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BM3000 for something to do

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Craig
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Craig replied on Tue, Apr 10 2018 12:32 PM

Having replaced 4 off 1N4148 diodes to no avail and so I have decided to replace TR44 & TR47. These are identified as BC310/BC311 respectively, these dont seem to be readily available so I would like to use NTE128 & NTE129........can anyone (Martin....) confirm the suitability of these as replacements?

Craig

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Beo_Jean replied on Tue, Apr 10 2018 1:12 PM

Craig:

Having replaced 4 off 1N4148 diodes to no avail and so I have decided to replace TR44 & TR47. These are identified as BC310/BC311 respectively, these dont seem to be readily available so I would like to use NTE128 & NTE129........can anyone (Martin....) confirm the suitability of these as replacements?

Craig

I would avoid NTE as much as possible in my restorations but the 2N5320 / 2N5322 will work just fine.

 

Craig
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Well....good news and bad, replaced the "new" trimmer I fitted to the right channel and now I can get the required 100mA total current signal without the trimmer turned fully clockwise, naturally I have parts on order that I may not now require. 


Craig
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however having now completed the replacement of the electrolytic caps and most of the tantals I find the mysterious "rustling" has returned to the left channel!.........as I havent had a speaker connected for a while I'm not 100% sure at what point this came back, it sounds sort of like RF pickup, I've disconnected the input signal to the amp at the 1uf cap (499) and the noise persist, so its being generated within the amp section I suspect it could be the tantals I have yet to replace....these are odd looking devils, yellow with white spots.....are these special in some way? come on Martin ;~)


Beo_Jean
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If you still accept help from other members; the positive lead of the tantal is located on the right side when the painted dot if facing you.  Otherwise, nothing special.  KEMET is a good brand BTW.


Craig
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Craig replied on Thu, Apr 12 2018 5:56 AM

Charles.....

Of course I'm more than happy to receive help from other members, I'm just trying to lure Martin back into the thread ;~) ................so looking at your post I can simply replace these odd looking fellows with 4.7uf tantals, they are identified as such in the schematic so I will go ahead.

Regards Craig

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Beo_Jean replied on Thu, Apr 12 2018 1:20 PM

Craig,

Personally, capacitors from 5uF and less, especially into the signal path, I replace them with films.  The WIMA 4.7uF 63V should fit just fine into PCB.

Craig
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Craig replied on Thu, Apr 12 2018 1:33 PM

I do quite like the look of the film capacitors, not that anyone ever see's them of course, however I have already bought tantals so I will go ahead and replace all four of these.....then go looking for the source of the noise with my scope ;¬) 

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Craig,

If you are at troubleshooting the crackling noise, you should pay attention to those black bullet transistors, they are known to be noisy.  I learned that when I restored my Beolab 5000.  I circled the ones I could spot from your picture.  Let me know the part number and I'll find you the equivalent.


Craig
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Craig replied on Thu, Apr 12 2018 8:56 PM

Ok.....another instalment on the saga of the 3000-2, the transistors Charles has identified are SGS87235, however I have removed them and switched them from the right to the left channel and the "rustling" sound persists.....I've also switched TR43 and TR58 as well as replacing the odd looking 4,7k tantals......all to no avail. It has eliminated a number of suspects........I will follow up tomorrow before the wine takes hold ;¬)

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Beo_Jean replied on Thu, Apr 12 2018 9:08 PM

Couple of questions Craig,

- Are the orange capacitors I see on the picture are on the noisy channel?  Unless you changed them already?

- Are you sure the noise comes from the output stage and not from the preamp or tone board?  Can you just disconnect the preamp coupling capacitor to the output stage and see if the noise persist?

- Are the tone controls and volume pots cleaned?

 

Craig
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Ok....thought I had replied to this last night put its disappeared.....non the less, here is an updated picture, I have been concentrating on the left side of the board (left channel) so there are still a couple of the odd tantals to replace on the right. I have tried disconnecting the output board by removing the grey/blue wire from the 1.0uf tantal (499)....I also removed the other side too for good measure, but the noise persists. I have cleaned the slider pots with alcohol and the gave then a squirt of servisol........ 


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Beo_Jean replied on Fri, Apr 13 2018 3:08 PM

Did you swap the output transistors?

Otherwise, it's time to fire-up the scope I think! Smile

Craig
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Ok...the scope is now involved, injected 2khz signal and went looking for it on both channels before I reconnected input signals to the output amp L & R sides, looked really good on both and was very steady...I know the pic is poor but the signals where both sound.


Craig
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Craig replied on Sat, Apr 14 2018 10:33 AM

Ok...as someone once said the way forward is to employ "halfing" as a way forward....went straight to the output caps and had a look, both traces where in good shape, but the left side waveform was wobbling all over the place, I'm assuming this is a funtion of the noise I hear on this channel?

Craig
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moved to the 4,7uf cap at the input to the bass/treble pots...got two good solid and stable waveforms...


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Craig replied on Sat, Apr 14 2018 10:41 AM

Measuring at the output side of the bass/treble pots at the 10uf cap I still get the two waveforms but the left channel is wobbling all over, playing with the bass slider makes it shake more......so I will look closer at the slider.

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Beo_Jean replied on Sat, Apr 14 2018 2:46 PM

If ever you need them Martin has both Treble and Bass pots and listed on eBay! 

I feel you're moving in the right direction

 

 

Craig
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With no signal injected and just the noise coming from the right channel I get these traces at the 4,7uf caps (433&521), input to the bass/treble network.


Craig
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not good pics I know.....but good enough for this, when I connect to the 10uf caps (449 & 536), output from the bass/treble network I get this, right channel on top.


Craig
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Craig replied on Sat, Apr 14 2018 3:02 PM

This is the noise I'm hearing.....the trace lives and breathes along wirh the changes in pitch and volume coming from the right speaker, this is the break point I think....still not sure about the cause, I have removed and checked the bass slider and it measures up fine with no suggestion of poor contact as it's adjusted from min to max.

Craig
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further measurements revealed I was getting the noise on the base of TR55 (right) but not on TR41 (left).....I wasn't getting the noise on the base of TR54 (right) or the base of TR40 (left). As I dont have the correct replacement trasistors I swapped TR54 and TR40 around......now the noise has gone from the right channel.......but it's also not to be heard in the left channel either! this is the trace I now get from the 10uf caps at 449 and 536........


Craig
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Craig replied on Sat, Apr 14 2018 4:39 PM

Bit of an anti-climax I'm afraid.........I can only guess at a poor solder joint on TR54, I'm pleased to have bottomed it out non the less.

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Beo_Jean replied on Sat, Apr 14 2018 5:14 PM

How do you measure the carbon potentiometers Craig?  With a digital multimeter?

Can I suggest you use an analog instead?  The total resistance of tracks is important however the slider contacts to signal ground is also very important to check but with a digital multimeter it's difficult to see if there is a problem.

Put one lead of the analog meter to the signal input of the pot and the other lead to the signal ground pin of the pot.  Then move slowly the slider, if the resistance is changing smoothly then the contact is good however if the meter needle jumps erratically then you have a problem.  It can be only dirt, dead spot or wear of the suspected track. 

 

Craig
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Craig replied on Mon, Apr 16 2018 10:49 AM

Actually I almost bought a AVO 8 a couple of months ago from Ebay....only because I felt a little nostalgic when I spotted it, we used them extensivly when I was an apprentice at ICI....and I notice there are still a couple for sale right now.

Craig
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Following on with the issue I was having with the right channel not getting up to 100mA without the trimmer wound clockwise quite a bit I received two replacement transistors so thought I may as well fit them and see if I can get the pot closer to midway. So I fitted BC141-16 & BC161-16 in place of what turned out to be texas instruments BFT80 & BFT40, I was expecting to find BC310 & BC311.......the texas instrument parts measured up ok but with hfe readings of 131 and 103......is this too far apart? I don't know, I do know that I am now able to get the 100mA with the trimmer turned a little more towards its central position.....but not massively, it's sat around 70% travel.....so I'm going to settle for that. 


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Beo_Jean replied on Tue, Apr 17 2018 2:22 PM

Craig,

I think you found very good replacements with the BC141 & 161, only the collector-emitter maximum voltage which is 10V lower but all other specs are pretty good and should last forever.  If you can get closer match for the HFE, it would be better but not dramatic.

Craig
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Just reading back over this post I can see I have been getting lost with my right and left channels!......originally the left channel had the "rustling" and the right channel wasn't able to draw the 100mA load. Now The right channel can provide the load....and I have resolved the "rustle" on the left, so all is clear and well.Moving on with recapping the pre amp.


Craig
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Craig replied on Fri, Apr 20 2018 7:47 AM

Having said all that.....following on from the recap of the pre amp I fed a little Santana into the unit and started to clear up a little as I enjoyed Carlos and the boys.....imaging my astonishment when, after about 10 mins the "rustling" returned! and as you can guess it now comes from the left channel, I know who is responsible so will dig him out and consign him to the trash can.....should have known a dry joint was a bit optimistic.

Craig
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Craig replied on Fri, Apr 20 2018 8:18 AM

Quick check through the random transistors I have reveals a BC182B NPN is available, I could use a little guidance on its suitability as a replacement for the rouge TR54 device BC173B....?

Craig

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tournedos replied on Fri, Apr 20 2018 8:23 AM
Craig:

Quick check through the random transistors I have reveals a BC182B NPN is available, I could use a little guidance on its suitability as a replacement for the rouge TR54 device BC173B....?

Craig

A definite maybe!

--mika

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Beo_Jean replied on Fri, Apr 20 2018 12:01 PM

Go with the BC549B or 550B.

 

Craig
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Craig replied on Sun, Apr 22 2018 8:47 AM

Ok rogue junction replaced and a test period (over 30mins) has established that the "rustling sound that is probably an easy fix" now seems to have been eliminated...............been a bit of a saga, but an enjoyable journey.....only the cosmetic side of things now.

Craig
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following on from the success of the scope fault finding I have recovered the woodwork with some left over veneer, could have sanded the scratches and marks out of the original to be honest.....but wheres the fun in that. Sanded down and first application of shellac applied


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cozza replied on Sun, May 20 2018 10:22 PM

Hi Craig.

Rustling / crackling sound from a bad transistor or diode junction may be localised using electronic freeze spray on each suspect component, without the extra work of swapping components from channel to channel.

Alternating freeze spray, waiting for change then with a little heat from a gas soldering iron or SMD rework station is usually sufficient to isolate a defective device. The rustling / crackling will change in intensity with temperature. This is a pretty standard faultfinding procedure for audio equipment.

HTH in future endeavours.

Colin.

Craig
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Craig replied on Mon, May 21 2018 12:59 PM

Thats interesting....I have heard of this but have steered away from it as I know if you supercool something it passes a lot more current, I have always worried in case damage to the downstream components could occur?

Craig

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cozza replied on Thu, May 24 2018 10:08 PM

Never had an issue with this method in 20+ years.. its a bit of a time saver. A drop or two of freeze is enough usually, but the important thing is to apply and wait.. it can take 10 to 20 seconds to cool the junction in the device through the case, and therefore to hear the change in the noisy part. Keep volume to a minimum and use a non beloved speaker for test, just in case you get any DC crackling.

Nowadays my favourite time saving tool in the arsenal is a high resolution thermal imaging camera, but that's another story.

 

 

 

 

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Dillen replied on Fri, May 25 2018 7:21 AM

The -40 degC of the cooling spray is far from supercool (-200 degC or lower).
It will not significantly affect any currentflow.
You can safely use it.

Martin

Craig
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Craig replied on Fri, May 25 2018 7:26 AM

Thanks for that.........It's certainly something I will try the next time I have such a fault, swapping/replacing components is time consuming it's true.

Craig

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Finally got around to finishing the case.........


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