 9:06 PM

Do you have a scope Craig?

Here's what I do when I don't have or can't trust the value.

- Shoot a 1Khz signal thru the input.

- All tone controls to 0

- Volume control to zero

Check the scope and increase the volume just enough to get the sine wave shape on the scope.  Increase or decrease the quiescent pot until the sine wave looks undistorted.  Once you found the sweet spot, just give it another shot to increase the value by 3-5 mV.

Now load the outputs (Ex: 4 Ohm) and increase the volume just before clipping and monitor the heatsink.  It should get hot but it should stabilize after a while.  If the heatsink continues to increase in temp (thermal runaway phenomenon) that means the current is too high.

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Craig replied on Tue, Apr 3 2018 9:35 AM

Charles that sounds a little drastic.......thermal runaway, dont want to go there unless I really need to. I have changed the emitter trimmers for new piher encapsulated ones and set them at the same value as the old ones, give or take, and powered up. Adjusted each channel to 21mA as per the previous setting, thing is max to min adustment wont give me 100mA described in the service manual........I'm leaning towards the conclusion that the manual is throwing me a curved ball.

solderon29
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I seem to remember that this measurement has to be done after the unit has been on for some time,as the circuit needs to stabilise?

I think the setting is quite critical too?

What you're measuring is the total consumption of the stage of course,not just the quiescent current of the output transistors,as is usual with later models.

I don't have the manual to hand,but I think the drill is:volume at min,no speakers connected,then monitor the current for at least 10 minutes.Tweak the pot until everything stabilises at 100mA?

Hope this helps?

Nick

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Craig replied on Tue, Apr 3 2018 10:45 AM

Cheers Nick......just read the manual again and it does indicate a period of 10 mins prior to adjusting the pots, I will give it a go this evening, I thought it was just the quiescent current for the output transistors......looking at the circuit diagram more closely I can see the measurment point is the collector of TR44 (TR59) which is indeed the whole stage power consumption. The 100mA makes more sense now.....

Regards Craig

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Craig:

Charles that sounds a little drastic.......thermal runaway, dont want to go there unless I really need to. I have changed the emitter trimmers for new piher encapsulated ones and set them at the same value as the old ones, give or take, and powered up. Adjusted each channel to 21mA as per the previous setting, thing is max to min adustment wont give me 100mA described in the service manual........I'm leaning towards the conclusion that the manual is throwing me a curved ball.

Craig,

The proposed procedure is perfectly acceptable when you do not have at all or you do not have the right parameters in hand and you have the right working instruments.  Moreover, this is the technique that manufacturers use to write adjustment parameters in manuals.

Concerning the thermal runaway, of course, you have to show a little discernment and not wait until you can cook an egg on your heatsink before reacting.  Smile

The quiescent adjustment is important where the output stage transistors should never be turned off when the signal is low, hence the name "idle current", causing distortion, but not too high either not to cause a high volume temperature runaway.

I had to use this procedure in few cases but relied on a temperature sensor instead of my 50 old hard working hands...

But hey, if you saved another Beomaster from the trash bin, that's what really matters! 

 

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Craig replied on Tue, Apr 3 2018 2:48 PM

Charles

Now that I have an understanding of what is going on I will set up the 100mA when the unit has had chance to stabilise and then inject the 1khz signal and have a look with my scope at the waveform as you advise, measuring the mV over one of the emitter resistors and see what I get..........

Regards Craig

Craig
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Ok......powered up with the meter connected up to the left channel, left for 10 mins and was able to adjust the trimmer to 100mA


Craig
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However when I carried out the same procedure on the right channel I couldn't get more than 21mA 


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Craig replied on Tue, Apr 3 2018 9:08 PM

When checking for mV across the emitter resistors I get 10mV across the left channel resistors and nothing across the right channel resistors, however when speakers are connected up both channels sound fine....and the "rustling" sound has disappeared.....I have cleaned the switches and linear sliders which may be the reason for that.

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Beo_Jean replied on Tue, Apr 3 2018 10:00 PM

Craig,

I've never been able to get a stable readings on any DC coupled output capacitors amplifiers I worked on.  However loading the outputs with 4 ohm dummies, I was able to get stable readings.

But I can't explain why you have that big of a difference between both channels and obviously something is wrong.

Good luck 

Craig
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The manual identifies TR45/TR57 as responsible for the no load current.....so I will start by looking there this evening.

 


Craig
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Tested TR45 and it's checked out fine........


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Something for which I was quite gratefull.......it lives behind this shield, didnt want the hassle of removing it needlessly Smile


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Removed the right channel idle current trimmer and tested it, all looks good, replaced it and tried for the 100mA again.....I can now adjust this up to around 60mA but no further, however I can now set both channels to 11mV across the emitter resistors.


Craig
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And the right channel.....it should be noted that on this side the trimmer is adjusted to its maximum rotation (to the right) to achieve this reading, something is still not 100% 


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Krolroger replied on Fri, Apr 6 2018 12:01 PM

I'm particularly interested in your fault finding as I'm trying to fix up a 3000-2 with a blown channel.  I learn a lot from you and many other contributors, especially Martin.

But in respect of measuring idle current on these, Martin posted this a few years back:

"You can set the idle current by simply measuring the voltage across one of
the emitter resistors. This way you don't have to break the circuit...
10mV sounds about right".

From the photos, it looks like you're measuring the voltage drop across the resistor pair (R491/493) which might give rise to erroneous readings as there is a mid point connection between them.

Happy to be corrected if wrong.

Simon

 

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Craig replied on Fri, Apr 6 2018 12:32 PM

Simon

I'm also on a learning curve with this 3000-2. The 11.0mV that I'm measuring across both emitter resistors on the left channel is a result of setting the complete left channel power consumption, at idle, to the 100mA as prescribed by the service manual. If I measure across one resistor instead of both I would expect to see half the voltage (5.5mV)......this reading seems reasonable to me based on work Ihave done on BM4400 & BM6000 units.

My current issue is not been able to get the prescribed 100mA out of the left channel but I do have 11.0mV over the emitter resistors (with the trimmer fully wound round clockwise). I still suspect TR45 even though its tested ok.......I will get to the bottom of this eventually.

Craig

 

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Ok.....replaced the high speed diode IN4148 with a spare (496) that sits in the idle current circuit, it tested reasonable I thought at 0.85v.....however when I reassembled everything and powered up I now have the 100mA i'm looking for on the right channel, I'm not 100% happy as I have only turned the trimmer down a small amount to get to 100mA compared to the left channel........but I will carry on with the recap.


solderon29
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It might be worth replacing all those 1N4148's while you are at it.They can be "little tinkers",for causing this sort of odd problem in output stages.My experience has been that despite rigorous testing by instruments,components can still break down in circuit.

I've just been presented with a BM3000-2 with a faulty  output stage channel,so will be able to "refresh my memory" on these,and pass on any observations on site.

Regards,

Nick

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Craig replied on Sun, Apr 8 2018 12:18 PM

That's sound advice Nick.....I think I have enough 1N4148's to do just that so will go ahead, certainly wont do any harm.

Regards Craig

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